EXPER~E~AL ANALYSES OF PATCH SELECTION BY FORAGING …€¦ · EXPER~E~AL ANALYSES OF PATCH SELECTION BY FORAGING BLACK SURFPERCH (EMZJZOTOCA JACKSONZ Agazzi)’ SALLY J. HOLBROOK

3. Exp. Mar. Biol. Ecol., 1984, Voi. 19, pp. 39-64 Elsevier

39

JEM 283

EXPER~E~AL ANALYSES OF PATCH SELECTION BY FORAGING

BLACK SURFPERCH (EMZJZOTOCA JACKSONZ Agazzi)’

SALLY J. HOLBROOK and RUSSELL J. SCHMITT

Department of Biokogikal Sciences and The Marine Science Institute, University of Calfomia, Santa Barbara, CA 93106, U.S.A.

Abstract: The dynamics of microhabitat use by foraging adult and juvenile black surfperch (Ernbiotoca jac~oni Agazzi) were explored. Detailed observations of black surfperch feeding at Santa Catalina Island, California, revealed that adults and young-of-year juveniles co-occurred in the same habitat but used different algal substrata as foraging sites. Juveniles selected invertebrate prey almost exclusively from the surface of fohose algae. The occurrence of young E. jachoni was highly correlated with that of foliose algae. Adults tended to bite most frequently from turf, a low-growing matrix of plants, colonial animals, and debris covering the rocky substratum. The abundance of adults was negatively correlated with the occurrence of fohose algae. Adults and juveniles showed marked, but different, preferences in their utilization of taxa of algae as foraging substrata. Certain algae (e.g., Zonaria farlowii Setchell & Gardner) were preferred while other taxa (e.g., Surgussum palmeri Gnus) were avoided by both age groups. However, most types of algae were preferred by one group but not the other. To test the hypothesis that knowledge of aigal substratum composition allows prediction of fish occurrence and foraging behavior in a patch, algal cover on 2 x 2 m* areas of bottom was manip~at~ creating plots dominated by turf, Zona~a~r~owi~, or Sargassum palmeri. Fish occurrence could be accurately predicted on the basis of abundance of fohose algae, but forarging activity of fish was highly dependent on the algal taxon that dominated the patch. Differential prey availabilities among foraging substrata provided some insight into the patterns of foraging patch preferences displayed by adult and juvenile Embiotoca jacksoni.

INTRODUCTION

Many vertebrate species exercise choice in selecting habitats with the result that their populations are associated with only a limited array of the available types (Partridge, 1978). Motile species can potentially select qualitatively different habitats for differing activities such as foraging, reproduction, and predator avoidance (Wiens, 1976). Habitat utilization can be influenced or determined by a complex of biotic and abiotic conditions, such as competition (e.g., Holbrook, 1979; Rosenzweig, 1979; Sale, 1979), predation (Crook, 1965 ; Fraser & Cerri, 1982), resource availability (Werner, 1977 ; Werner et al.,

1981), and aspects of the physical environment (Fry, 1951; Hilden, 1965; Terry & Stephens, 1976). Although theory as well as laboratory and field investigations have examined the means and consequences of habitat use patterns (e.g., Wecker, 1963; Levins, 1968; Rosenzweig, 1973, 1979; Krebs et al., 1974; Levin & Paine, 1974; Murdoch & Oaten, 1975; Charnov, 1976; Werner & Hall, 1976, 1977, 1979; Wiens,

’ Contribution No. 73 from the Catalina Marine Science Center.

0022-0981/84/$03.00 0 1984 Elsevier Science Publishers B.V.

40 SALLY J. HOLBROOK AND RUSSELL .I. SCHMIT1

1976; Zach & Falls, 1976; Cowie, 1977; Krebs et ul., 1978; Mittelbach, 1981; Werner

et al., 198 1; Fraser & Cerri, 1982), much remains to be learned about patch identification and selection. This paper deals with what constitutes a feeding patch to an animal in

a coarse-grained environment. Patch selection can be fluency by various con~tra~ts operating at several levels:

a p~ticul~ habitat may be used to avoid predators while patches within that area may be selectively used as foraging sites. Theoretical and empirical studies of foraging patch selection have identified differences in availability of prey among areas as a central factor determining preferences. For example, Werner & Hall (1976, 1977, 1979) and Werner et al. (1981) attempted to predict shifts between habitats by sunfishes (Centrar- chidae) based on changing pro&abilities of harvesting prey in various habitats. How-

ever, Mittelbach (1981) suggested that risk of predation tempered foraging habitat util~ation during certain life stages of these fishes. Predation risk has been found to influence microhabitat use by other freshwater fish (Fraser & Cerri, 1982).

We examined the dynamics of microhabitat use by black surfperch, Embiotoca jacksoni Agazzi (Pisces: Embiotocidae), at Santa Catalina Island off the coast of southern California. These fish are microcarnivorous, feeding on invertebrates associated with algae and a matrix of colonial animals, plants, and debris ( = turf) that cover the benthos (Ebeling & Bray, 1976; Schmitt & Coyer, 1982: Laur & Ebeling, 1983). E. ja~k~oF?j can live up to 7 yr and reach a standard length (SL) of 300 mm. Females bear large young ( % 50 mm SL) once each year. At birth, black surfperch immediately assume a microcarnivorous diet, taking the same general prey taxa as adults. Adults, however, are capable of winnowing unwanted debris such as sand, algae, and small food articles, from their mouths, retaining, and swallowing desired food items. Juveniles (up to 1 yr in age) rarely exhibit this behavior. At Santa Catalina Island, young of the year and adults intermingle in shallow water (I-10 m) over rocky areas while foraging. A congener, E. ~~teralis Agazzi, which is known to influence spatial patterns of black surfperch (Hixon, 1979, 1980), does not occur at Santa Catalina (Schmitt & Coyer, 1983).

Both adult and juvenile (young-of-year) black surfperch groups occupied the same heterogeneous habitat that consisted of a mosaic of small-scale (l-2 m2) patches of turf and species of foliose algae. Initially we investigated the extent to which adults and juveniles displayed similar patterns of distribution and abundance among microhabitat patches. Using these data, we tested predictions about patch selection by adults and juveniles based only on consideration of foraging activity among patch types.

METHODS

Field work was conducted on the lee side of Santa Catalina island, California (33 0 27’ N : 118 o 20’W). Ubse~ations and expe~ments were carried out in a small cove west of Lionshead during June-November 1981, and in 3ig Fisherman Cove during

PATCH SELE~ION BY FORAGING S~RFPERCH 41

March 1982. Fish for dietary analyses and algae for analyses of prey av~lability were collected at Lionshead and at nearby Isthmus Reef. Substrata were composed predomi- nantly of boulders and cobble on benchrock. The bottom sloped gradually to sand at depths of 12-15 m.

A variety of foliose algae and enc~s~g coralline algae, as well as a matrix of plants and colonial animals composing %rf’, occurred on the rocky subs~atum. Small patches (l-2 m’) of turf steeped with areas dominated by foliose algae such as Cystoseiru negfecta (Turner), S~~~s~~ palmeri Grun, Zonaria far~o~j~ Setchell & Gardner, and ~ict~opte~ sp. The ~di~du~ algae were < 50 cm in height. The composition of the subs~atum from which surfperch fed was delineated at Lionshead by eight 50-m line transects. The subs~atum type at one r~do~y chosen point per meter was specified, for a total of 50 points per transect. At each sapling point, the taxon (usually species) of alga or, alternatively, the nature of the bottom (e.g., sand, bare rock, rock covered with encrusting coralline algae, turf) was determined (Table I). All transects were placed parallel to the shore at depths of 3-10 m.

Analyses of otoliths (Schmitt & Coyer, unpubl. data) and studies of growth (Schmitt & Holbrook, unpubl. data) indicated that ~~biot~u j~c~~oni at Santa Catalina attain a standard length of x 120 mm by the end of their fast year. Black surfperch were placed into two age classes based on size: adults (all animals > 1 yr old at time of obse~ation~ and juveniles (young-of-yes).

Fourteen 2 x 2 m plots were established at Lionshead for study of patch utilization. Six of the Lionshead plots were situated where the substratum was dom~ated by turf (p~rn~ly erect coralline algae) and encrusting algae. These plots were in shallow (a 3 m) water. The other eight Lionshead plots were situated nearby in slightly deeper water (4-5 m) in areas where foliose algae predominated. Plots were positioned ~bitr~ly several meters apart. The spacing between the plots and their positioning were such that fish could and did approach plots from any direction. The plot size (2 x 2 m) utilized was small enough so that visiting fish did not spend much time in a single plot compared to nearby areas; the longest visit we recorded on an unm~ipulated plot was 159 sec. Plot spacing enabled fish to visit repeatedly several or even all of the plots as well as much of the su~ound~g area during a daily foraging bout. Both adult and juvenile black surfperch were abund~t ~ou~out the Lionshead study area. Individuals of all ages foraged actively t~ou~out the daylight hours over the entire range of rocky subtid~ to depths of = 12 m.

Subs~atum composition of each Lionshead plot was deters by 2 m long line transects (n = 2/plot), In each lo-cm interval of the line transect, the alga or bottom type below a randomly selected point was iden~~, yielding a total of 40 sampling points per plot. The duration of visit and feeding activity (bites~m~) of any black surfperch entering ( = “trespass”) the Lionshead plots were subsequently monitored. Divers with underwater stopwatches hovered adjacent to the plots for randomly selected 30-min periods during 0800-1400. During each obse~ation period, the start time of each trespass by a fish, time and location of bites taken on the plot, ~nnows to remove

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PATCH SELECTION BY FORAGING SURFPERCH 43

unwanted debris from the mouth (if any), and exit time were recorded. Black surfperch were undisturbed by SCUBA divers that were as close as 1 m.

Actively foraging individuals at Lionshead were followed by SCUBA divers. Fish were observed between 0800-1400 during July and August 1981. A total of 84 fish (48 adults and 36 juveniles) were watched for periods of up to 5 min each. A total of 1840 bites (n = 960 for adults, n = 880 for juveniles) during > 6 h of direct observation of feeding fish were recorded. The standard length of each observed fish was estimated; samples of fish subsequently speared verified that visual estimates were within 10% of actual length. The frequency distributions of bites taken were compared with availability. Selectivity of algal substrata was calculated using the preference index (g) of Manly (1974; see also Chesson, 1978, 1983). This measure has several advantages over other indices, because the g does not change with relative density of patch types unless predator behavior varies (Chesson, 1983). Electivity values calculated from Manly’s index are therefore not influenced by unequal availabilities of patch categories (Chesson, 1983).

The eight Lionshead plots dominated by foliose algae (all at 4-5 m depth) were used for a short-term habitat selection experiment. The object of the experiment was to create relatively small, uniform patches of various substratum types and observe patterns of use by adults and juveniles in these microhabitats. During the 4-day experiment, the algal cover on four of the eight plots was manipulated each day. Cover on the other four plots remained unmanipulated throughout. Since the plots were located only a few meters apart, foraging fish could select between unmanipulated and manipulated plots (i.e., plots were not independent). The plots were small enough and the length of visits by fish to them were brief enough (see below) that satiation of an individual fish was not likely to occur as a result of a visit to any single plot. Data from observations of freely foraging fish revealed that fish overutilized (“preferred”) some foraging substrata and avoided others. The experiments involved four types: (1) unmanipulated plots (mixed algae and turf), (2) turf (preferred by adults and avoided by juveniles), (3) Zonariu furlowii (preferred by both adults and juveniles), and (4) Sargassum palmen’ (avoided by both adults and juveniles). Of the eight plots, four were manipulated for the same substratum type (e.g., turf) each day and four remained unmanipulated.

During each substratum manipulation, every one of the eight plots was observed (to document fish trespasses, bites, winnows) for a total of 90 min during 0800-1400. A plot was observed in 30-min intervals by each of three divers whose order of rotation through the plots was determined by a random number table. On the first day, all plots were observed in their unmanipulated state. Following these observations, all foliose algae taller than 5 cm was cut (at the holdfast) and removed from the general area on the four manipulation plots. This “turf exposure” reduced three-dimensional algal structure and exposed turf and encrusting coralline algae. The other four plots remained unmanipulated. Intervening areas were undisturbed during the experiments. The scale of the manipulation was such that fish already present in the area could respond to the patches; we have no evidence that the substratum alteration attracted fish from outside

44 SALLY J. HOLBROOK AND RUSSELL J. SCHMITI

the local area. Further, observations on the plots were made the day following each m~ipulation, avoiding possible effects of short-term disturbance of the fish by divers actively working on the bottom. After 90 mm of observation of the turf exposure experiment, three-dimensional components of the turf (mainly erect coralline algae) were removed from the four plots. Small rocks from outlying areas (at least 50 m from the experimental area) that were covered with Zonaria farlowii were then closely packed onto the four manipulation plots. Z. farlowii is a spreading, foliose dga and the addition of cobbles covered with this species effectively covered the remn~ts of the turf substratum. Fish foraging data for the 2. farlo wii experiment were collected the following day. Subsequent to observation, the Zonaria-covered rocks were removed and cobbles laden with Sargassum palmeri were carefully placed on the four manipulation plots. On the next day, the eight plots were observed once again for 90 min each.

Since the experimental design was such that individu~ fish could choose between plot types (unm~ipulat~ versus rn~~~ulat~ on a given day), we used G-tests (Sokal & Rohlf, 1969) to compare proportion of fish visits, proportion of bites taken, and total time spent by fish on manipulated and unmanipulated plots. The initial (prior to manipulation) observations on the eight plots were used to generate expected frequencies for these analyses, since there was some between-plot variance in fish activity. We assumed that once in a plot, subs~uent feeding activity of an ~ndi~du~ fish was independent of its activity on any other plot, so trespass time and feeding rate (bites/trespass; bites/mm) of individual fish on plots were analyzed by analysis of variance (ANOVA).

During March 1982, two short-term experiments were conducted in Big Fisherman Cove. The first tested the effect of three-dim~sion~ structure on patch selection and foraging activity. This involved addition of three-dimensional structure (plastic plants) to turf areas. The artificial plants did not contain prey items and were not used as foraging locations by the fish. On each of five successive days, one of tive 1-m’ plots dominated by turf was observed for 30 min during the period 0900-1200. The size of each trespassing tish was estimated and the trespass time, number and times of bites and winnows, and substrata upon which bites were taken were recorded. Following observation, twenty 30-cm tall artificial plants constructed from molded green plastic were placed on the plot. Each artificial plant was weighted at the bottom with a 2-0~ lead weight and added to the plot in a haphazard pattern. The plants were spaced far enough apart (= 20 cm) so that fish could (and did) swim easily between them and gain access to the turf. The surface area covered by the bases of artificial plants was c 10% of the plot. Each plant rose to a height of 15 cm on a single “stipe’” and branched into three ‘*blades” each 5 cm long and 2 cm in diameter. The plot containing the artificial plants was then observed for a second 30-min interval and fish activity was recorded. Visitation and foraging data were analyzed by a randomized block design ANOVA.

A second experiment investigated the ability of adult ~rn~io~~a ja~~~ni to distinguish between otherwise identical patches that differed only in prey abundance. The experiment involved presentation of two prey rich and two prey poor Cystoseira plants to


actively foraging individuals. Cystoseiru plants were collected in Big Fisherman Cove and carefully placed on a shallow (5 m) bench. At 0900 on the first day, four plants of similar size and shape were placed on the substratum in a square pattern, 20 cm apart. Bites taken on each plant were recorded as black surfperch encountered and foraged from them. Bite frequencies on the four plants were used as a basis for comparisons of those taken during the following day. At that time, two of the plants were taken to a boat and most of the invertebrate prey on them was removed by vigorous washing in sea water. The prey removed were saved for later analysis. To control for handling, the remaining two plants were treated in a similar manner, except prey were not removed. All four plants were then arranged on the bottom as on the first day, and during 0900 to 1100 the response of foraging fish to the four plants was monitored. At the end of the observation period, the four plants were placed in plastic bags underwater and taken to the laboratory where all remaining prey items were removed. The experiment was repeated during the following 2 days using four additional Cystoseiru plants.

Fish collected for dietary analyses were taken by divers using hand spears between 0900-1200 at Lionshead and Isthmus Reef (n = 44 adults and n = 80 juveniles). Speared fish were taken immediately to the laboratory; guts were removed and placed in 10% buffered formalin. The contents of the entire gut of each fish were examined under a dissecting microscope. All items representing 15 taxonomic groupings (gammarid amphipods, sphaeromatid isopods, idoteid isopods, jaeropsid isopods, shrimp, mysids, caprellid amphipods, crabs, urchins, ophiuroids, bivalves, gastropods, tanaids, copepods, and ostracods) were identified and counted. The lengths (or widths when appropriate) of a sample of up to 100 individuals per taxon were measured using an ocular micrometer. Length-weight relationships developed by Coyer (1979) enabled determination of biomass of each of the measured prey items in every fish. Items in a taxon were placed in a series of 9 prey weight classes (by OS-mg increments) to facilitate between-fish comparisons.

Samples of algae were obtained in June and July 198 1 from Lionshead and Isthmus Reef so that prey available to foraging fish could be assessed. We collected individual plants of a variety of taxa by carefully placing a plastic bag over a plant underwater and cutting the holdfast. The bags were sealed underwater. In the laboratory, invertebrates were washed from each alga and preserved in a solution of 10% buffered formalin. Taxonomic and size class analyses of items on the algae were carried out as above. A total of 60 specimens of algae representing a range of substrata and taxa (turf, erect coralline algae, Cystoseira neglecta, Zonaria farlowii, Dictyopterik undulata Holmes, Sar- gassumpalmeri, Plocamium cartilagium (L.), Pterocladia sp., Gigartina spinosa (Kutzing) were included in the analyses.

46 SALLY J. HOLBROOK AND RUSSELL J. SCHM1T-l

RESULTS

FISH DISTRIBUTION AND LEVEL OF FORAGING PATCH DISCRIMINATION

Spearman rank correlation analyses revealed that the number of trespasses by adults on 2 x 2 m plots was inversely correlated with the amount of foliose algae (r = - 0.8 1; t = 4.82; P -=z 0.01). By contrast, the number of juvenile visits was positively associated with algal cover (r = 0.63; t = 2.81; P < 0.05) (Fig. 1).

ADULTS JUVENILES (2 120 mm) I I (c 120 mm)

< 50% Fleshy Algae

30 20 IO Number of fish

trespasses per hour

10 20 Number of fish

trespasses per hour

Fig. 1. Mean numbers of visits (“trespasses”) per hour ( & 95 ‘?, confidence intervals) by adult and juvenile black surEperch to undisturbed natural plots (2 x 2 m each) at Lionshead: the plots are grouped according to the percent cover of fohose algae as determined by random sampling within each of the 14 plots; the

c: 50’; foliose algae category was comprised of six plots, while eight comprised the >, 5Q”;, category.

While foraging, black surfperch swam slowly near the bottom, moving continuously from place to place. Often fish stopped and took one or several bites from the same spot. However, Spearman rank correlations between the total number of bites made by either adult or juvenile fish and the percent cover of foliose algae in a plot were not statistically significant, suggesting that surfperch used a more detailed set of criteria during foraging periods than amount of foliose algae present in a local area. We examined the possibility that fish discriminated among structurally similar foraging substrata. Fish preferences for bushy, rosette-like, and planar types are shown in Table I. Adults overused planar surfaces as foraging sites and underused bush-like taxa. Juveniles displayed preference for the bush-like category and avoided planar substrata. Both adults and juveniles used rosette-like algae about in proportion to their availabihty. These overall preferences did not result from unifo~ preferences for each of the types within a category. Statistical comparisons of the av~ab~ty of each foraging substrate with its use by fish revealed that 75 ?; were either over- or underutilized (12 of 16 single classification G-tests were significant at P < 0.01). Electivity values confirmed this pattern (Table I). For example, among planar surfaces, only turf was preferred by ad&s, whereas bare rock (or rock covered by encrusting coralline algae) was not favored. Juveniles dispfayed the opposite preferences for planar substrata. As a group, rosette-like substrata were used about in the proportion of their relative abundances. However, within the category, certain algal


species (e.g., Zonariufarlowii) were highly favored by both adult and juvenile Embiotoca jucksoni, while mo~holo~c~y similar algae (e.g., ~ac~y~ictyo~ coriaceum (Holmes), ~~~~rni~ c~~ti~u~~~rn) were not preferred. These data strongly suggest that foraging sites were not chosen solely on the basis of their general structural features, but rather that particular substrata were selected and others avoided.

INFLUENCE OF EXPERIMENTALLY ALTERED AVAILABILITY OF SUBSTRATA

An algal tailoring experiment explored the link between preferences for selected foraging substrata and the general pattern of distribution of adults and juveniles observed on 2 x 2 m plots (Fig. 1). Our efforts to alter availability of algal species on plots were successful (Table II). About 50 % of the area of the plots was covered by the substratum being manipulated.

TABLE II

Relative availability of substratum types in plots used in the algal tailoring experiment: data for manipulated (n = 4) and for unmanipulated (n = 4) plots are combined; a total of 40 random point samples were taken

per plot to establish substratum composition.

Treatment

Substratum type

Unmanipulated plots Manipulated plots

No No Turf Zonarta Sargassum manipulation manipulation exposure addition addition

Turf/erect coralline algae

Cystoseira negiecta Sargassum palmeri Zonaria farlowii Dictyopteris undulata Sand/bare rock Encrusting coralline

algae Other taxa”

0.119 0.150 0.469 0.256 0.144 0.088 0.156 0.038” 0.006 0.013 0.294 0.363 0.000 0.031 0.650 0.119 0.081 0.000 0.531 0.025 0.144 0.056 0.000 0.025 0.013 0.075 0.050 0.044 0.019 0.006

0.123 O.t38 0.444 0.131 0.144 0.038 0.006 0.000 0.000 0.006

a Holdfast only. ’ Includes Phyllospadix sp., Colpomenia sp., Pachydictyon coriaceum.

Tailoring of algae had significant effects on the occurrence of fish (Table III). Exposure of turf resulted in a dramatic shii in visits by adults to m~ip~ated plots; in addition, the total number of recorded trespasses by adults nearly doubled. The turf exposure had no effect on the pattern of plot visitation by juveniles. Addition of Zonaria furlowiz’ resulted in a significant shift in proportion of visits to manipulated plots by both adults and juveniles. The Sargassum palmeri treatment, however, did not alter the visitation pattern of either fish group. The results were generally consistent with patterns of preference revealed by foraging observations (Table I).

The total amount of time black surfperch spent on plots was also affected by the

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le b

lack

sur

fper

ch

on m

anip

ulat

ed

and

unm

anip

ulat

ed

plot

s du

ring

the

alg

al

tailo

ring

exp

erim

ent:

the

expe

cted

nu

mbe

r of

tre

spas

ses

or s

econ

ds

spen

t on

eac

h pl

ot t

ype

for

a tr

eatm

ent

was

gen

erat

ed

usin

g th

e fr

eque

ncy

of v

isits

on

plot

s be

fore

alte

ratio

ns

occu

rred

; di

ffer

ence

s be

twee

n pl

ot t

ypes

with

in a

tre

atm

ent

wer

e an

alyz

ed

by s

ingl

e cl

assi

fica

tion

G-t

ests

; *

P <

0.0

25;

** P

< 0

.005

; ar

row

s in

dica

te

dire

ctio

n of

dev

iatio

n fr

om e

xpec

ted

prop

ortio

n in

man

ipul

ated

pl

ots.

Adu

lts

Juve

nile

s _”

^._

_.~

..-

- -.

.. U

nman

ipul

ated

M

anip

ulat

ed

G

Unm

anip

ulat

ed

Man

ipul

ated

G

T

reat

men

t pl

ots

plot

s va

lue

plot

s pl

ots

vafu

e __

_“__

.___

__.._

._

_

Num

ber

of f

ish

tres

pass

es

No

man

ipul

atio

n (c

ontr

ol)

10

14

61

30

- T

urf

expo

sure

1

387

33.4

1**

68

46

2.69

NS

Zon

ariu

ad

ditio

n 11

33

7

5.34

’ 65

56

1 9.

16**

Sa

rgas

sum

ad

ditio

n 5

14

1.93

NS

80

35

0.35

NS

. -_

_-

Tot

al n

umbe

r of

sec

onds

in

plo

ts

No

man

ipul

atio

n (c

ontr

ol)

215

380

2315

98

8 T

urf

expo

sure

13

15

46t

1260

.87*

* 24

23

1345

t 56

.33*

* Z

onar

ia

addi

tion

325

599

0.34

NS

2030

16

17T

33

2.80

**

Sarg

assu

m

addi

tion

62

27:

? 5 f

.89*

* 27

34

1006

!.

17.4

9**


experimental treatments (Table III). Compared with the proportion of time split between manipulated and unmanipulated areas before substrata were altered, adults and juveniles spent a dispropo~ionate amount of time on plots where turf was exposed. Addition of Zonaria had a similar influence on juveniles but not adults. The Surgussum treatment decreased the proportion of time spent by juveniles on manipulated quadrats, although adults spent relatively more time in Sargussum areas. Analyses of the mean duration of fish trespasses revealed no significant differences in average number of seconds either adults or juveniles spent on a m~ipu~ated or unm~ip~ated plot for any treatment (Table IV). However, the trend was toward longer visits on plots containing

TABLE IV

The mean duration of a fish visit on plots during the algal tailoring experiment: previous analyses indicated the average trespass times were not different between manipulated and unma~pulated quadrats prior to substratum alteration; two-way ANOVA with unequal but propo~onal subgroups model was used; turf exposure treatment was not included in analysis of adults because only one visit occurred on the

unmanipulated plots; means connected by lines do not differ (P > 0.05) by SNK test.

Duration of a trespass on manipulated plots (s) Substratum treatment

Adults Zonaria addition Turf exposure No treatment Sargasmn addition

42.8 41:6 27.1 19.8

Juveniles Zonariu addition No treatment Turf exposure Sargassum addition

32.9 32.9 2713 23.2

ANOVA tables

Source of variation d.f. MS F Significance

Adults Plot type (manipulated

vs. unmanipulated) Substratum treatment Interaction Error

Juveniles Plot type (manipulated

vs. unmanipulated) Substratum treatment Interaction Error

1 22.71 0.04 NS 2 323.50 0.52 NS 2 579.07 0.92 NS

51 627.41

1 1826.70 2.39 NS 3 895.78 1.17 NS 3 496.94 0.65 NS

352 765.65

favored substrata. Patterns in the total amount of time spent on quadrats were probably strongly influenced by the frequency of visits that indi~du~s made to various subs~atum types (Table III).

Not only were visitation patterns altered by substratum manipulations, foraging

50 SALLY J. HOLBROOK AND RUSSELL J. SCHMII-I

behaviors of both adult and juvenile Embiotoca jacksoni varied among treatments. The probability that a fish would eat once in a plot was affected by algal composition (Table V). Addition of Sargassum significantly reduced the proportion of juvenile trespassers that fed and had a similar influence on adult fish. No other treatment had a statistically detectable effect.

TABLE V

The proportion ofEmbiotoca jacksoni that entered a plot but did not eat during the algal tailoring experiment:

the number of such trespassers is given in parentheses (see Table III for total number of visitors); due to sample size constraints, only data for juveniles were analyzed using a 2 x 2 frequency table;

*** P < 0.005.

Proportion of trespassers that did not feed

Substratum treatment Unmanipulated plots Manipulated plots G value

Adults

No treatment Turf exposure

Zonariu addition Sargassum addition

0 (0) 0 (0) 0 (0) 0.13 (5) 0.18 (2) 0.33 (11) 0.20 (I) 0.43 (6)

Juveniles No treatment Turf exposure Zonaria addition Sargassum addition

0.12 (7) 0.13 (4) 0.008 NS 0.12 (8) 0.11 (5) 0.204 NS 0.08 (5) 0.11 (6) 0.034 NS 0.04 (3) 0.27 (11) 1 I .302***

The mean number of bites made per trespass differed significantly for both groups of fish depending upon the substratum composition of the patches (Table VI). Adults took significantly more bites per visit on turf than on other types. Among the manipulated substrata, juveniles took most bites per trespass on Zonaria and least on Sargassum. Finally, the foraging rate (bites per minute) was also influenced by the algae in the patches (Table VII). No difference was detected in the feeding rates of adults, although the trend was toward faster rates on more favored substrata. Juveniles fed significantly faster on Zonaria than on turf or Sargassum. No changes were detected in the average number of bites made per trespass or bites per minute by adults or juveniles on unmanipulated plots throughout the experiment (Tables VI, VII). There- fore, we concluded that changes in these parameters on the manipulated quadrats resulted from alterations in substrata.

The total foraging effort of black surfperch on the experimental plots should reflect trends in the frequency and duration of visits, as well as the average number of bites taken per trespass. Indeed, for a given treatment, the relative feeding effort of fish was greater on manipulated than unmanipulated plots when favored substrata were provided, and were lower when unpreferred substrata were made available. Adults took relatively more total bites from areas where turf was exposed (G = 346.01; P < 0.005)


and where Zonaria was provided (G = 16.62; P < 0.005). Addition of Sargassum had no effect on adults in this regard (G = 1.30; NS). By contrast, exposure of turf (G = 4.73; P < 0.05) and presence of Sargassum (G = 64.52; P < 0.005) significantly decreased the overall foraging effort made by juveniles on manipulated plots. Addition

TABLE VI

The number of bites taken per trespass by black surfperch during the algal tailoring experiment: data were log transformed to ensure homogeneity of variance; means are back transformed; the mean number of bites per trespass did not differ between manipulated and unmanipulated areas before substrata were altered; (adults: t = 1.55, NS; juveniles: r = 0.69, NS); means not connected by line differ at P < 0.05 by SNK

test.

Bites per trespass on manipulated plots Substratum treatment

Adults

Juveniles

Turf exposure Zonaria addition No treatment Sargassum addition 6.5 2.2 2.1 1.7

No treatment Zonaria addition Turf exposure Saxassum addition 4.2 3.6 2.i 2.5

ANOVA tables


Adults Manipulated plots

Among substratum treatments Error

Unmanipulated plotsa Among substratum treatments Error

Juveniles Manipulated plots


Unmanipulated plots Among substratum treatments Error

3 1.096 58 0.161

2 0.076 0.590 NS 19 0.129

3 0.315 126 0.117

3 0.057 0.588 NS 247 0.097

6.824 P < 0.001

2.69 P < 0.05

a Turf exposure not included.

of Zonaria, however, resulted in an increased proportion of bites taken on manipulated quadrats (G = 39.16; P < 0.005). All of the major trends in foraging behavior elicited by alteration of substratum availability were consistent with predictions generated from observations of freely foraging black surfperch (Table I).

52 SALLY J. HOLBROOK AND RUSSELL J. SCHMIT’I

Tns~t VII

The feeding rates of black surfperch on plots during the algal tailoring experiment: the number of bites per minute differed between manipulated and unmanipulated plots before substrata were altered (P < 0.05 for

both groups of fish; means not connected by lines differ at P < 0.05 by SNK test.

Bites per minute on manipulated plots

Substratum treatment

Adults

Juveniles

Turf exposure

10.5

Zonaria addition 11.9

Zonaria addition No treatment 10.0 8.4

No treatment ‘Turf exposure 10.3 7.2 ~~

ANOVA tables

Source of variation

Adults Manipulated plots


Unmanipulated plots” Among substratum treatments

Error

Juveniles

Manipulated plots Among substratum treatments Error

Unmanipulated plots Among substratum treatments Error

d.f. MS F

3 29.6 1 0.611 73 48.44

2 65.32 1.1s

19 56.85

3 243.42 4.03 133 60.38

3 IS.54 0.87 248 17.94

Sargassum addition

1.3

Sargassum addition

63

Significance

NS

NS

P < 0.001

NS

B Turf treatment not included.

THE ROLE OF PREY AVAILABILITY IN SELECTION OF FORAGING SUBSTRATA

The diets of adult and juvenile black surfperch at Santa Catalina Island are summarized in Table VIII; further details can be found in Schmitt & Coyer (1983). Four taxa (gammarid amphipods, isopods, caridean shrimps, and crabs) comprised > 900~ of the diets by weight of both fish groups. Gammarid amphipods accounted for > 709, of the diets by number and biomass. An array of other prey taxa contributed little to the diets, and were not included in the analyses of prey availability within patches. The primary difference between young and adult Embiotoca jacksoniconcerned the size of prey consumed (Table VIII). Adults took relatively larger prey (x = 1.02 mg) on average than did juveniles (F = 0.45 mg). Adult guts also contained substantially more items (_? = 598.9 prey) than young fish (x = 291.7 prey).


Variation in types, abundances, sizes and/or accessibility of prey among substrata are potential explanations for the preference of certain algal patches over others. To explore patterns of prey availability, individual plant samples were placed into three categories

TABLE VIII

The diets of adult and juvenile black surfperch at Santa Catalina Island: given are the proportion of prey taxa that comprise fish diets by number and by biomass; also presented are the frequency of prey falling into nine size classes and the proportion of prey biomass in those size categories; n = 44 adult and 80

juvenile fish.

Prey taxa

Gammarid amphipods Isopods Caridean shrimps Crabs Caprellid amphipods Tanaids Copepods Ostracods Bivalves Gastropods

Proportion by number Proportion by biomass

Adults Juveniles Adults Juveniles

0.768 0.693 0.681 0.770 0.104 0.086 0.113 0.089 0.011 0.006 0.080 0.040 0.005 0.004 0.054 0.023 0.020 0.008 0.019 0.024 0.011 0.025 0.003 0.007 0.002 0.086 0.001 0.020 0.009 0.066 0.002 0.017 0.006 0.009 0.005 0.008 0.058 0.017 0.032 0.012

Size class (mg)

0.01-0.50 0.309 0.638 0.087 0.378 0.51-1.00 0.464 0.309 0.393 0.455 1.01-1.50 0.134 0.034 0.179 0.067 1.5 I-2.00 0.041 0.006 0.072 0.020 2.01-2.50 0.018 0.002 0.042 0.006 2.51-3.00 0.008 0.001 0.020 0.005 3.01-3.50 0.006 0.000 0.020 0.000 3.5 l-4.00 0.004 0.00 1 0.018 0.003 > 4.00 0.017 0.008 0.171 0.068

based on their relative preference by fish: those preferred by adults (turf and erect coralline algae), those favored by juveniles (Zonaria furlowii, Dictyopteris unduluta and Cystoseira neglecta), and those neither fish favored (Sargassum palmeri, Plocamium cartilagineum, Gigartina spinosa and Pterocladia sp.). Analysis of the mean number of prey (per g of substratum) on series of individual plants representing each category revealed no significant differences. A similar result was found when gammarid amphipods were considered alone (Table IX). Each alga supported an average of 1092 prey items.

Some differences were detected in the amount of prey biomass available. While no statistical differences were detected when all four prey taxa were considered together, the trend was toward less prey biomass (per g wet weight) on unfavored substrata

TABLE IX

The number of major prey items found on various foraging substrata: major prey include gammarid amphipods, isopods, shrimps, and crabs; the substratum types are those favored by adults, those favored by juveniles, and substrata neither fish favored; see text for algal species comprising each category; gammarid amphipods are also considered alone; n = 20 samples of each substratum type; coeflicients of variation were calculated on ~transformed data; data were log transformed in ANOVA to make variances homogeneous; means presented are back transformed; means not different by SNK test are connected.

Number of prey/g substratum

-._ ._._..._ __- Major prey items

x No.& substratum cv

Adult substrata c

11.2 105.97;

Juvenile substrata &preferred substrata

14.6 14.3 - _...._.. - __~ ..--- 59.0% 51.69;)

Gammarid amphipods %’ No./g substratum cv

8.6 13.3 ~-“.----._~_ 104.49; 60.8%

ANOVA tables

10.5 ^._._. 62.8%

___..___.. ._ ____


Major prey items Substratum type Error

Gammarid amphipods Substratum type Error

7 5;

0.076 0.63 NS 0.122

2 0.119 1.60 NS 57 0.112

TABLE X

The amount of prey biomass available on various types of substrata: presented are means for major prey taxa and for gammarid amphipods alone; data were log transformed for ANOVA to make variances homogeneous; means are back transformed; coefficients of variation were calculated from untransformed

data; means not connected by line differ at P < 0.05 by SNK test.

Mg prey/g substratum ___~.~~______~~~___-...______

Adult substrata Juvenile substrata

Major prey items Mg prey/g substratum 12.6 11.7 _.- _ ---. CV 92.3 z 61.7%

Gammarid amphipods Mg prey/g substratum 6.2_.-_ .-..--. _L! CV 101.3% 58.37;

-_---- ---.--.- ~ ANOVA tables

-. -. __l..-..---

Source of variation d.f. MS F ____________~.___-______~~~---

Major prey items Substratum type 2 0.038 0.440 Error 57 0.086

Gammarid amphipods Substratum type 2 0.275 3.600 ErRX 57 0.076

Unpreferred substrata

9.2 49. I y)

4.2 59.4 ‘;,

___.._-.__.-.--._--

Significance

NS

P < 0.001


(Table X). The average biomass of available gammarid amphipods did diITer among categories, with substrata preferred by juveniles containing the greatest biomass of gammarids (Table X).

TABLE XI

Mean size of prey associated with various algal substrata: given are mean size of all major taxa and of gammarid amphipods alone; means not connected by line differ at P -c 0.05 by SNK test.

Mean prey size (mg)

Adult substrata Juvenile substrata Unpreferred substrata

Major prey items X size (mg)

Gamma~d spends X size (mg)

0.82 0.65 0.59

0.63 0.59 0.41

ANOVA tables

Source of variation d.f. MS F Signific~ce

Major prey items Substratum types Error

2 0.301 4.740 P < 0.025 57 0.064

Gammarid amphipods Substratum types Error

2 0.268 5.551 P < 0.001 57 0.048

Substrata favored by adults exhibited a larger mean prey size (Table XI). Overall, prey averaged nearly 30% heavier on turf and erect coralline algae than on foliose algae, The mean size of gammarid amphipods also varied among the three categories; the mean gammarid size was z 30% smaller on substrata neither fish preferred (Table XI). The distribution of prey biomass (Table XII) revealed that more than 70% of available biomass occurred in the two smallest weight classes on unfavored substrata. This compared with x55-60% for algae preferred by adults and juveniles.

One of the striking features of adult-favored substrata was the extreme between- sample variation in number of prey and in the amount of prey biomass. Coefficients of variation for these parameters (Tables IX, X) indicated that patches of turf were much more variable with regard to mount of prey available than were foliose algae. It seemed possible that adult black surfperch did not sample patches of tu~r~domly (as we did), but rather identified and selectively used prey rich areas within that substrate type, We expe~ment~y ~ves~gat~ whether adult ~~~iu~~a jacks~~~ could d~erenti~ly use otherwise identical patches that differed only in the amount of prey available. Adults took significantly fewer bites from foliose algae after prey densities were reduced (Table XIII). Aproximately 90% of all prey items were removed from manipulated

56 SALLY J. HOLBROOK AND RUSSELL 3. SCHMITI

plants, and the number of bites from those algae decreased by 75-85 “/, (compared with the frequency of use on those plants before prey were removed). The number of bites taken from plants where prey were not manipulated was not reduced as a result of prey

depletion or our handling, suggesting that the disparity in bites between treatments was indeed caused by prey reduction.

TABLE XII

The size-frequency distribution of major prey found on various algal substrata: also given is the proportion

of prey biomass comprising each weight category. --__

Weight class Adult substrata Juvenile substrata Ilnpreferred substrata

Proportion of prey by number

0.01-0.50 0.613 O.hY3 0.5YS

0.51-1.00 0.262 0.207 0.320

1.01-1.50 0.062 0.054 0.039

1.51-2.00 0.024 0.018 0.037

2.01-2.50 0.012 0.010 0.005

2.51-3.00 0.007 0.005 0.003

3.01-3.50 0.003 0.001 0.00 I

3.51-4.00 0.004 0.002 0.002

14.00 0.0 13 0.010 0.008

Proportion of prey by biomass

0.01-0.50 0.200 0.51-1.00 0.360 1.01-1.50 0.133 1.5 I-2.00 0.069 2.01-2.50 0.044 2.5 I-3.00 0.027 3.01-3.50 0.0 15 3.51-4.00 0.018 > 4.00 0.136

0.28 1 0.137 0.296 0.469 0.1 I6 0.09 1 0.056 0.039 0.039 0.030 0.026 0.017 0.008 0.005 0.015 0.016 0. I66 0.097

Adults greatly underused algae that had strong three-dimensional structure (Table I). Furthermore, adults were rarely found in areas dominated by such algae (Fig. l), even though suitable prey patches occurred beneath these plants (see Turf Exposure Treat- ment; Tables III-VII). We examined whether the presence of three-dimensional structure on an otherwise suitable plot reduced the visitation pattern and foraging effort of adult E. jacksoni. Addition of sparsely distributed artificial plants on a plot of turf did not affect the number of adult trespasses, although consistent between-plot differences existed (Table XIV). However, adults that entered a plot with vertical structure stayed a significantly shorter time than on the same plot without artificial plants. Of most importance, though, is the fact that adults took significantly fewer bites on natural areas after the addition of vertical structure (Table XIV).

TA

BLE

XII

I

Prey

cha

ract

eris

tics

and

rela

tive

use

of f

olio

se a

lgae

in

a pr

ey r

educ

tion

expe

rim

ent:

the

num

ber

of b

ites

mad

e by

adu

lt bl

ack

surf

perc

h on

Cys

fow

ira

negl

ectu

w

ere

reco

rded

be

fore

and

aft

er p

rey

wer

e re

duce

d fr

om e

xper

imen

tal

plan

ts;

two

plan

ts

per

trea

tmen

t pe

r tr

ial

wer

e us

ed;

the

expe

cted

nu

mbe

r of

bite

s ta

ken

from

man

ipul

ated

an

d un

man

ipul

ated

pl

ants

w

as g

ener

ated

fr

om t

he r

elat

ive

use

of p

lant

s be

fore

pre

y w

ere

redu

ced;

de

viat

ion

from

exp

ecta

tion

was

tes

ted

usin

g 2

x 2

cont

inge

ncy

tabl

es;

M,

man

ipul

ated

pl

ants

; U

, un

man

ipul

ated

pl

ants

; **

* P

c 0.

005.

Prey

cha

ract

eris

tics

Tri

al 1

Pr

ey r

emov

ed

Prey

rem

aini

ng

afte

r ex

peri

men

t

Tri

al 2

Prey

rem

oved

Pr

ey r

emai

ning

af

ter

expe

rim

ent

x N

o. p

rey/

g pl

ant

M

U

0.58

$06

0.67

0.76

0.06

1.

50

X m

g pr

ey/g

pla

nt

M

U

0.96

0.04

0.

73

0.93

0.09

3.

40

f pr

ey s

ize

(mg)

M

U

1.84

0.92

1.

05

1.23

1.33

2.

33

X p

lant

siz

e (g

)

M

U

533.

5 62

1.0

740.

0 90

2.5

Fora

ging

ac

tivity

(no

. bi

tes)

Tri

al 1

B

efor

e pr

ey r

educ

tion

Aft

er p

rey

redu

ctio

n

Man

ipul

ated

pl

ants

U

nman

ipul

ated

pl

ants

61

51

23

50

Sign

ific

ance

G =

9.

56+

*+

Tri

al 2

B

efor

e pr

ey r

educ

tion

AA

er p

rey

redu

ctio

n 93

48

16

61

G

= 4

8.14

***

58 SALLY J. HOLBROOK AND RUSSELL J. SCHMITT

T~i3i.i: XIV

.The influence of added vertical structure (plastic plants) on number of adult black surfperch visits per observer hour, length oftrespass time, and number offish bites taken per observer hour: a randomized block

ANOVA was employed (see text for details); n = 5 plots.

Fish activity .~ .-- _ _ ~.___._ ._

I? No. trespasses,’ X trespass time Total bites/ State of plot observer hour (s) observer hour -_-I___-_“._.-” .._ ___._. ~. -._

Without structure 15.0 15.4 25.0 With structure $2.6 h.3 1.2

-.... - _ _- _... - . .._. -_.. _ .__ _._ _ _ ._ _ ANOVA tables


Number of trespasses Vertical structure Blocks {piots) Remainder

Trespass time Vertical structure Blocks (plots) Remainder

Number of bites Vertical structure Blocks (plots) Remainder

-

I 14.4 4 65.9 4 2.9

I X6.1 4 14.7 I 13.5

! 1416.1 4 83.6 4 X6.6

4.966 NS 22.724 P c 0.01

15.231 P < 0.025 1.085 NS

16.352 P < 0.025 0.965 NS

DISCUSSION

One obstacle encountered in the study of habitat selection has been a realistic determ~ation of what constitutes a patch to the organism (Wiens, 1976). Often the task facing the investigator is to estabIish what physical elements and spatial scale comprise a patch, based primarily on the response of the organism to its environment (Wiens, 1976). Confounding the process is the fact that patch scdes and elements can vary with function (e.g., feeding, reproduction) and with time (Wiens, 1976; Schluter, 1982). For example, Schluter (1982) used the foraging response of Darwin’s finches to clusters of different seeds as an indication of patch types. Among freshwater fishes, Werner & Hall (1979), Werner et al. (1981), and Mittelbach (1981) ah considered gross habitat type (e.g., ve~tation, open water, sand bottom) as patches that were selectively used as feeding locations. While the spatial scale of defined patches differed tremendously between finches and fishes, in both cases an explanation of patterns of habitat use based on biologically meaningfiil differences (prey availability) among patches was sought.

One of the goals of the current study was to estabhsh the level of patch identification by foraging black surfperch. Studies of habitat partitioning and selection by fish have


tended to document the use of relatively broad scale habitat types such as different sections of marine reefs or various portions of the littoral zone in lakes (reviewed by Sale, 1979). This approach, while useful, has not provided much insight into the dynamics of microhabitat use or of local patch selection. The habitat occupied by black surfperch at Santa Catalina island - shallow subtidal rocky reefs - was composed of a mosaic of small patches ((2 m”) of low foliose algae and turf. We found that the amount of foliose algae in 2 x 2 m patches was a sufficient predictor of the occurrence of adults or juveniles (Fig. 1). Although foraging patch preferences displayed by adults and juveniles were broadly consistent with three-dimensional structure, the cues for patch discrimination were more subtle than simple plant growth form. Experimental alteration of algal substrata indicated that both adult and juvenile E. jacksoni discriminated between foraging patches at the level of algal species (Tables III-VII). Foraging activities of fish varied among algal substrata; favored algae were subject to greater foraging effort than less favored ones. Furthermore, fish were able to distinguish a prey-rich patch from an otherwise identical one where prey were reduced (Table XIII).

Local habitat partitioning frequently occurs between conspecific juvenile and adult fish (Clarke, 1977; Itzkowitz, 1977, Werner er al., 1977; Helfman, 1978; Keast et al.., 1978; Williams, 1978; Robertson etal., 1979; Brandt, 1980; Hixon, 1980; Leum & Choat, 1980; Waldner & Robertson, 1980; Fraser & Cerri, 1982). The degree of habitat d~erentiation can range from ~~upa~on of totally separate habitats by adults and juveniles to relatively broad but not complete habitat co-occurrence. Microhabitat partitioning by age-classes of fish has been infrequently explored. Adult and juvenile E. jucksoni overlapped spatially at Santa Catalina, although each used a particular array of microhabitat patches. We suspect that if there had been stronger zonation of patch types at Santa Catalina (e.g., foliose algae replacing turf with depth), the between-habitat spatial segregation of adult and juvenile fish would have been more complete. As it is, the dist~bution~ mosaic of patches ~ont~buted little to spatial separation of foraging adults and juveniles beyond the scale of a few centimeters. Indeed, fish of all ages were frequently seen travelling together and foraging within touch of one another, yet utilizing qualitatively different foraging substrata.

An implicit assumption in studies of habitat selection is that various patches differ in “quality” which ultimately can influence the fitness of the user (Wiens, 1976; Partridge, 1978). Indeed, dit3’erence in the “pro~tabi~ty” of foraging in various areas has been forwarded as an explanation for feeding patch selection in several empirical and theoretical investigations (e.g., MacArthur & Pianka, 1966; Schoener, 1971; Estabrook & Dunham, 1976; Pulliam, 1974; Charnov, 1976; Wiens, 1976; Pyke et al., 1977; Werner & Hall, 1977, 1979; Werner et al., 1981; Mittelbach, 1981; Schluter, 1982). There were two patterns of microhabitat use by foraging black surfperch that must be considered in this regard. First, why were certain algal substrata not favored by either adult or juvenile fish? Secondly, why did adult and juvenile fish display qualitatively different patterns of patch choice? Differences in ch~a~te~stics of prey among algal species may have been involved in the pattern of patch choice displayed by btack

ho SALLY J. HOLBROOK AND RUSSELL J. SCHMIT’T

surfperch. The taxonomic composition and density of invertebrates on algae can vary between plant taxa (Krecker, 1939 for freshwater algae; Colman, 1939; Wieser, 1952 for marine intertidal algae; Rosine, 1955; Hicks, 1977 for subtidal algae). Presumably prey differences are due to species-specific characteristics of algae, including presence of toxins, as well as the presence of predators (Nelson, 1979a,b; Stoner, 1980b). As a group, the species of algae that adult and juvenile fish underused supported less biomass of favored prey, as well as prey of smaller size, than other types (Tables X, XI). Algal blade morphology and plant biomass have been argued to be critical determinants of invertebrate density and diversity (Young et al., 1976; Heck & Wetstone, 1977; Young & Young, 1978; Hicks, 1980; Stoner, 1980a). If there was a clear relationship between algal size and/or morphology with characteristics of associated invertebrate prey (e.g., visibility, accessibility), we would predict that black surfperch would respond similarly to algal species that were much alike in morphology. This was clearly not the case for E. jacksoni (Table I). The general pattern was that adult and juvenile black surfperch tended to use foraging patches that were relatively prey rich. The prey reduction experiment suggested that, within a species of algae, fish discriminated and selectively used plants supporting more prey (Table XIII).

There were some differences in the characteristics of prey available on substrata favored by adults and those preferred by juveniles. For example, the largest overall prey occurred on types preferred by adults (Table X). Further, turf was extremely variable in the amount of prey it contained; density and biomass of available prey varied significantly more on a small spatial scale than on foliose algae. Since adults discriminated relatively prey-rich patches (Table XIII), we suspect that adults are capable of exploiting turf areas high in prey density. However, the primary difference between patches preferred by adults and juveniles involved the degree of vertical structure. The dissimilar foraging methods employed by adults and juveniles might explain the difference in patch types used. Typically, adults took one or more mouthfuls of turf and winnowed out unwanted items such as sand and debris. Remaining invertebrates were then ingested (Schmitt & Coyer, 1982; Laur & Ebeling, 1983) leading to a posteriori prey selection. Adults were also capable of picking prey from the surfaces of algae, possibly to obtain individual prey that were first located visually. Juvenile E. jacksoni were incapable of winnowing and only picked prey from algal surfaces. Turf, which was rich in prey, contained a large amount of unwanted material and could probably only be effectively exploited using a winnowing feeding mode. Winnowing behavior develops slowly during the first year of lie (Schmitt & Holbrook, unpubl. data), eventually enabling individuals to utilize turf more frequently for foraging. Addition of vertical structure on an otherwise suitable patch of turf significantly decreased the foraging effort of adults in those areas (Tabie XIV). In addition, suitable patches occurred beneath foliose algae (Tables III-VII) but were not used by foraging adults. At present, we can only speculate that vertical structure interfered with the mechanics of prey harvesting by adult E. jacksoni.

We hypothesize that differences in the type of foraging patches favored by adult and

PATCH SELECTIONBYFORAGINGSURFPERCH 61

juvenile fish involves, at least in part, ontogenetic differences in the ability to harvest prey. Other factors can contribute to habitat partitioning of foraging fishes. For example, Mittelbach (198 1) explored the relationship between the profitability (maximum energetic intake) of feeding in various lake habitats and the utilization of those areas by different size classes of bluegill sunfish (Lepomis macrochinrs). Large bluegills switched habitats as foraging profitability changed through time. Small fish, however, remained in one habitat (vegetation) regardless of potentially increased energetic returns from other areas, presumably because of the protection against predators afforded by vegetation. We do not discount the potential influence of predation on patch utilization patterns of Embiotoca jacksoni. The occurrence of young fish in areas dominated by foliose algae probably confers a strong advantage in avoiding piscivores. Predation on adult surfperch is low (Bray & Hixon, 1978; Hixon, 1979), but in the first months of life, young surfperch are probably extremely vulnerable to a variety of predators. We have observed young of the year black surfperch (60-70 mm SL) being captured by adult kelp bass, Paralubrux cluthrurus Girard. The advantage of using shaded areas, such as those provided by foliose algae, has been pointed out by Helfman (1981). The opportunity for escape and concealment in larger algae provides an advantage to juveniles that would not exist if they foraged on the more exposed turf areas. It is not clear to what extent risk of predation influences either adult or juvenile patterns of patch use.

Embiotoca jacksoni have a complex foraging behavior that involves recognition and utilization of particular foraging substrata within the array available. Patterns of both foraging and microhabitat selection change ontogenetically, probably because foraging eficiency and risk of predation change as fish grow. Black surfperch actively select certain patches as foraging locations, and they are capable of responding rapidly to alterations in availability of patches and/or prey. The pattern of patch use by adult and juvenile fish is consistent with an explanation based on characteristics of available prey. Much needs to be learned regarding the causes and fitness consequences of patch selection by these fish. Nevertheless, the distributional mosaic of patches strongly influences the distribution patterns of adults and juveniles and has a complex influence on the foraging activities of the fish. The behavioral plasticity that characterizes E. jacksoni no doubt enhances their ability to utilize the patchily distributed resources of their environment.

ACKNOWLEDGEMENTS

We thank J. Bence, R. Dean, A. Ebeling, M. Hixon, M. Keough, W. Murdoch, and C. Osenberg for critical discussions and comments on the manuscript. M. Bercovitch and C. Osenberg assisted with the field work. A. Breyer, P. Chiu, and B. Downes assisted in the laboratory. Financial support was provided by grants from the National Science Foundation (OCE-8110150) and the Academic Senate of the University of California, Santa Barbara. Dr. R. Given and the staff of the Catalina Marine Science Center provided hospitality and logistic support.

62 SALLY .I. HOLBROOK AND RUSSELL J. SCHMITT

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EXPER~E~AL ANALYSES OF PATCH SELECTION BY FORAGING …€¦ · EXPER~E~AL ANALYSES OF PATCH SELECTION BY FORAGING BLACK SURFPERCH (EMZJZOTOCA JACKSONZ Agazzi)’ SALLY J. HOLBROOK