Predator response to releases of American shad larvae in the Susquehanna River basin

Ecologv of Freshwuter Fish 1998: 7: 192-199 Printed in Denmark , All rights reserved

Copyright D Munksgaurd 1998

ECOLOGY OF FRESHWATER FISH

ISSN 0906-6691

Predator response to releases of American shad larvae in the Susquehanna River basin

Johnson JH, Ringler NH. Predator response to releases of American shad larvae in the Susquehanna River basin. Ecology of Freshwater Fish 1998: 7: 192-199. 0 Munksgaard, 1998

Abstract - Predation on American shad (Alosa sapidissima) larvae within the first two hours of release was examined from 1989 to 1992 on 31 occasions at stocking sites in the Susquehanna River basin. Twenty-two fish species consumed shad larvae; the dominant predators were spotfin shiner (Cyprinella spiloptera), mimic shiner (Notropis volucellus) and juvenile smallmouth bass (Micropterus dolomieu). The number of shad larvae found in predator stomachs ranged from 0 to 900. Mortality of shad larvae at the stocking site was usually less than 2%. The greatest mortality (9.6%) occurred at the highest stocking level (1.5 million larvae). Highly variable predation rates and release levels of shad insufficient to achieve predator satiation hindered the ability to determine a specific type of functional response of predators. Predator numbers increased with stocking density, indicating short-term aggregation at the release site. Be- cause of practical problems associated with releasing the large numbers of larvae that would be required to satiate predators, routine stocking at these levels is probably unreasonable. Releases of 400,000 to 700,000 larvae may reduce predation by offsetting depensatory mechanisms that operate on small releases and the effects of increased predation due to predator aggregation on large releases. Night stocking may reduce predation on larval shad at the release site.

Un resumen en espaiiol se incluye detras del texto principal de este articulo.

J. H. Johnson', N. H. Ringlefl Research and Development Laboratory, U.S.

Geological Survey, Biological Resources Division, Wellsboro, Pennsylvania, 2Department of Environmental and Forest Biology, State University of New York, College of Environmental Science and Forestry, Syracuse, New York. USA

Key words: Alosa sapidissima; American shad; predation; stocking

James H. Johnson, Tunison Laboratory of Aquatic Science, U.S. Geological Survey, Biological Resources Division, 3075 Gracie Road, Cortland, NY 13045, USA

Accepted for publication July 3, 1998

Introduction

Of the quantitative factors that affect predation rates, predator and prey densities have been considered the most important (Holling 1966). Natu- ral events or anthropogenic perturbations that dra- matically alter predator o r prey densities may offer unique insights into predator-prey relationships. A common fisheries enhancement technique that al- ters predator and prey densities is the stocking of hatchery-reared fish. Knowledge of predator-prey interactions, especially when using juvenile hatchery fish, can help guide stocking practices (Fresh & Schroder 1987) and may be essential to successful hatchery programs (Peterman & Gatto 1978).

Increases in prey density can elicit short-term behavioral responses by predators. Changes in predator density in relation to prey densities are known as the numerical o r aggregative response (Solomon

1949), whereas the functional response reflects changes in the number of prey consumed per predator per unit time relative to prey density (Murdoch 1973). Functional response relationships can be linear (Type I), decelerating (Type 11) or sigmoid (Type 111) (Holling 1959, 1966). The asymptote of the functional response can be considered the maximum number of prey a predator can consume per unit time and is regulated by satiation level and/or handling time. The asymptote may increase as predators grow because of larger gut capacities or im- proved hunting capability (Murdoch 1973).

We examined predation on larval American shad (Alosu sapidissima (Wilson)) released into the Juniata River, a tributary of the Susquehanna River. For the past decade, an average of about 11 million American shad larvae have been stocked annually into the upper Susquehanna during May and June as part of a multi-agency, basin-wide ef-

192

Predator response to shad larvae

quently, to prevent mortality on shad larvae during the collection of predators, it was necessary to allow the larvae time to disburse. The efficiency of the Shindler-Patalas sampler in collecting larvae was not determined. It is likely, however, that larger larvae (i.e., 216 mm) were less susceptible to capture than small larvae with this method. Water velocity measurements were made with a Marsh- McBirney water current meter I , 3 and 5 m from shore along two transects (at 0.6 depth) located within the release site. Water turbidity was determined with a Markson turbidimeter.

All seined fish were immediately placed in 10% formalin. American shad larvae and other prey found in stomachs were identified, counted, and measured under magnification ranging from X7 to X60. For cyprinids that lack a true stomach, only the contents of the anterior one-third of the stomach were examined. All American shad larvae identified in predator stomachs were considered of hatchery origin, as natural reproduction of shad is minimal because the only adults that ascend the Juniata were those that are transported above dams on the Susquehanna River.

The functional response of important predator species and total predators was depicted by plotting the average number of American shad larvae consumed per predator against the number of larvae released. The estimated number of larvae released was provided by hatchery personnel. Sufficient data were available to examine the functional response of spotfin shiner (1989-1992), mimic shiner (1989- 1990), and total predators (1989-1992). Five of the 3 1 releases of American shad larvae that were examined were made at night; these data were not used in determining functional response. Spotfin shiner dominated predators numerically and provided the most observations (26) for evaluation of the functional response.

The aggregative response (considered here a short-term numerical response) of predators was described by plotting predator numbers at the stocking site against the number of American shad larvae released. Predator population numbers within 1 h following stocking were estimated using values of seine capture efficiency for major predator groups. Capture efficiency estimates were computed by dividing the number of recaptured marked fish (R) by the total number of marked fish (M). Fin clipping (top of caudal fin) and six immersion dyes were evaluated to determine the best marking technique. Immersion dyes, which greatly reduced fish handling and associated stress, were subjectively judged to be superior to fin clipping as a marking technique. Bismark brown Y was the most effective of the immersion dyes in staining fish in the laboratory and was selected for

fort to restore the species. American shad were ex- tirpated from most of the Susquehanna River basin when impassable dams precluded access to up- river areas (Meade 1976). Larval American shad (about 18 days old and 6-16 mm) are used in the restoration program because of high mortality of juvenile shad from handling and transportation stress during stocking (Johnson & Dropkin 1992). We sought to describe the short-term behavioral responses of fish predators at sites where American shad were released in the Susquehanna River.

Material and methods

Field efforts were carried out from 1990 to 1992 in the Juniata River, a historically important spawning tributary for American shad. The pri- mary release site for American shad larvae was at Thompsontown, Pennsylvania in the upper Sus- quehanna basin. In this area the Juniata is about 125 m wide and averages about 1 m deep. Dis- charge during May and June ranges from 21 to 482 m3/s and averages about 162 m3/s (U.S. Geological Survey 1989-1 992). Predator collections were made on 26 occasions following diurnal releases of American shad larvae. Three additional releases were monitored in 1989, one at Thompsontown and the others at Montgomery Ferry on the Sus- quehanna River. Releases were made from buckets in about 0.5 m of water within 10 m of shore. All collections of predators occurred within 2 h after routine stocking of American shad larvae by personnel from the Pennsylvania Fish and Boat Com- mission (PFBC). Stocking sites were selected by PFBC hatchery personnel based on access.

About 30 min following each release a 7.6- mX 1.2-m (4.8-mm mesh) seine was hauled through the stocking site. Seining was standardized at usually five (occasionally 10) hauls and conducted in a manner to characterize predation on larvae at the stocking site (i.e., point of release to about 30 m downstream). Most larval shad drifted downstream with the current and departed the stocking site within an hour after release. The downstream movement of American shad larvae upon release was determined using drift nets below the release site and Shindler-Patalas 12-L plankton samples at the sites. Three Shindler-Patalas samples were taken beginning 30 min after release and at 15 min intervals thereafter until most shad larvae had dis- bursed from the release site. The Shindler-Patalas samples were taken to ensure that most American shad larvae had departed the release site prior to the collection of predators with the small mesh seine (i.e., 4.8 mm). Although the seine mesh was adequate to collect small predator (i.e., <20 mm), it unfortunately also captured shad larvae. Conse-

1 93

Johnson & Ringler

Table 1. Fishes that consumed American shad larvae in the Susquehanna River basin, 19891992.

Number Size range Common name Genus and species examined (total length, mm)

1. Carp Cyprinus carpi0 L. 74 21-27 2. Central stoneroller Campostoma anomalum (Rafinesque) 1 30 3. Cutlips minnow Exoglossum maxillingua (Lesueur) 1 43 4. Spotfin shiner Cyprinella spiloptera (Cope) 2,335 20-1 06 5. Common shiner Luxilus cornutus (Mitchell) 63 47-71 6. Spottail shiner Notropis hudsonius (C I in to n ) 78 19-78 7. Mimic shiner N. volucellus (Cope) 916 18-87 8. Rosyface shiner N. rubellus (Agassiz) 54 22-69 9. Bluntnose minnow Pimephales notatus (Rafinesque) 293 35-85

10. Blacknose dace Rhinichthys atratulus (Hermann) 1 26 11. Creek chub Semotilus atromaculafus (Mitchell) 296 33-42 12. Fallfish S. corporalis (Mitchell) 141 18-96 13. Banded killifish Fundulus diaphanus (Lesueur) 104 32-94

15. Rock bass Ambloplites rupesfris (Rafinesque) 85 45-1 13 16. Redbreast sunfish Lepomis aurifus (L.) 198 31-122 17. Pumpkinseed L. gibbosus (L.) 1 84

14. Mottled sculpin Coftus bairdi Girard 1 23

18. Bluegill L. macrochirus Rafinesque 59 39-97 19. Smallmouth bass Micropterus dolomieu Laepde 202 15-225 20. Largemouth bass M. salmoides (Laepde) 5 37-50 21. Tessellated darter Etheostoma olmstedi Storer 8 19-28 22. Walleye Stizostedion vitreum (Mitchell) 48 24-74

Table 2. Pearson correlation coefficients and probabilities for variables relating to the average number of shad consumed per predator (UP) and the total number of predators at the stocking site in the Juniata River, Pennsylvania, 1989-1992. Sample size is in parentheses.

Variable

Number of Shad Number of Water S/P shad stocked density predators Temp. Turbidity velocity (26) (26) (17) (26) (23) (23) (23)

~~ ~

Average number of American shad consumed per predator (SIP) ___ 0.70* 0.12 0.28 -0.01 0.02 -0.24 Probability <0.001 0.657 0.163 0.953 0.930 0.264 Number of predators 0.28 0.46* -0.33 _-- -0.20 0 15 -0.18 Probability 0.163 0.019 0.199 0.366 0.506 0.399

'* Significant correlations (P<0.05).

field trials at a treatment level of 50 ppm for 15 min. To determine capture efficiency, all fish col- lected in one seine haul were marked, identified, counted and released. A second seine haul was made at the site 3 to 4 h after the initial release to collect marked fish. Capture efficiency trials were conducted in areas with similar habitat characteristics (ie., depth, velocity, substrate, vegetation) to the stocking site. Because some releases of Ameri- can shad larvae were made at night, nocturnal capture efficiency estimates were made to examine the possibility of differences in efficiency between diurnal and nocturnal periods'. For purposes of com- parison, similar numbers of shad larvae were re-

Experimental night stocking trials were implemented by the Pennsylvania Fish and Boat Commission in 1990 and 1991 to attempt to improve the survival of American shad larvae.

leased during diurnal periods within 12 h of each of the four nocturnal releases.

Mortality curves of American shad larvae were derived by plotting the percentage of larvae eaten by all predators against the number of larvae released. The total number of shad larvae lost to predation at the release site was estimated by multi- plying the average number of larvae consumed per predator by the estimated total number of predators. The percentage mortality was determined by dividing the total number of American shad larvae consumed at the stocking site following each release by the total number of larvae released.

Regression analysis was used to examine functional response, aggregative response and percentage mortality (Zar 1984; SAS 1987). The data were examined for outliers using Poultier's criterion at a significance level of 0.05 (SAS 1992). Outliers were


12

not detected for the models that best fit the behavioral response data. One outlier was identified in the percent mortality data (9.3% at a stocking level of 83,000 larvae). Re-examination of the data, however, revealed no computational errors, so the data point was not deleted from the analysis. Percent mortality data were heteroscedastic but data trans- formations (i.e. log, power, inverse, square root, arcsine) were not effective in achieving homoscedasticity, nor did they improve R2 values. Consequently, regression analysis, which is robust to deviations from homoscedasticity (Zar 1984), was performed on non-transformed (heteroscedastic) data.

We used Pearson correlation analysis (SAS 1987) to compare consumption estimates to the number of shad stocked, number of predators present, shad densities (i.e., Shindler-Patalas samples), water turbidity, water velocity and water temperature. Differences in the average number of shad larvae consumed per predator between years and between day and night periods were compared using the Fisher least significant difference test (Milliken & Johnson 1984; SAS 1987).

Y =6.99dX+4.73~13X+ 1.41 I

adjusted R' = 0.54 Fw.01

Results

Twenty-two predator species consumed American shad larvae (Table 1). Spotfin shiners, mimic shiners and juvenile ( O + , 1+) smallmouth bass were the major predators. The number of shad larvae found in predator stomachs averaged 6.0 (diurnal and nocturnal releases) and ranged from 0 to 900. The average number of American shad larvae consumed per predator at the release site decreased yearly from 1989 to 1992, but the differences were not significant. The average number of shad consumed per predator was (P=O.OOOl) positively correlated (r=0.70) with the number of larvae stocked. None of the other variables (i.e., estimated shad density, number of predators, turbidity, temperature, or water velocity) were correlated (P>0.05) with the average number of American shad larvae consumed per predator (Table 2).

The functional response of mimic shiner was examined using 11 observations. A quadratic model provided the best fit when examining the relationship between prey consumption and prey density for both cyprinids and all predator species (Fig. 1). However, R2 values were not high, ranging from 0.51 (all predators) to 0.56 (mimic shiner) (Fig. 1).

The aggregative response of the four predator groups was examined with data from the same sample dates used for examining functional response. Seine capture efficiencies used to estimate predator numbers at the stocking site ranged from 5% for yearling smallmouth bass to 13% for cypri-

-0

.c v)

c

m e

m 4

f

O C a .

Spotfin shiner (a)

, , . . I . . , . .

203 400 600 800 loo0 12m 14m 1600

Y = 1.706X + 9.2e-12X + 3.46 adjusted R' = 0.56, P4.01

14

I

t I /

/

30

L

25 m W

\ E 20 Q) Q

$ 15 -

Y = 1.83e-3X + 1.12e-5X2 + 2.67 adjusted R2 = 0.51

PXO.01 i I /

t I

I / /

c 0 200 400 600 800 1000 1200 1400 I600

Shad Larvae Released (xl000) Fig. I. Average number of American shad larvae consumed per spotfin shiner (a), mimic shimer (b) and total predators (c) in relation to the number of shad larvae released. The boxes represent data points and the line represents the fitted quadratic model.

nids (Table 3) . The aggregative response for total predators and spotfin shiner was best approxi- mated by a positive linear relationship, but R2

195

Johnson & Ringler

2500 --

5 2000 IE 0 8 1500 P

2 1000

.c

6

500

0

1 000 Y = 437.8054+0.W166x R' = 0.368 P = 0.010

500

-~

--

--

--

011-'+-- I I a 0 400000 a00000 1200000 16ooooo

3000 Spotfin shiner @)

Y = 123.4578+0.00103x Ra= 0 2 3 5 P = 0.012

I +.--

400000 8OWOO 1200000 1600000

Number of shad larvae stocked

Fig. 2. Number of total predators (a) and spotfin shiners (b) at the stocking site in relation to the number of American shad larvae released. The boxes represent data points and the line represents the fitted quadratic model.

values were low, 0.37 and 0.24, respectively (Fig. 2). The slope of the relationship for mimic shiner was not significantly different (P>0.05) from zero. Correlation analysis showed a significant positive relationship (aggregative response) (u=0.46) between the total number of predators and number of shad larvae stocked (Table 2).

The percentage mortality of American shad larvae exhibited a quadratic relationship with an R2 of 0.35 (Fig. 3). Predation following the five night stockings was low. On three of these occasions the average number of shad larvae consumed per predator following night stocking was significantly less ( fY0 .05) than during the day (Table 4). Predation following nocturnal releases of American shad larvae averaged 0.3 shad per predator, whereas shad consumption following paired diurnal releases averaged 5.7 shad per predator, and for all 26 diurnal releases consumption averaged 8.6 larvae per predator.

Table 3. Seine capture efficiency estimates in the Juniata River, Pennsylvania, 1989-1 992.

Number Capture efficiency (%)

Predator species or group of trials Mean (SD) Range

Day Cyprinids 23 13.2 (5.7) 5.6-25.3 Subyearling smallmouth bass 8 8.0 (4.8) 0.0-15.4 Yearling smallmouth bass 10 4.9 (6.5) 0.0-14.3 Sunfisha 12 7.7 (4.3) 0.0-17.7 Yearling walleye 6 6.8 (5.6) 0.0-12.5 Miscellaneousb 6 6.0 (7.0) 0.0-16.7

Cyprinids 10 12.3 (5.4) 4.5-21.4 Subyearling smallmouth bass 7 10.3 (5.1) 0.0-15.4

Night

Yearling smallmouth bass 6 4.5 (6.9) 0.C-14.3 Sunfisha . 6 9.6 (6.1) 0.0-18.1 Yearling walleye -_ _ _ _ _ _ _ Miscellaneous -_ _ _ _ ---

a Includes all centrarchids other than smallmouth bass. Primarily includes banded killifish, mottled sculpin, and tessellated darter.

Table 4. Average number of American shad larvae consumed per predator following paired releases during day and night periods. A paired day release was not made with the June 28,1990 release. The number of predators examined is in parentheses.

Number of Average number of shad Date Release time shad released consumed per predator

~~

06-28-90 05-30-91 05-30-91 06-03-91 06-04-91 06-06-91 06-06-91 06-17-91 06-18-91

Night Night Day Night Day Night Day Night Day

154,000 439,000 363,000 642,000 655,000 632,000 682,000 393,000 381,000

0.23 (1 53) 0.48 (196) 0.63 (247) 0.30 (277) 1.35* (288) 0.16 (46) 0.52* (64) 0.04 (209) 13.0* (165)

* Significantly different between day and night (k0.05).

10- -~ Y = 9 6e-7x + 8.8e-12x2 + 3.726 I - R' = 0.353 /

,- -, - 0 - I , L L I i ~ -

0 200000 400000 600000 800000 1000000 1200000 1400000 1EQOOOO

Shad larvae stocking levels Fig. 3. The percentage mortality of American shad larvae at the stocking site in relation to the number of shad larvae released. The boxes represent data points and the line represents the fitted quadratic model.

196


the functional response for examining predation on juvenile salmonids. Although they did not elab- orate, this seems logical because the percentage mortality includes the prey consumed per predator and the number of predators. Holling (1959) also believed that the quantification of predation should include functional response and numerical response components. Ringler & Brodowski (1983) considered their functional response analysis primarily as a contribution to identifying maximum predation rate, apart from the shape of the response curve. Ruggerone & Rogers (1984) suggested that if the numerical (aggregative) response of predators varies with prey density, then the percentage mortality may increase with increased prey abundance up to some threshold prey density. Be- yond this threshold, the percentage mortality would decrease.

Although predators exhibited a positive linear aggregative response in relation to the number of American shad larvae released, threshold densities may not have been met because the percentage mortality increased at the highest stocking levels. The percentage mortality of shad larvae at the stocking site decreased up to stocking densities of 700,000 but was lowest at releases of 400,000 to 700,000 larvae. Above 700,000, the percentage mortality increased. The trend of increased percentage mortality above 700,000 is supported by only four data points and of these, the percentage mortality at the highest stocking density has the most profound effect. The highest percentage mortality, 9.6%, occurred at the largest stocking level of 1.5 million. This observation greatly influenced the upward inflection in Fig. 3 at higher release levels. A high rate of mortality at releases up to 400,000 may be due to depensatory mechanisms operating at small releases, whereas an increase in mortality rate at releases above 700,000 may reflect the effects of predator aggregation at the stocking site. This interpretation is suggested by the positive linear relationship between the number of predators and the number of shad larvae released; percent mortality may increase if there is a strong numerical response of predators (Wootton 1990).

The use of capture efficiency estimates with seines is thought to improve abundance estimates of small fishes (Parsley et al. 1989). For predators on American shad larvae, capture efficiency of the seine differed among species and groups of species. Capture efficiency values can vary due to habitat differences such as benthic or mid-water occu- pancy (Lyons 1986; Parsley et al. 1989; Pierce et al. 1990). For expediency, minor predators such as banded killifish, mottled sculpin and tessellated darter were grouped together for capture efficiency estimates even though they occupy different areas

197

Discussion

Although quadratic models provided the best fit to our consumption and prey density information, the data were insufficient to determine a specific type of functional response. Petersen & DeAngelis (1992) have emphasized the difficulty of selecting a functional response type from field data because of highly variable predation rates. It is also poss- ible that too few American shad larvae were released to satiate predators. Peterman & Gatto (1978) and Ruggerone & Rogers (1984) concluded that predators on juvenile Pacific salmon were operating on the ascending limb of the functional response curve where juvenile salmonids were poten- tially the most vulnerable.

The small size (i.e., 6-16 mm) of American shad larvae may explain why predators could be operating at the low end of their functional response. Small prey size greatly increases the number of potential predators (Zaret 1980) and requires more prey to satiate predators (Ivlev 1961). Predators of American shad larvae in the Susquehanna River basin ranged from 18 to 225 mm in length. This size range probably includes more than 90% of all the fish predators in the river. Consequently, because of the large number of potential predators, it may be difficult to stock sufficient numbers of shad larvae to move higher on the functional response curve. Furthermore, because there is a positive linear aggregative response of total number of predators, releasing larger numbers of American shad larvae at one time would probably cause further aggregation of predators at the stocking site. If this occurs, the possibility of satiating predators with higher release levels may be reduced (Ruggerone & Rogers 1984).

At the stocking site, predators probably would encounter large numbers of American shad larvae soon after release. Consequently, knowledge of the foraging behavior of predators when they are ex- posed to patches or schools of prey may be useful in helping identify their functional response. Al- though schooling behavior has distinct anti-predator advantages (Taylor 1984; Pitcher 1986) and is considered to be an important consideration for some functional response determinations (Petersen & DeAngelis 1992), it is probably not a factor in the case of 18-day-old larvae because American shad are not known to school at this age (Ross & Backman 1992a, b). Fish predators encountering patches of recently released Ameri- can shad larvae would probably concentrate their attack in the densest patches of larvae (Murdoch & Oaten 1975).

Peterman & Gatto (1978) considered percent mortality to be a more sensitive relationship than

Johnson & Ringler

of the water column. Efficiency estimates did not differ much for each predator group between diurnal and nocturnal periods. Although the visibility of the seine to fish may be reduced at night (this would act to increase capture efficiency), operator efficiency was also reduced at night. Nocturnal de- ployment increased the frequency of bottom snags and caused the operators to proceed more slowly than during the day. These factors reduce capture efficiency (Pierce et al. 1990).

Light intensity has been shown to affect predation on juvenile salmonids. Predation by some species of sculpin has been observed to be greater at night than during the day (Hamilton 1981; Mace 1983). Hamilton (1981) found that predation by staghorn sculpin (Leptocottus armatus Girard) on juvenile salmonids was affected more by light intensity than by salmon density. Predation by staghorn sculpins was higher during moonlit nights than cloudy nights (Patten 1971; Ginetz & Larkin 1976). Patten (1971) and Hamilton (1981) suggested that higher predation rates at night may be due to better visual acuity by staghorn sculpin than salmon under low light conditions. Our data indicate that predation on American shad larvae at the release site was lower following nocturnal releases. Consequently, at least at the release site, night stocking may help to increase survival of stocked shad by reducing predation.

Mortality of shad larvae at the stocking site was usually less than 2%, indicating that predation at the stocking site is often not a significant source of mortality. Most releases of American shad larvae were probably on the ascending limb of the functional response curve, where predators may be operating at high consumption rates. Considerably higher numbers of shad larvae would need to be released at one time to satiate predators because (1) the shad larvae are very small and individual predators have been found to contain up to 900 shad larvae, (2) the number of available predators is large and (3) an apparent aggregative response was exhibited by predators at the release site in response to higher numbers of shad released. Be- cause of practical problems associated with trans- porting and releasing the large number of Ameri- can shad larvae that would be required to satiate predators (i.e., large stocking numbers would sub- stantially slow release operations, thus increasing stress and mortality on shad larvae), routine stocking at this level is not a reasonable alternative. The percentage mortality was lowest at releases of 400,000 to 700,000 larvae. Although data are limited, night stocking may reduce predation at the release site. During the last 2 years of this investi- gation (i.e., 1991-1992), however, predation at the release site may have been insufficient to achieve

any measurable benefit from a large- scale night stocking program.

1. En las zonas de repoblacion de la cuenca del rio Suaquehan- na, estudiamos en 31 ocasiones (aiios 1989-1992) la predacion ejercida sobre larvas de Alosu sapidissirnu en las dos primeras horas de liberacibn. Encontramos que 22 especies de p e e s se alimentaron de dichas larvas. Los predadores dominantes fue- ron Cyprinella spiloptera, No tro pis volucellus y juveniles de Mi- cropterus dulornieu. Encontramos entre 0 y 900 larvas en 10s estomagos de 10s predadores. 2. La mortalidad de las larvas en las zonas de repoblacion fuk, a1 menos, el 2Y0. La mortalidad mas alta (9.6%) fue detectada en 10s mayores niveles de repoblacibn (1.5 millones de larvas). Tasas de predacion variables junto a niveles de repoblacion in- suficientes que permitan alimentarse hasta saciarse a 10s predadores, dificultaron la determinacion del tip0 especifico de res- puesta funcional de 10s predadores. El numero de predadores aument6 con la densidad de la repoblaci6n, lo que indica una agregaci6n a corto plazo en la zona de repoblacion. 3. Problemas practicos relacionados con el gran numero de larvas que se necesitarian liberar para saciar a 10s predadores, sugieren no realizar estas repoblaciones a gran escala de forma rutinaria. Repoblaciones intermedias de 400,000 a 700,000 larvas podrian disniinuir la predacion ya que se reducirian tanto 10s mecanismos que actiian sobre liberaciones pequeiias como 10s efectos de agregacion de predadores que se generan en liberaciones grandes. Repoblaciones nocturnas tambikn podrian re- ducir la predacion sobre las larvas de A. sapidissirnu.

Acknowledgments We thank D. Dropkin for assistance in the field and laboratory, L. Mengel and J. McKenna for statistical support, D. Radaker for preparing figures, S. Bencus for typing the manuscript and D. Steward, R. Werner, M. Mitchell, R. Ross and D. Rottiers for reviewing the manuscript. We also wish to thank M. Hend- ricks and the staff of the Pennsylvania Fish and Boat Com- mission’s VanDyke Fish Hatchery for their cooperation.

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Documents

Predator response to releases of American shad larvae in the Susquehanna River basin