Examining Use of Habitat Fragments by Larval and Juvenile

Examining Use of Habitat Fragments by Larval and Juvenile Fishes in Lake Texoma River-

Reservoir Interface Zones

By

Morgan D. Gilbert, B.S.

A Thesis

In

Wildlife, Aquatic, and Wildland Science and Management

Submitted to the Graduate Faculty in

Partial Fulfillment of

the Requirements for

the Degree of

MASTER OF SCIENCES

Dr. Allison Pease

Chair of Committee

Dr. Timothy Grabowski

Dr. Gene Wilde

Dr. Mark Sheridan

Dean of the Graduate School

May, 2016

Copyright 2016, Morgan D. Gilbert

Texas Tech University, Morgan Gilbert, May 2016

ii

ACKNOWLEDGMENTS

No project is accomplished in isolation. This thesis is the product of the combined

efforts of many people. I’d like to first thank my partner, Katie Leuenberger, without

whose love and support none of this would have been possible. I would also like to thank

my advisor, Dr. Allison Pease, for more reasons than can be recounted here, but

especially for her expertise and understanding.

I would like to acknowledge the Gulf Coast Prairie Landscape Conservation

Cooperative for funding and supporting this project (GCPLCC 2013-04). I would also

thank the Oklahoma Department of Wildlife conservation (ODWC) and Texas Parks and

Wildlife Department (TPWD), particularly Cliff Sager, Matt Mauck and Richard Snow

(ODWC), David Buckmeier and Dr. Nate Smith (TPWD) as well as Dr. Gene Wilde, Dr.

Tim Grabowski and Matt Acre (TTU) for their assistance with sampling design. Finally, I

would like to thank Jared Breaux, Cassie Vaughan, Jade Stytz, Zach Redinger, Dylan

Sebek, and Gim McLarren for their technical assistance.

This work was carried out under the auspices of TTU Animal Care and Use

Committee (13113-12).


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TABLE OF CONTENTS

ACKNOWLEDGMENTS………………………………………………………………ii

ABSTRACT……………………………………………………………………………..iv

LIST OF TABLES…………………………………………………………………........v

LIST OF FIGURES……………………………………………………………………..vi

1. INTRODUCTION………………………...…………………………………………..1

2. METHODS……………………………………………………………………………7

3. RESULTS……………………………...……………………………………………..13

4. DISCUSSION…………………….………………………………………………….32

LITERATURE CITED………………………….…………………………………….40


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ABSTRACT

Lake Texoma is home to several isolated coves walled off by sedimentation as a

result of reservoir aging. The habitat fragments are relatively new features on the

landscape, isolated from the reservoir and taking diverse forms. These fragments have

been formed on the arms of two physicochemically distinct rivers entering Lake Texoma

(Red and Washita). Fragmented coves are located within the river-reservoir interface, a

highly productive and ecologically important transitional zone. I examined the structure

of young-of-the-year (YOY) fish assemblages utilizing these habitats and investigated the

influence of environmental factors on taxonomic and guild composition. Sampling was

carried out from March through August in 2014 and 2015 using light traps and push nets

to target larval and juvenile fishes. Differences in YOY fish abundance and assemblage

structure were observed between river arms and individual fragments. Analyses using

non-metric multidimensional scaling (NMS) and a follow-up analysis of similarity

(ANOSIM) revealed significant differences in the structure of larval assemblages

between years, and diversity was higher in the very wet, flooded 2015 season. While

habitat generalists were dominant throughout our study area, some fragments provided

habitat to species that rely upon river floodplain habitats for reproduction, especially

during the year with more extensive hydrological connectivity. This work should provide

managers with insights into the role that these novel habitats play in supplementing

reservoir fish assemblages.


v

LIST OF TABLES

Table 1. Summarized predictions and indicators by family of common fishes in Lake

Texoma.…………………………………………………………………………...….......6

Table 2. Summary of species captured by gear type. + indicates that the gear successfully

collected specimens of this taxa, while - indicates no collections with that gear type.

Light traps had the broadest success in detecting YOY fishes compared to both push nets

and fyke nets….……………………………………………………………………….....11

Table 3. Summary of keys and sources utilized for the identification of larval fishes

collected during sampling, including level of taxonomic identification……………….13

Table 4. Summary of habitat variables by site, including dominant substrate by site and

maximum dissolved oxygen (mg/l), temperature (°C), conductivity (μS/cm), and Secchi

depth (centimeters) by site.................................................................…………………...21

Table 5. Abundances of larval specimens collected during 2014 grouped by family. Very

large numbers of Dorosoma (Clupeidae) and Menidia (Atherinopsidae) (>90%) were

captured during sampling. …………………………………………………………......26

Table 6. Abundances of larval specimens collected during 2015 grouped by family. Very

large numbers of Dorosoma (Clupeidae) and Menidia (Atherinopsidae) (>90%) were

captured during sampling…………………………………………………………….....31

Table 7. Analysis of Similarity (ANOSIM) and Similarity Percentages (SIMPER)

statistics comparing the two sampling seasons (2014 and 2015) and individual years

samples by river arm (Red and Washita) and season (Spring and Summer). * indicates a

significant difference between categories…………………………………………..……38


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LIST OF FIGURES

Figure 1. Regional map showing the location of Lake Texoma with inset displaying

reservoir fragment sites sampled……………...………………………………………8

Figure 2. Log-transformed mean daily discharge (in CFS) for the Washita and Red Rivers

during 2014 sampling. Daily discharge measurements were retrieved from the

Gainesville gage on the Red River and the Durwood gage on the Washita River……..16

Figure 3: Log-transformed mean daily discharge (in CFS) for the Washita and Red Rivers

during 2015 sampling. Daily discharge measurements were retrieved from the

Gainesville gage on the Red River and the Durwood gage on the Washita River. Flows

spiked to very high levels in both the Red and Washita rivers beginning around May and

continuing through the end of sampling….……………………………..………………17

Figure 4. Mean (± SE) dissolved oxygen (mg/l), temperature (C), Secchi depth

(centimeters), and conductivity (μS/cm) measured during sampling events in 2014. Both

dissolved oxygen and temperature followed seasonal gradients, while Secchi depth and

conductivity varied between the Red and Washita river arms.… ……………………..19

Figure 5: Mean (± SE) dissolved oxygen (mg/l), temperature (°C), Secchi depth

(centimeters), and conductivity (μS/cm) measured during sampling events in 2015.

Dissolved oxygen and temperature followed a seasonal gradient, while Secchi depth and

conductivity varied between river arms. Both the Red and Washita rivers and associated

fragments were somewhat lower in turbidity and conductivity in 2015……….…….…20

Figure 6. Relative abundance of larval taxa in 2014 from light traps. Dorosoma and

Menidia were dominant in spring and summer, respectively, while other taxa were

collected in much smaller proportions…..…………………………………………….23

Figure 7. Phenology of appearance of larval taxa during 2014 sampling. Larvae were

collected at Washita River arm sites earlier than those on the Red River arm, and certain

taxa (Morone and Carpiodes) were detected only at Washita River arm sites………….24

Figure 8. Larval light trap catch per unit effort (CPUE, measured as abundance/minutes

of night), averaged across sites by river arm for the 2014 sampling season. Densities of

larval specimens tended to be similar between river arms, with the exception of Lepomis

and Morone species, which had higher CPUE on the Washita River arm. …….………25

Figure 9. Relative abundance of larval taxa in 2015 from light traps and push nets

combined. Dorosoma and Menidia once again dominated, but taxa such as Lepomis were

collected in higher proportional abundances compared to 2014……………………...28

Figure 10. Phenology of appearance of larval taxa during 2015 sampling. Specimens

were detected earlier in sampling and persisted for longer compared to 2014. Some taxa

collected only on the Washita River arm in 2014 – particularly Morone – were detected

on the Red River arm in 2015....……… …………………………………………...29


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Figure 11. Larval light trap catch per unit effort, averaged across sites by river arm, for

the 2015 sampling season. Densities of some taxa differed markedly between the Red and

Washita river arms. Dorosoma were collected in much higher densities on the Red River

arm, while Morone and Lepomis species were found in higher densities on the Washita

River arm.…………..……………………………………………………………………30

Figure 12. Non-metric multidimensional scaling of 2014 light trap abundance data,

categorized by river arm. ANOSIM found no evidence that assemblages present in Red or

Washita river arm sites differed significantly from one another (R=-0.009,

p=0.554)…....………………………………………………………………..…….….… 33

Figure 13. Non-metric multidimensional scaling ordination of 2014 light trap abundance

data, categorized by season. ANOSIM found no evidence that assemblages collected in

spring and summer significantly from one another (R=0.009,

p=0.354).………………………………………………………………...….……………33


categorized by reservoir fragment size (1 = >1 km2, 2 = 1-2 km

2, 3 = >2 km

2). ANOSIM

found no evidence to suggest that assemblage structure differed between small,

intermediate, and large fragments (R=-0.047,

p=0.790)………………..…………………..………………………………………..34


categorized by river arm. ANOSIM revealed that the Red and Washita river arm sites

differed significantly in assemblage structure (R=0.151, p=0.011). A SIMPER analysis

showed that the taxa driving the observed dissimilarity were Menidia, Dorosoma,

Lepomis macrochirus, and Morone………………………………………………..……35


categorized by season. ANOSIM found evidence to suggest that the Spring and Summer

assemblages differed significantly in assemblage structure (R=0.113, p=0.0430) and the

SIMPER analysis showed that the taxa driving the observed dissimilarity were Menidia,

Dorosoma, Lepomis macrochirus, and Pomoxis. .........………………… ……………..36


categorized by reservoir fragment size (1 = < 1 km2, 2 = 1-2 km

2, 3 = >2 km

2).

ANOMSIM found no evidence of a significant difference in assemblage structure

between different sizes of reservoir fragments (R=0.032, p=0.229).…… ……………36

Figure 18. Non-metric multidimensional scaling of 2014 and 2015 light trap abundance

data combined, categorized by sampling year. ANOSIM revealed that the 2014 and 2015

assemblages differed significantly in structure (R=0.214, p=0.001) and a the SIMPER

analysis showed that the taxa driving the observed dissimilarity were Dorosoma,

Menidia, Pimephales, and Lepomis macrochirus.……………… ………….………….37


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1. INTRODUCTION

In their seminal paper, Junk et al. (1989) state that “the principal driving force

responsible for the existence, productivity, and interactions of the major biota in river-floodplain

systems is the flood pulse.” In the domain of fisheries, this is especially apparent in assemblages

utilizing river-floodplain systems, where streams with greater latitudinal connectivity exhibit

greater diversity and productivity than those without (Ward et al. 1999; Gutreuter et al. 2000).

The timing and magnitude of flood pulse events is one of the major influences structuring

habitats within these systems and, consequently, the presence and abundance of fish species

within them. Junk et al. (1989) attributed this phenomenon in part to increased habitat area, but

further research has also revealed other mechanisms behind the supplementary effects of access

to floodplain habitats. Thorp et al. (1998) described the importance of terrestrial carbon inputs

from floodplain habitats as an important factor for productivity, with floodplain habitats

providing high nutrient loads into riverine ecosystems. This has the effect of increasing algal and

invertebrate abundance, thereby providing more prey for growing larval fishes and reducing

competition between them (Bayley 1995; Winemiller 2004). Larval fishes also benefit from

floodplain habitats as refugia from predation. The presence of abundant structure in the form of

debris and terrestrial vegetation provides shelter from predators present within floodplain

habitats (Copp 1992). Simultaneously, the relatively shallow and ephemeral nature of these

habitats makes it difficult for large-bodied piscivorous fishes to make use of them, reducing the

impact of predatory activity. These mechanisms provide for high-quality nursery habitat for a

variety of fish taxa in floodplain rivers. Both structure and resource abundance brought about by

higher productivity allow for greater survival of early life stages.


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The benefits of the productive nursery habitats found within floodplains are lost without

access to the floodplains, as flow regulation reduces the connections between river and

floodplain. Without this source of food and shelter for young fish, impounded rivers are often

dominated by more competitive lentic or generalist species both within the reservoir and in

adjacent river habitats (Herbert and Gelwick 2003; Guenther and Spade 2006). Even if a species

is otherwise well-adapted to the reservoir environment during its adult life, lack of access to

floodplains or other suitable habitats for reproduction and early growth of offspring may

diminish populations (Miranda and Bettoli 2010). However, some habitats associated with

reservoirs may be suitable for riverine, floodplain associated fishes, allowing such species to

persist alongside the dominant reservoir assemblage (Freitas-Terra et al. 2010).

The reservoir environment is not homogenous (Gido et al. 2002; Miranda et al. 2008),

and the transitional area where a river enters the reservoir – the river-reservoir interface (RRI) –

has come under increased scrutiny as a hotspot of diversity and productivity within regulated

river ecosystems (e.g., Kaemingk et al. 2007; Buckmeier et al. 2014). As a river enters the

reservoir, the water slows down, and suspended matter within it is deposited. This results in

higher inputs of nutrients into the RRI than the main body of the reservoir, which leads to high

productivity and species richness for both fish (Freitas-Terra et al. 2010) and zooplankton

(Nogueria 2008) assemblages. The deposition of sediments also changes the structure of the RRI

as sedimentation results in the development of diverse, hydrologically connected habitats

(Kaemingk 2007), including reservoir fragments – coves and backwaters walled off by sediment

deposition and only intermittently connected to the rest of the system.

Reservoir fragments in the RRI are of particular interest because they are isolated habitats

intermittently connected to the river or reservoir during times of high flow. These novel habitats


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potentially replicate some features of the nursery habitats previously provided by natural

floodplain pools, being structurally complex, nutrient-rich and shallow, slow-moving waters that

may provide many benefits to fish species making use of them (Kaemingk et al.. 2007; Graeb et

al.. 2009). The structural characteristics of reservoir fragments, combined with the highly

productive waters of the RRI and the proximity of upstream river habitats, make these habitats a

potential nursery for riverine fish, and may make them a desirable feature to be conserved or

cultivated by reservoir fisheries managers.

Lake Texoma, a reservoir on the Texas-Oklahoma border, has a diverse and well-

developed set of reservoir fragments in the RRI zones (Patton and Lyday 2008), ideal for

studying the factors that impact fish assemblage structure in these habitats. With two major

rivers, the Red River and the Washita River, feeding it, two RRI’s are present, each with

reservoir fragments of varying size, morphology, and age adjacent to them. Additionally, the

Red and Washita rivers present two distinct systems, each with abiotic factors that potentially

influence the fish assemblage structure. The Red River is a major tributary of the Mississippi,

and generally has higher discharge (USGS 2007), sediment load and conductivity (Kleiner 2010)

than the Washita. The Washita River is more entrenched within its channel (Baker et al. 2009),

and has a lower conductivity and suspended sediments. Its course is also highly altered as it

approaches the reservoir, channeling the river into Cumberland Cut, an artificial mud canal with

very little meander and abundant erosion that directs it into the main body of Lake Texoma

(Gilbert, personal observation). The reservoir fragments found on the Washita river arm are

located adjacent to Cumberland Cut. Lake Texoma is a major destination for anglers, and the

abundant populations of sport fish species, particularly Striped Bass (Morone saxatillis), are of

major economic impact in the region. Guide services and other services centered on recreational


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fishing for Striped Bass provide approximately $20 million to the local economy and have an

estimated total impact (including goods and services purchased by anglers unrelated to fishing)

of just over 57 million dollars (Schorr et al.1995; TPWD 2013). Understanding the role of

reservoir fragments in the reproduction and growth of this and other sport fish species will

provide important information for managers tasked with maintaining and enhancing Lake

Texoma recreational fisheries.

Patton and Lyday (2008) examined the morphology of reservoir fragments on the

Washita River arm of Lake Texoma and patterns in the structure of fish assemblages utilizing

them. Their study, which focused on adult fishes, found that the isolation of the transitional zone

of the reservoir into reservoir fragments resulted in differences in the assemblages utilizing those

sites as compared to non-fragmented reservoir habitats. They suggested that some form of

ecological succession was occurring in these fish assemblages. More information is needed on

how these reservoir fragments are used by early life stages of fishes, and the degree to which

they provide nursery habitat for species and guilds of concern for fisheries managers. I have

sought to fill this need by examining young-of-the-year (YOY) assemblages of fishes utilizing

the fragments of Lake Texoma. To this end, I pursued a series of objectives:

1) Determine the composition of the larval assemblage utilizing reservoir fragment habitats in

the RRI zones of the Red and Washita river arms of Lake Texoma.

2) Examine what local habitat variables lead to differences in YOY assemblage structure.

3) Examine the role of temporal variation in connectivity on assemblage structure over two

years.


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At the outset of this study, the reservoir fragments of the Texoma RRI zones had

experienced isolation due to low water levels owing to drought conditions in the region. Such

isolation was likely to favor taxa capable of enduring periods of high temperatures, low water

levels, and generally less favorable conditions. In these circumstances, I predicted that

dominance by generalist, reservoir species like Gizzard Shad (Dorosoma cepidianum) and Inland

Silverside (Menidia beryllina) would be observed across these habitats (Table 1). The presence

of other taxa – particularly Alligator Gar (Atractosteus spatula), some centrarchids, and some

cyprinid taxa such as Notropis and Cyprinella species– might indicate that the fragments were

providing adequate nursery habitat for fishes associated with river floodplain habitats. These taxa

were expected to be more abundant in seasons with higher hydrological connectivity. The second

year of the study (2015) was an exceptionally wet year, allowing for a powerful contrast with the

more typical, drier first year (2014). By examining the differences between the two years, I was

able to investigate changes in YOY fish assemblages during periods of isolation versus periods

of extensive connectivity.


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TABLE 1. Summarized predictions and indicators by family of common fishes in Lake Texoma.

Fish taxon Notes on ecology and predictions for habitat use

Cluepeidae Primarily Dorosoma cepidianum but also including Dorosoma petenense. Prefers warm, plankton-rich habitats. Spawns in rock and gravel habitats and avoids

muddy areas. Capable of tolerating very high water temperatures (up to 35°C).

Important to the diet of Morone saxatilis. Likely to persist in unfavorable

conditions (Gelwick et al. 2001).

Atherinopsidae Primarily Menidia beryllinia. Very common in large impoundments. Favors sandy

bottoms and spawns in areas of abundant vegetation. Highly tolerant of high

temperatures, low dissolved oxygen, and high conductivity (Gilbert and Lee 1982). Likely to persist in unfavorable conditions.

Pimephales Primarily Pimephales vigilax. Common in large impoundments, favoring mud

bottoms and areas with abundant floating vegetation. Resistant to high

temperatures, low dissolved oxygen, and prefers high turbidity habitats (Rutledge and Beitinger 1989). Likely to persist in unfavorable conditions.

Notropis Prefers sandy or silty substrates in areas of moderate flows, habitats with clear

waters (Edwards 1997). Presence may indicate interactions with riverine

assemblage through hydrological connection and colonization.

Morone Includes both Morone saxatillis and Morone chrysops. Major component of the

reservoir assemblage. Presence in reservoir fragments indicates connection to the

reservoir (especially for M. saxatilis), abundant prey for adults and young of year, and habitat favorable for their reproduction – sandy, rocky areas preferred over

muddy, silty ones. (Matthews et al. 1992)

Centrarchidae Includes representatives of Lepomis and Pomoxis. Many species (e.g., Pomoxis

annularis, Lepomis gulosus) tend to be found in high abundance in oxbow lakes and other backwaters (Rigs and Bonn 1959). A high relative abundance of these

fishes may indicate habitats have characteristics in common with natural

backwaters and isolated pools, provide adequate structural cover, and possess water quality factors desirable to these species (particularly low turbidity).

Lepisosteidae Large riverine fishes associated with floodplain habitats. As a taxon that favors

oxbow and other slack-water riverine habitats, gar may serve as an indicator of

high-quality “floodplain-like” habitat. Atractosteus spatula in particular, as a species with high site fidelity, may indicate that a reservoir fragment has

experienced regular connections with the greater RRI.

Catostomidae Historically found in coves of Lake Texoma prior to fragmentation (Riggs and

Bonn 1959). Carpiodes carpio is commonly found in silt-bottomed, low flow pools and backwaters that experience frequent connectivity to river habitats (Lee

and Plantania 1980). High relative abundance may indicate similarity and

connectivity to the reservoir assemblage.


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2. METHODS

Study Site Description

Lake Texoma is a 30,224 ha reservoir (TWDB 2014) at the confluence of the Red and

Washita rivers, located approximately 200 km south of Oklahoma City, Oklahoma, and 120 km

north of Dallas-Fort Worth, Texas. The reservoir lies along the Oklahoma-Texas border, with the

majority of the reservoir (70%) on the Oklahoma side. Denison Dam was completed in 1944 for

hydropower and flood control. The resulting reservoir is the 12th largest in the United States, and

is a major recreational site across the Texas-Oklahoma region (USACE 2013). Like many other

reservoirs built for flood control, Lake Texoma has undergone sedimentation as it has aged.

Much of its 101,362 km2 drainage is composed of agricultural lands subject to considerable

erosion (Matthews et al. 2005). This has resulted in previously navigable areas becoming

impassable to boats and the fragmentation of the RRI into isolated, shallow habitats. Large coves

have been walled off by deposited sediment, cutting them off from Lake Texoma and the RRI

(Patton and Lyday 2008). I chose reservoir fragments for study on both the Red and Washita

river arms of the Texoma RRI zone. Reservoir fragments were chosen based on accessibility,

navigability, and location within the RRI zone. Additionally, two adjacent main-channel river

sites were selected; one on the Red River, and another on the Washita River. These sites were

selected for proximity to other study sites and accessibility. Table 4 summarizes reservoir

fragment sites.


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Figure 2. Regional map showing the location of Lake Texoma with inset displaying reservoir

fragment sites sampled.

Summary of sampling methods

Following methods described by Kelso et al. (2007), I used three methods to thoroughly

sample both reservoir fragments and adjacent riverine systems. Larval light traps, larval push


9

nets, and fyke nets were all deployed in the habitats and seasons best suited for each gear type as

outlined below. There were two distinct phases in sampling, as changing seasons and shifts larval

and juvenile abundance required different sampling methods. In the first phase of the first

sampling season, running from March through July 2014, two sampling trips occurred each

month. Sites during that period were sampled with light traps and push nets. In the second phase

of sampling, running from August through November 2014, sampling was undertaken once

monthly with fyke nets. The second sampling season in 2015 ran from March through July, and

involved the use of light traps and push nets only, omitting the fyke net sampling later in the

season. All sampling was conducted aboard an aluminum johnboat equipped with a mud motor.

Early season: Light traps and push nets

Light traps operate by attracting fish larvae and juveniles to a light source – in this case, a

chemical light stick. The animals enter through narrow slits and enter a collection chamber.

Because of the design of the trap and the small size of the entrance, escape is unlikely. Light

trapping has been shown to be effective in sampling phototactic species, particularly in areas of

complex habitat where other sampling methods would be ineffective (Humphries et al. 2002).

Additionally, they tend to be more effective in capturing larger larvae and juveniles when

compared to other methods (Hernandez and Shaw 2003; Kelso et al. 2007). Three light traps

were deployed at each sampling site. A 30-m shoreline reach was selected for each site at

random from a pool of potential sample sites prior to each sampling visit and the traps were set

in such a way as to sample the variety of mesohabitats available in each shoreline selected. Traps

were secured in place using rebar in areas where the water depth was between 30 and 100 cm.

Geographic coordinates (GPS), depth, and notes on adjacent cover and substrate type were


10

recorded for each trap. Traps were placed no more than four hours before sunset, and then

retrieved after sunrise the next day. Using industrial grade light sticks with a12-hour duration,

the traps sampled throughout the night. Fishes collected were preserved in Excell Plus™ tissue

fixative and returned to the Texas Tech University campus for sorting and identification.

Push nets are a common sampling gear for collecting young fishes (Kelso et al. 2007),

and they operate by being secured alongside a moving boat and sampling along a transect. Fishes

are caught by the net and drawn into a collection assembly by the force of the water moving

through it. For this study, nets with a 50–cm diameter opening, 1.5 meters in length, with a 500

micron mesh were used. They are particularly effective in capturing fish in the earlier larval

stages, which are less likely to be able to avoid the net, and the nets do not discriminate based on

phototactic behavior (Kelso 2007). Push net transects began from points randomly selected from

a pool of potential transect starts. Transects then ran parallel to shore at approximately 1.5 m/s

for three minutes, with the only deviations from a straight line being to avoid extremely shallow

areas or accumulations of vegetation that would foul the sampling gear. A GPS coordinate of the

starting and ending points, an Oceanic Dynamics flowmeter reading, an approximate distance

traveled, and notes on any adult fish captured and released were collected for each transect. Push

net transect effort was standardized by sampling at approximately the same speed for the same

transect, and effort was quantified by volume sampled. Fishes collected were preserved and

returned to the Texas Tech University campus for sorting and identification.

Late season: Fyke netting

Late in the sampling season of the first year (September – November 2014), light traps

and push nets became less effective as the population of larval fish declined. Fyke nets are an


11

effective gear type for the capture of juvenile and adult fish that are capable of sampling both

open and structurally complex habitats. Fyke nets were set using the same randomized selection

of shoreline reaches as used for light traps. A single net was set in a randomly selected shoreline

reach in each study site (for river and reservoir fragment sites). The gear was left over night, and

samples recovered and processed the following day. Fishes were identified on site and released,

with a small subsample (<5%) retained and preserved in formalin for verification.

Bias in sampling

Light traps were the most successful sampling method (Table 2). A few rare taxa, particularly

Lepisosteidae and Ictalurus, were not captured in push nets. Fyke nets failed to capture

Dorosoma, likely due to their pelagic habitat use, and Ctenopharyngdon idella, likely due to

their relative rarity.

Table 2: Summary of species captured by gear type. + indicates that the gear successfully collected

specimens of this taxon, while - indicates no collections with that gear type. Light traps had the broadest

success in detecting YOY fishes compared to both push nets and fyke nets.

Fish taxon Light Trap Push Net Fyke Net Notes

Lepisosteidae + - + Floodplain associated

Dorosoma + + - Generalist

Cyprinella + + + Riverine

Pimephales + + + Generalist

Notropis + + + Floodplain associated

Ctenopharyngdon idella + - - Reservoir associated

Carpiodes carpio + - + Riverine

Ictalurus + - + Generalist

Menidia beryllina + + + Generalist

Lepomis + + + Floodplain associated

Morone + + + Reservoir associated

Pomoxis + + + Floodplain associated


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Habitat data collection

During all sampling trips, local habitat data was collected with a water quality meter.

Local habitat measurements included dissolved oxygen, conductivity, water temperature, and

Secchi depth. Measurements were taken the morning during light trap collection, from a point

between 15 and 20 m from shore. The water quality meter probe was submerged at a uniform

depth of 1 m. For all sites and sampling dates, I took notes on local weather conditions, the

presence of anglers, and any other factors that may have been pertinent to sampling.

Additionally, at river sampling sites I measured instantaneous discharge (in cubic meters per

second) using a Hach flowmeter. U.S. Army Corp of Engineers stream gauge readings at

USACE GSVT2 (Red River, Gainesville, Texas) and DURO2 (Washita River, Durwood,

Oklahoma) were used to assess mean daily discharge during sampling and to assemble a

hydrograph.

Specimen Identification

Larval samples returned to TTU from light trap and push net collection were first sorted

according to the taxonomic family before later being identified to the lowest possible taxonomic

level. Wallus et al. (2006 and 2008) was utilized extensively in the identification of

Centrarchidae, and Moronidae. Catostomidae were identified using Kay et al. (1994), Snyder et

al. (2008) was utilized for the identification of Menidia and Dorosoma, and a combination of

Taber (1969) and Wallus et al. (2014) was used to identify specimens of family Cyprinidae.

Juvenile samples from fyke netting were identified on-site to the species level before release

using Thomas et al. (2007) and Miller and Robison (2004).


13

Table 3: Summary of keys and sources utilized for the identification of larval fishes collected

during sampling, including level of taxonomic identification.

Taxon Source

Dorosoma (Genus) Snyder et al. (2008)

Menidia (Genus) Snyder et al. (2008)

Notropis (Genus) Taber (1969) and Wallus et al. (2014)

Cyprinella (Genus) Taber (1969) and Wallus et al. (2014)

Pimephales (Genus) Taber (1969) and Wallus et al.(2014)

Ctenopharyngdon idella (Species) Wallus et al. (2014)

Carpoides carpio (species) Kay et al. (1994)

Morone(genus) Wallus et al. (2006)

Lepomis gulosus (species) Wallus et al. (2008)

Lepomis macrochirus (species) Wallus et al. (2008)

Lepomis megalotis (species) Wallus et al. (2008)

Pomoxis (Genus) Wallus et al. (2008)

Analytical methods

To examine patterns in the structure of fish assemblages utilizing reservoir fragments and

adjacent riverine habitats, I used non-metric multidimensional scaling (NMS), an ordination

method. For the NMS analyses, I used matrices of log-transformed abundance from light trap

samples, and points (individual sampling events) were plotted along two primary axes by their

similarity.


14

Efficiency of fish collection with the push nets proved to be patchy due to difficulties

associated with sampling shallow, debris-rich habitats. Thus, the push net abundance data were

not analyzed using these methods. Not all sites were sampled using push nets at each sampling

event due to issues with boat accessibility. Additionally, push nets captured fewer fish taxa than

light traps. They tended to capture relatively few rare taxa, with samples being dominated by the

highly abundant Menidia and Dorosoma. Because fyke nets were used only in 2014, those data

were also omitted from statistical analysis. Because light traps were consistently and effectively

used to sample throughout both years and across both river arms, the trap data were selected as

the primary focus for ordination.

The NMS ordinations were examined for evidence of clustering by categories using

Analysis of Similarity (ANOSIM), and the taxa driving differences between groups were

identified using Similarity Percentages (SIMPER). The NMS ordinations were analyzed

separately for 2014 and 2015 and with sites categorized and color coded based on the season of

individual samples (spring or summer), the river arm (Red or Washita), and the size of the

fragment – with size category 1 being the smallest (< 1 km2), size category 2 being intermediate

(1-2 s km2) and size category 3 covering the very largest reservoir fragments (>2 km

2). An

ANOSIM was used to detect any clustering or separation based on these a priori categories, and

if a sufficient p-value was found to indicate statistically distinct clusters, a SIMPER analysis was

used to detect which taxa were driving these differences. The NMS, ANOSIM, and SIMPER

analyses were carried out using the PRIMER v7 statistical package (Clarke and Gorley 2015).


15

3. RESULTS

Hydrography & Connectivity

The two sampling years, 2014 and 2015, showed a strong contrast, with 2014

experiencing minimal rainfall, low flows (Figure 2) and virtually no hydrological connections

between fragments and river arms. In 2015, the region received very heavy rainfall during the

sampling season and consequent high flows (Figure 3) and abundant connections between river

and reservoir fragment habitats were observed.

Figure 2. Log-transformed mean daily discharge (in CFS) for the Washita and Red Rivers during

2014 sampling. Daily discharge measurements were retrieved from the Gainesville gage on the

Red River and the Durwood gage on the Washita River.

0

1

2

3

4

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6

3/15

/20

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3/29

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4/19

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/20

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/201

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/20

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8/23

/20

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8/30

/20

14

log

mea

n d

aily

CFS

Date

Washita

Red


16

Figure 3: Log-transformed mean daily discharge (in CFS) for the Washita and Red Rivers during

2015 sampling. Daily discharge measurements were retrieved from the Gainesville gage on the

Red River and the Durwood gage on the Washita River. Flows spiked to very high levels in both

the Red and Washita rivers beginning around May and continuing through the end of sampling.

The hydrological conditions observed in 2014 were associated with the drought that

shaped local climate factors in the region since 2011 (Shideler et al. 2012). Sparse rainfall

occurred late during the 2014 sampling season and led to some peaks in discharge, particularly

along the Washita River arm. Flows in both the Washita and Red River were generally low in

spring and summer of 2014 (Figure 2). The Washita River peaked in June at 10,100 CFS before

falling. The Red River displayed little variation in flows during the 2014 season, with the highest

peak occurring very late in sampling at 1,210 CFS in August. With low water levels across both

river arms, no natural connections between reservoir fragments and other bodies of water

occurred even during peak flows. A single artificial connection – a small ditch constructed for

0

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0/2

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5

log

me

an d

aily

CFS

Date

Washita

Red


17

airboat access – was observed at Lebanon pool, but was so small (<10 cm wide) that it likely had

minimal effect through the 2014 season.

In stark contrast to 2014, 2015 was an exceptionally wet year. Heavy rainfall began in

early May and continued through the end of the season. This resulted in exceptionally high flows

in both the Red and Washita rivers (Figure 3). The Red River maintained very high flows

throughout the season (peaking at 67,200 CFS in June), and while the Washita River reached

even higher flows (peaking at 106,000 CFS in June), it showed a greater degree of variation with

intermittent peaks followed by declines. Connections between reservoir fragments and the

reservoir were established beginning in May, with no fragment avoiding connection and many

becoming highly connected coves of Lake Texoma.

Habitat variables

While a strong difference in flows occurred between 2014 and 2015, and connectivity

and total wetted area increased, other local habitat conditions were less distinct between years.

Several patterns in habitat variables were apparent in 2014 (Figure 4, Table 2). Dissolved oxygen

declined from spring into summer across both river arms. Likewise, temperature generally

increased until reaching a peak in July and declining through August. Neither the Red River nor

the Washita River was distinct from one another in these variables. Turbidity, as measured by

Secchi depth, increased as the 2014 sampling season progressed. Sites on the Washita River arm

were generally less turbid than those on the Red River arm early in the season. The only variable

that did not change at an observable level over the course of sampling was conductivity. Sites on

the Red River arm displayed distinctly higher conductivity than Washita River arm sites. Similar

patterns were apparent in 2015 (Figure 5). Oxygen declined over the course of sampling in both


18

the Red and Washita rivers, with a commensurate increase in temperatures in both river arms.

Again, the Washita River arm sites were less turbid than the Red in 2015, while the Red had

higher conductivity.

Figure 4. Mean (± SE) dissolved oxygen (mg/l), temperature (C), Secchi depth (centimeters), and

conductivity (μS/cm) measured during sampling events in 2014. Both dissolved oxygen and

temperature followed seasonal gradients, while Secchi depth and conductivity varied between the

Red and Washita river arms.


19

Figure 5: Mean (± SE) dissolved oxygen (mg/l), temperature (°C), Secchi depth (centimeters),

and conductivity (μS/cm) measured during sampling events in 2015. Dissolved oxygen and

temperature followed a seasonal gradient, while Secchi depth and conductivity varied between

river arms. Both the Red and Washita rivers and associated fragments were somewhat lower in

turbidity and conductivity in 2015.

Some differences in physical habitat were apparent between reservoir fragments, even

those located near one another on the same river arm (Table 4). Widowmoore was the largest and

deepest site, but lacked structural cover when compared to the neighboring Kansas Creek site.

Lebanon Pool and Cumberland Pool were similar in size, morphology, and proximate land use

(predominantly recreational). Hickory Creek was very shallow but also possessed abundant

cover and structure in the form of logs and other debris. Some sites were fed, intermittently or

perennially, by small creeks – including Kansas Creek, Widowmoore, Briar Creek, and Hickory

Creek. Sites also differed in terms of local substrate and other habitat variables (Table 4).


20

Table 4: Summary of habitat variables by site, including dominant substrate by site and maximum dissolved oxygen (mg/l),

temperature (°C), conductivity (μS/cm), and Secchi depth (centimeters) by site.

Site River

Arm

Dominant

Substrate

Max

Do

2014

Max

Do

2015

Max

Temp

2014

Max

Temp

2015

Max Conductivity

2014

Max Conductivity

2014

Max Secchi

Depth

2014

Max Secchi

Depth

2015

WIDOWMOORE Washita Sand 9.99 9.87 29.9 28.6 424.5 322.3 94 72

KANSAS CREEK Washita Gravel 9.66 9.88 30.3 27.6 344.4 260.5 54 36

CUMBERLAND

POOL

Washita Mud 10.66 9.96 28.1 27.5 1048 865.2 74 68

LEBANON POOL Red Mud 9.89 9.73 28.1 27.8 2752 2354 30 23

HICKORY CREEK Red Mud 9.28 9.29 26.5 25.9 2692 2522 37 31

WILSON CREEK Red Sand 7.55 7.72 26.2 26.2 2495 2065 48 36

BRIAR CREEK Red Mud 9.35 9.21 27.5 27.6 1579 1235 34 20


21

Relative abundance of larvae

In 2014, I collected 18,950 larvae across all sites using light traps. Dorosoma

dominated abundance in the spring in both river arms (Figure 6, Table 3). Menidia

beryllina were also highly abundant in both arms. During the spring, larval Lepomis

species (particularly Lepomis macrochirus) and Notropis and Cyprinella species were

also collected frequently in Washita sites. The Red River arm produced somewhat more

specimens, but these were overwhelmingly Dorosoma and Menidia, with few other taxa

represented. Morone larvae were only collected in the Washita River arm in 2014, with

no detections in the Red River arm in either spring or summer. In the summer of 2014,

Menidia became the most dominant taxon across both river arms, and Dorosoma

remained somewhat abundant. The proportion of Lepomis and Pomoxis increased over

the course of sampling in the Washita River arm, while the total dominance of Dorosoma

and Menidia in the Red river arm declined with the detection of a broader array of taxa,

including Notropis, Pimephales, Ictalurus, Lepomis and Pomoxis. Generally, larvae were

collected on Washita River arm sites before the Red River arm in 2014 (Figure 7), and

the Washita sites continued to yield larvae for a longer period of time. Certain taxa,

particularly Lepisosteidae, Morone, and Carpiodes carpio, were only detected only on

the Washita river arm in 2014. Morone were detected earlier in the Washita and not at all

in the Red River arm in 2014 (Figure 7). Lepomis larvae were also collected in greater

numbers overall in the Washita River arm (Figure 8). Generally, the Washita River sites

contained higher abundances of larvae earlier in the season compared to sites on the Red

River arm in 2014. However, greater total abundances of larvae were collected from the

Red River arm, largely due to higher numbers of Menidia larvae.


22

Figure 6. Relative abundance of larval taxa in 2014 from light traps. Dorosoma and

Menidia were dominant in spring and summer, respectively, while other taxa were

collected in much smaller proportions

0

1000

2000

3000

4000

5000

6000

7000

8000

Spring Washita Spring Red Summer Washita Summer Red

Tota

l Cat

ch

Season and River Arm

Cyprinids Centrarchids Clupeids Lepisosteids Ictalurids Atherinopsids Moronids Catastomids


23

Figure 7. Phenology of appearance of larval taxa during 2014 sampling. Larvae were

collected at Washita River arm sites earlier than those on the Red River arm, and certain

taxa (Morone and Carpiodes) were detected only at Washita River arm sites


24

Figure 8. Larval light trap catch per unit effort (CPUE, measured as abundance/minutes

of night), averaged across sites by river arm for the 2014 sampling season. Densities of

larval specimens tended to be similar between river arms, with the exception of Lepomis

and Morone species, which had higher CPUE on the Washita River arm.


25

Table 5: Abundances of larval specimens collected during 2014 grouped by family. Very large numbers of Dorosoma (Clupeidae) and

Menidia (Atherinopsidae) (>90%) were captured during sampling.

Cyprinidae Centrarchidae Clupeidae Lepisosteidae Ictaluridae Atherinopsidae Moronidae Catostomidae

Spring

Washita

152 110 2319 4 4 1048 86 5

Spring

Red

10 3 3118 0 0 1121 1 0

Summer

Washita

72 286 395 0 0 2444 7 2

Summer

Red

238 123 1761 20 124 5562 0 4


26

In 2015 I collected 12,202 larvae over the course of sampling. The lower overall catch

was due in part to sampling difficulties associated with exceptionally high rainfall and some sites

being omitted due to inaccessibility caused by high waters and barriers to travel by boat or truck.

A similar pattern in Menidia and Dorosoma abundance occurred in 2015 as in 2014, with

Dorosoma dominating the spring samples with Menidia second most abundant, and the latter

became most abundant in the summer (Figures 10 and 11). Morone were captured in both river

arms in 2015, although in greater abundance on the Washita River arm. Larval Lepomis and

Pomoxis were also detected in sites where they had not been collected in 2014 (Table 4). Some

taxa were detected earlier in 2015 than in 2014 (e.g., Notropis), while most others appeared at

roughly the same time in both arms. Once again, Lepisosteidae and Carpiodes carpio were only

detected in the Washita River arm sites, but Morone larvae were collected in Red River arm sites

for the first time in 2015 sampling. Notably, the Red River arm sites produced denser

abundances of Dorosoma in 2015, while densities of Menidia beryllina and Lepomis were

roughly similar across the river arms. Morone, Notropis, and Cyprinella were collected in higher

abundances on the Washita River arm throughout 2015 sampling.


27

Figure 9. Relative abundance of larval taxa in 2015 from light traps and push nets combined.

Dorosoma and Menidia once again dominated, but taxa such as Lepomis were collected in higher

proportional abundances compared to 2014.

0

1000

2000

3000

4000

5000

6000

7000

8000

Spring Washita Spring Red Summer Washita Summer Red

Tota

l Cat

ch

Season and River Arm

Cyprinids Centrarchids Clupeids Lepisosteids Ictalurids Atherinopsids Moronids Catastomids


28

Figure 10. Phenology of appearance of larval taxa during 2015 sampling. Specimens were

detected earlier in sampling and persisted for longer compared to 2014. Some taxa collected only

on the Washita River arm in 2014 – particularly Morone – were detected on the Red River arm in

2015.


29

Figure 11. Larval light trap catch per unit effort, averaged across sites by river arm, for the 2015

sampling season. Densities of some taxa differed markedly between the Red and Washita river

arms. Dorosoma were collected in much higher densities on the Red River arm, while Morone

and Lepomis species were found in higher densities on the Washita River arm.


30

Table 6: Abundances of larval specimens collected during 2015 grouped by family. Very large numbers of Dorosoma (Clupeidae) and

Menidia (Atherinopsidae) (>90%) were captured during sampling.

Cyprinidae Centrarchidae Clupeidae Lepisosteidae Ictaluridae Atherinopsidae Moronidae Catostomidae

Spring

Washita

23 19 421 17 1 333 33 3

Spring Red 8 53 2302 0 0 1002 23 0

Summer

Washita

35 126 156 0 1 1356 13 0

Summer Red 67 180 1341 0 16 4666 7 0


31

Patterns in assemblage structure

The optimal two-dimensional NMS solution using abundances from larval light traps in

2014 had a final stress of 0.12 (Figure 12). Sites on the left side of axis 1 were mostly on the

Red River arm and were associated with relatively high abundances of Pimephales. The top of

axis 2 was associated with some sites containing high abundances of Notropis, particularly

Cumberland pool and Lebanon pool, which have a greater likelihood of hydrological

connectivity than neighboring sites. The bottom of axis 2 was associated with sites containing

rarer taxa, particularly Carpiodes carpio and Morone, driven primarily by higher abundances

captured at Kansas Creek on the Washita River arm. Sampling events in the Red or Washita river

channel sites tended to cluster with one another away from the reservoir fragment samples. The

ANOSIM comparison of assemblage structure based on river arm indicated no significant

grouping (R =-0.009, p = 0.554), and that the Red and Washita rivers were more similar than

different in 2014. The ANOSIM comparing assemblage structure based on season for 2014

found no significant groupings (R = 0.009, p = 0.354), indicating that the spring and summer

seasonal assemblages did not differ significantly (Figure 13). The optimal 2-dimensional NMS

solution of larval fish abundance in reservoir fragments only (without river channel sites) for

2014 had a final stress of 0.11 (Figure 14). The ANOSIM based on size categories of reservoir

fragments found no significant groupings (R = -0.047, p =0 .79), indicating that the larval

assemblages present in fragments of all sizes were roughly similar.


32

Figure 12. Non-metric multidimensional scaling of 2014 light trap abundance data, categorized

by river arm. ANOSIM found no evidence that assemblages present in Red or Washita river arm

sites differed significantly from one another (R=-0.009, p=0.554)

Figure 13. Non-metric multidimensional scaling ordination of 2014 light trap abundance data,

categorized by season. ANOSIM found no evidence that assemblages collected in spring and

summer significantly from one another (R=0.009, p=0.354).


33


by reservoir fragment size (1 = >1 km2, 2 = 1-2 km

2, 3 = >2 km

2). ANOSIM found no evidence

to suggest that assemblage structure differed between small, intermediate, and large fragments

(R=-0.047, p=0.790)

The optimal 2-dimensional NMS ordination of sites based on the abundance of larvae in

2015 had a stress of 0.1 (Figure 15). In general, axis 1 separated Washita River channel sites

from a subset of Red River reservoir fragments. Carpiodes carpio and Cyprinella abundance was

associated with sites plotted on the right side of axis 1. The left side of axis 1 was associated

with sites with overall low diversity and abundance. Sites plotted high on axis 2 were associated

with relatively high abundances of Morone on the Washita River arm. As in 2014, samples from

sites in the Red or Washita river channel tended to cluster with one another away from the

reservoir fragment samples. The ANOSIM comparing sites based on river arm revealed

significant grouping by river arm (R = 0.151, p = 0.011), indicating that significant differences in

assemblage structure existed between river arms in 2015. The SIMPER analysis indicated that

the principal taxa driving the differences in assemblage structure between river arms were

Dorosoma, Menidia beryllina, Lepomis macrochirus, and Morone (Table 5). The ANOSIM

comparing assemblage structure of sites based on season indicated significant seasonal grouping


34

(R = 0.113, p = 0.043), providing some evidence that the assemblages utilizing reservoir

fragment habitats differed between spring and summer (Figure 16). The SIMPER analysis

indicated that the taxa driving this difference were Menidia beryllina, Dorosoma, Lepomis

macrochirus, and Pomoxis (Table 5). The optimal 2-dimensional NMS ordination of larval fish

abundance in reservoir fragment sites only (without river channel sites) for 2015 had a final

stress of 0.11 (Figure 17). The ANOSIM (R = 0.032, p = 0.229) indicated no significant

grouping based on reservoir fragment size classes.


by river arm. ANOSIM revealed that the Red and Washita river arm sites differed significantly in

assemblage structure (R=0.151, p=0.011). A SIMPER analysis showed that the taxa driving the

observed dissimilarity were Menidia, Dorosoma, Lepomis macrochirus, and Morone


35


by season. ANOSIM found evidence to suggest that the Spring and Summer assemblages

differed significantly in assemblage structure (R=0.113, p=0.0430) and the SIMPER analysis

showed that the taxa driving the observed dissimilarity were Menidia, Dorosoma, Lepomis

macrochirus, and Pomoxis.


by reservoir fragment size (1 = < 1 km2, 2 = 1-2 km

2, 3 = >2 km

2). ANOMSIM found no

evidence of a significant difference in assemblage structure between different sizes of reservoir

fragments (R=0.032, p=0.229).


36

The optimal 2-dimensional NMS solution based on larval abundances at sites across both

years combined had a final stress of 0.12 (Figure 18). Axis 1 was associated with 2014 sampling

events from sites containing Carpiodes carpio on the right side and 2015 samples from sites

containing Morone on the left side. Sites plotted on the far right side of axis 2 were associated

with abundances of Cyprinidae (particularly Cyprinella, Pimephales and Ctenopharyngodon

idella) captured during both sampling periods. The ANOSIM based on sampling year showed

that the assemblages collected in 2014 and 2015 differed significantly (R = 0.214, p = 0.001).

SIMPER found that the taxa driving the differences in assemblage structure between years were

primarily Dorosoma, Menidia beryllina, Pimephales, and Lepomis macrochirus (Table 5).

Figure 18. Non-metric multidimensional scaling of 2014 and 2015 light trap abundance data

combined, categorized by sampling year. ANOSIM revealed that the 2014 and 2015 assemblages

differed significantly in structure (R=0.214, p=0.001) and a the SIMPER analysis showed that

the taxa driving the observed dissimilarity were Dorosoma, Menidia, Pimephales, and Lepomis

macrochirus.


37

Table 7: Analysis of Similarity (ANOSIM) and Similarity Percentages (SIMPER) statistics

comparing the two sampling seasons (2014 and 2015) and individual years samples by river arm

(Red and Washita) and season (Spring and Summer). * indicates a significant difference between

categories.

Comparison Sample Statistic

(R)

ANOSIM

p-value

SIMPER

dissimilarity

Dissimilar Taxa

Between 2014 & 2015 0.214 0.001* 68.40% Dorosoma (30.10%), Menidia

(28.22%), Pimephales (8.26%), Lepomis macrochirus (7.84%)

2014 Arms -0.009 .554 70.34% Menidia (35.31%), Dorosoma

(19.01%), Pimephales (14.90%),

Lepomis macrochirus (6.01%) 2014 Seasons 0.009 .354 70.87% Menidia (35.96%), Dorosoma

(20.40%), Pimephales (14.77)

2015 Arms 0.151 0.011* 53.83% Menidia (28.53%), Dorosoma

(25.19%), Lepomis macrochirus

(10.10%), Morone (8.83%)

2015 Seasons 0.113 0.0430* 51.97% Menidia (30.72%), Dorosoma

(19.38%), Lepomis macrochirus

(13.35%), Pomoxis (6.61%)

In general, river channel sites tended to be similar to both the reservoir fragments

adjacent to them, and with one another. Some clustering of riverine sampling sites was apparent

in both 2014 and 2014. Notropis and Ctenopharyngdon idella tended to influence riverine

sampling sites the most, potentially driving some of the observed similarity between the Red and

Washita rivers. Differences in abundances driven by particular reservoir fragment sites also had

some influence on assemblage structure. Kansas Creek was the most diverse and productive site

on the Washita River arm, contrasting with both Widowmoore and Cumberland Pool in

producing larger abundances of Lepomis and various Cyprinidae (particularly Notropis). Sites

that possessed very little structural cover and had either muddy or sandy substrates tended to be

the least productive, including Wilson Creek, where I collected only Menidia and Dorosoma

during 2014 sampling. Individual sites generally conformed to trends found on both river arms,

but there were exceptions. Widowmoore, despite being on the generally more diverse Washita

River arm produced fewer taxa than Briar Creek on the Red River arm.


38

4. DISCUSSION

Hydrological conditions in the 2014 and 2015 sampling seasons provided a stark contrast

between dry, isolated habitats in 2014 and wet, connected habitats in 2015. While in 2014 many

taxa were collected at only a few sites– including Morone and Lepomis species – many sites

were almost completely dominated by Dorosoma and Menidia beryllina larvae. Although sites in

the wet year (2015) were still dominated by these two taxa, the distribution of other species,

particularly Lepomis macrochirus and Morone species, spread to include sites where they had

not been detected previously. My results suggest that these habitats are providing nursery

habitats for a variety of fish taxa, but that the structure of larval assemblages using them varies

based on hydrological condition and habitat connectivity.

Variation in habitat conditions across river arms and years

Discharge of both the Red and Washita rivers varied significantly between 2014 and

2015, which had a major impact on connectivity and size of reservoir fragment habitats. In 2014,

these habitats were smaller and more isolated, while in 2015, fragments grew enormously and

experienced a high degree of connectivity with adjacent waters. Flooding is known to play a role

in abiotic conditions of both Lake Texoma and the rivers that feed it (Matthews 1984, Matthews

et al. 2004). Local abiotic conditions in 2014 varied across a seasonal gradient. Temperatures

rose from spring through summer, while dissolved oxygen fell. Turbidity was lower in the

Washita River arm than the Red, but both sets of sites became more turbid as sampling

continued. Previous studies have also shown that the Red and Washita rivers differ in turbidity,

conductivity and other chemical characteristics (Baldys 2009, Olairu et al. 2011). Conductivity

did not conform to a seasonal gradient and was higher at Red River sites throughout the sampling

period. In 2015, I observed similar patterns for local abiotic conditions. Temperatures were


39

somewhat lower than samples in 2014, particularly on the Washita River arm. Turbidity also rose

substantially after the flooding in 2015, especially during summer sampling. In 2015, there was a

massive increase in coarse particulate organic matter – mostly in the form of woody debris –

present in such quantities as to make push net sampling difficult or impossible because of fouling

and clogging of sampling gears. This is in agreement with the results of other studies that have

shown substantial differences in habitat and physiochemical conditions between wet and dry

years in river-reservoir ecosystems (Matthews et al. 2004, Kimmel et al. 1990).

Use of river-reservoir habitat fragments by YOY fishes

The overall abundance of larvae collected in the reservoir fragments was higher for 2014

than 2015. This was due in large part to difficulties associated with sampling in 2015’s flood

conditions, wherein challenges including the aforementioned net clogging, tornadoes and other

severe weather, and inaccessibility of some sites prevented sampling in some instances. Catch

per unit effort declined in 2015, also likely due to sampling difficulties. Such reduced sampling

efficiency related to flood conditions has been shown in previous studies of fish assemblages in

Lake Texoma (Gido et al. 2000). Assemblages in 2014 were dominated by Menidia and

Dorosoma, with traces of other taxa present. This pattern is in line with previous studies of fish

assemblages in Lake Texoma and its reservoir fragments (e.g. Gelwick and Matthews 1990;

Gido et al. 2002; Eggelton et al. 2005; Patton and Lyday 2008). Generally, the Washita River

arm had sites with greater diversity than the Red River arm, and taxa of management concern –

Morone particularly – were captured only on the Washita in 2014. Studies of adult fish

assemblages in Lake Texoma have generally shown differences in fish assemblage structure

between the two river arms of Lake Texoma (e.g., Gido et al. 2002; Eggelton et al. 2005).


40

Matthews et al. (2004) suggested that fish assemblage structure in Texoma rapidly changes with

environmental variation, and so the difference in local diversity I observed in 2014 might imply

that abiotic factors play an important role, especially during dry periods.

In 2015, Menidia beryllina and Dorosoma were also present in overwhelming numbers.

However, taxa that had not been detected previously on the Red River arm were captured soon

after the first major rain events. These included centrarchid species (Lepomis macrochirus and

Pomoxis), Morone and Notropis. Some of these are associated with the greater reservoir

assemblage (Morone) while others, such as Notropis, are associated with river floodplain habitats

(Turner et al. 1994; Phelps et al. 2015). While the Washita River arm habitats produced a large

numbers of Menidia and Dorosoma, CPUE of these taxa was lower compared to the Red River

sites, and the ratio of generalists to other taxa was less extreme in the Washita River arm in 2015

than in the Red River arm. This suggests that the Washita River arm sites supported a more

functionally diverse fish assemblage than that found in the Red River arm. The pattern in

Morone abundance across 2014 and 2015 – with low abundances in a few Washita sites in the

low-water year, followed by the rapid colonization of newly available habitat across both river

arms during the wet year – is corroborated by findings from monitoring of juvenile and adult

Morone saxatillis in Lake Texoma (Matt Mauck, ODWC, personal communication 2016). Based

on what is known about the ecology of Morone saxatillis in Lake Texoma, it is expected that

their abundance should increase during periods of high connectivity (Baker et al. 2009).The

overall increase in diversity of YOY fish assemblages that I observed within individual habitats

during periods of high connectivity is also in agreement with expectations based on studies of

river floodplain habitats (e.g. Zeug & Winemiller 2007).


41

Multivariate analysis of catch data revealed several groupings and contrasts across

habitats and across years. Unsurprisingly, the assemblages collected in 2014 and 2015 were

statistically distinct, due in large part to the shifting abundances of Lepomis macrochirus,

Pimephales, and other Morone. These differences can be attributed to the colonization of newly

available habitats by taxa that had previously been unable to use isolated reservoir fragment

habitats as nursery grounds. Many riverine fish taxa appear in seasonally isolated habitats, such

as floodplain backwaters, only when connectivity is sufficient (Tockner et al. 2000), as was

witnessed here. 2014 provided an example of a year with flows too low to generate a sufficient

flood pulse for floodplain guilds to penetrate reservoir fragment habitats, while 2015

demonstrated the importance of high flow events in providing access to habitats and nutrients

that would have been otherwise unavailable.

A significant difference in larval fish assemblage structure between the Red and Washita

River arms was detected by ANOSIM for the 2015 sampling season. This difference was likely

driven in part by physiochemical differences between the two rivers. Higher turbidity is

associated with reduced larval feeding efficiency (Matthews 1984; Breitberg 1988) and may

result in lower reproductive success in highly turbid systems. This may explain the greater

abundance for taxa like Lepomis macrochirus in the less turbid Washita sites. High conductivity

is associated with higher rates of growth of Dorosoma (Claramunt and Wahl 2000), which

supports my finding of greater relative abundance of that taxa in the Red River arm sites. Finally,

the ANOSIM analysis also detected a significant difference between the Spring and Summer

assemblage in 2015. Some degree of seasonal shift in assemblage structure is to be expected as

different taxa reproduce at different times (Matthews 2012). However, the timing of this shift in

structure coincides with the beginning of the heavy rainfall that characterized the 2015 sampling


42

season, so this clustering of spring and summer samples into two distinct groupings may be

driven by connectivity following the increase in discharge. Major flood events have been shown

in other studies to have a major impact in the structure of larval assemblages in river-reservoir

systems (Aghostino et al. 2004).The differences in assemblage structure across seasons in 2015

may also be related to the decline in sampling efficiency during the latter portion of the sampling

season.

Differences in local physicochemical factors between the river arms appear to have had

little effect on assemblage structure in 2014, however, where the assemblages using the Red and

Washita River arms were not statistically distinguishable. It is likely that, although some habitat

conditions differed between the two river arms (particularly conductivity and turbidity), other

factors constrained assemblage structure when the habitats were totally isolated in the dry year.

Conditions in 2014 were likely difficult enough – and had been for some time – that any

differences brought about by local physiochemical differences were minor compared to the

effects of long-term habitat isolation. This highlights once again the great importance that

hydrological connectivity appears to play in structuring the larval assemblages hosted by

reservoir fragment habitats. The high rains and rising waters of 2015 resulted in higher overall

diversity the local assemblages. This greater diversity may have allowed the influences of

physiochemical factors like conductivity and turbidity on assemblage structure to become more

apparent.

Despite the differences in assemblages I observed between years, seasons, and river arms,

one factor remains true: these reservoir fragments are dominated by habitat generalist taxa.

Menidia beryllina are tolerant of a variety of habitat conditions (Liensch and Matthews 2000),

including brackish water and high temperatures. Dorosoma species are found in a wide range of


43

habitats in large rivers and reservoirs, and are particularly tolerant of high turbidity and dissolved

solids (Williamson and Nelson 1985). Both Menidia beryllina and Dorosoma species reproduce

in large numbers and require few specific habitat factors beyond open waters or minimal cover

(e.g., aquatic vegetation) for successful reproduction (Willis 1987; Middaugh and Hemmer

1992). By the criteria of Winemiller and Rose (1992), both Dorosoma and Menidia beryllina can

be classified as reproductive opportunists. Both taxa are also generally planktivorous, favoring

zooplankton (Menidia) and phytoplankton (Dorosoma) and feeding only opportunistically on

insects (Bettoli et al. 1991; Dettmers & Stein 1992). In contrast, many of the other taxa involved

in this study that were less abundant and found in fewer sites had divergent requirements for

success, including Morone saxatilis, which require adequate habitat connectivity to thrive

(Crance 1984). The confluence of reproductive flexibility, tolerance of a wide variety of habitat

conditions, and a reliable source of nutrients all combine to grant an advantage to both Menidia

and Dorosoma, allowing them to persist even during long periods of isolation and unfavorable

conditions.

Comparisons with similar studies

Patton and Lyday (2008) sampled the same sites I examined on the Washita River Arm

for this study, but they focused on gradients of connection (from fully connected through degrees

of isolation) and adult assemblage structure. They found that connected sites were very similar to

one another but significantly different from isolated sites, in which highly pelagic species were

not found. Dorosoma and other species were found across all sites, but more connected sites

yielded greater abundances of Morone. This parallels my study, in which sites had higher

Morone abundance in the more hydrologically connected 2015 season compared to the dry,


44

isolated year. Their study also found that while species richness was similar between connected

and isolated sites, rarer species showed up more frequently in connected habitats, which is in

agreement with the results of my study. Certain aspects of Patton and Lyday’s investigation of

adult fish assemblages differed from the outcome of my study. For example, Pomoxis

abundance was high at Widowmoore Creek in their study, while they appeared in very small

numbers at that same site in 2014 or 2015 in my study. This difference may be due in part to

adult fish utilizing the habitat without reproducing there, or in changes to assemblage structure

over time.

Acre (2015) undertook a very similar study within the RRI zone of Lake Livingston on

the Trinity River in Texas. He found that young-of-year fish assemblages within these habitats

exhibited many of the same patterns detected in my study at Lake Texoma, including the

dominance of Dorosoma and Menidia beryllina in structuring the assemblage regardless of the

connectivity regime. In agreement with my study, Acre (2015) found that connections fostered

greater diversity and abundances within fragment sites, and he concluded that RRI backwaters

serve as nursery habitat when natural habitats are unavailable. He found that some floodplain-

associated taxa – like Atractosteus spatula and other species of Lepisosteidae – were absent

entirely from his study area. This contrasts with my study, where gar species were detected in

several instances and appeared to be utilizing some or all of reservoir fragment habitats in the

Texoma RRI zone. The more connected nature of the Livingstone RRI backwater sites may be

providing species like Atractosteus spatula with numerous other choices of reproductive and

nursery habitat.

A recent study by Zeug et al. (2005) investigated naturally occurring oxbow habitats on

the Brazos River, Texas, which are similar in many ways to the assemblage structure and


45

intermittently connected nature of the reservoir fragments at Lake Texoma. These habitats are

colonized during periods of connection and exhibit several gradients in terms of depth and

physicochemical factors such as conductivity. The authors found that local habitat features (e.g.,

depth) played a major role in structuring adult fish assemblages utilizing these habitats along

with the frequency of connectivity. In agreement with my study, habitats that experienced long

periods of isolation were dominated by more tolerant generalist taxa (e.g., Gambusia affinis).

The Brazos River oxbow habitats contained greater fish diversity and experienced more

extensive connectivity than the Texoma reservoir fragments during my study, and it may be that

the relatively deep and stable hydrological nature of the Lake Texoma reservoir fragments results

in habitats that are more similar to one another. This is in contrast with systems like the Brazos

River oxbows, where gradients of connectivity and other major differences between sites result

in greater variation across habitats.

Conclusions

In summary, the reservoir fragments at Lake Texoma do appear to be serving as nursery

habitats in some situations. Successful reproduction by a variety of taxa was observed and

occurred across river arms, environmental gradients, and years. But the taxa that were most

abundant in the reservoir fragments are perhaps not those that managers or conservationists

would prioritize. Many fish taxa utilized the fragment habitats as larvae, but those that dominated

tended to be those that will thrive in almost any waterway – generalists and opportunistic species

that excel at surviving and reproducing in a wide range of habitats, including in degraded

conditions. Unsurprisingly, habitat fragmentation and isolation in this system did not produce

habitats that hosted diverse assemblages of larval fish. However, during exceptionally wet years


46

like 2015, the reservoir fragments of Lake Texoma may serve as nursery habitats for a more

diverse assemblage, including taxa like Morone and Notropis, both of which are of interest as

species of management and conservation concern. These reservoir fragments seem to favor

habitat generalists during periods of isolation, while periods of flooding result in a rapid uptick

of diversity and abundance. This may be due in part to the opportunity for taxa to recolonize

previously isolated habitats and due to a typical response of riverine fish assemblages to high

waters – increased recruitment and dispersal among habitats (Junk et al. 1989; Humphries et al.

1999).

Data collected during the fyke netting phase of this study imply that many fishes found as

larvae within these habitats remain there into their juvenile and adult life history. However, it is

likely that the fish found to be remaining within these habitats in 2014 had little choice in their

dispersal as no connections existed to allow exchange between the reservoir fragment and

reservoir. Constraints on research resources prevented additional fyke netting in 2015, but

comparing larval habitat use with juvenile and adult distributions in situations of greater

connectivity between reservoir fragments and Lake Texoma would make an excellent avenue of

future research.

The patchy distribution of some taxa (e.g., Morone) may be due in part to the chaotic and

opportunistic manner in which newly available habitats are exploited in reservoirs (Matthews et

al. 2004). As these habitats are not generally available due to low water levels, fishes are

unlikely to develop site fidelity or otherwise become aware of its availability, making their

utilization the result of stochastic factors. Similar habitats that are unpredictably connected have

been shown to be used only opportunistically by fishes (e.g. Bunn et al. 2006; Sheldon et al.

2010). Reducing the length of time during which fragments are isolated may provide benefits to


47

sport fish species and species of conservation concern even greater than those suggested by this

study if the pattern of long isolation and brief connection was replaced with annual, predictable

connections between fragment and reservoir. Altering the current pattern of connectivity and

isolation is a challenging prospect, as climate change scenarios suggest future drought conditions

and the sedimentation problem at Lake Texoma is severe and ongoing (Sublette 1955, Atkinson

1999), and is only likely to increase into the future as the reservoir continues to age.

My findings for reservoir fragments at Lake Texoma are unlikely to be unique. The

effects of global climate change are projected to increase the severity and duration of droughts

worldwide (Dai 2013). The pattern of long periods of isolation punctuated by rare and extreme

flood events is likely to continue and intensify. Historical patterns of flow variation will become

less predictive of future flows, and we are likely to see isolated habitats become connected less

and less frequently. Further, as the thousands of impoundments and reservoirs worldwide age,

fragmentation and isolation are likely to occur in much the same way they have afflicted Lake

Texoma. This has serious implications for reservoir and fisheries managers, as habitat loss and

fragmentation are only likely to increase in severity and frequency. Further, the growing and

urbanizing world population will place additional water demand and stress on reservoirs,

necessitating drawdowns and exacerbating the problems associated with low water levels. While

some connections are likely to occur among fragmented reservoir habitats during periods of very

high discharge – as I saw in Texoma during 2015 – this is unlikely to be a solution in the long

term. Intense flooding increases water levels to connect these habitats, but it also ultimately

increases sedimentation in river-reservoir ecosystems, resulting in the creation of a higher,

broader barrier to further connections. Preventing or mitigating the factors that lead to

fragmentation and isolation include riparian restoration, the regulation of land use, and more


48

direct techniques like dredging. While expensive in both time and money, increasing

connectivity among river-reservoir habitats would likely increase their ecological and economic

value.


49

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Documents

Examining Use of Habitat Fragments by Larval and Juvenile