Growth, survival and size-selective predation mortality of larval and juvenile inland silversides, Menidia beryllina (Pisces; Atherinidae)

Journal

ELSEVIER

of Experimental Marine Biology 199 (1996) 165-177

JOURNAL OF EXPERIMENTAL MARMEBIOLOGY AND ECOLOGY

and Ecology,

Growth, survival and size-selective predation mortality of larval and juvenile inland silversides, Menidia beryllina (Pisces; Atherinidae) Timothy

R. Gleason*,

David A. Bengtson

Department of Zoology, University of Rhode Island, Kingston, RI, USA Received

1 May 1995; revised 28 August

1995; accepted

25 September

1995

Abstract A series of laboratory and field experiments were conducted to determine the relative importance of food limitation and predation as sources of mortality for Menidia betyllina (Cope) larvae and juveniles. Seven-day experiments using in situ mesocosms to exclude predators demonstrated significant growth (mean instantaneous growth rate of 0.122-0.135 day-‘) and survival (mean 88-89%) for M. beryllina larvae in a Rhode Island, USA, estuary. These results suggest that food was not limiting for growth or survival and, therefore, that predation is likely the primary source of mortality for young-of-the-year (YOY) M. beryllina. Predation experiments were conducted in laboratory aquaria and in in situ mesocosms to assess the size-selectivity of potential predators. Laboratory-reared striped bass, Morone suxutilis Walbaum and field-collected white perch, Morone americana Gmelin, crevalle jack, Curunx hippos Linnaeus and bluefish, Pomutomus sultutrix Linnaeus, were presented with a choice of two or three size classes of laboratory-reared M. beryllinu and allowed to feed for 3-24 h. For field-collected predators the experimental prey size range was similar to the size present in the field. Striped bass, white perch and crevalle jack selectively preyed on the smallest size classes. Bluefish, however, selectively preyed on the largest size class. These results suggest that size-specific survival of YOY M. beryllinu may vary spatially and temporally depending on the particular suite of predators encountered by individual populations or cohorts. However, in the estuary studied predation mortality appears to be directed towards the larger members of the M. beryllinu cohort. Keywords:

Growth rate; Menidiu; Mesocosm;

Prey selection;

Size-specific

predation

*Corresponding author. Present address: US Environmental Protection Agency, National Health and Environmental Effect Research Laboratory, Atlantic Ecology Division, 27 Tarzwell Drive, Narragansett, RI 02882, USA. Tel.: + l-401-7823033; Fax: + I-401-7823030; e-mail: [email protected] 0022-0981/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0022.0981(95)00194-8

166

T.R. Gleason,

D.A. Bengtson

I J. Exp. Mar.

Bid.

Ed.

199 (1996)

165177

1. Introduction It is generally accepted that fish populations vary considerably in size over time, both in the presence and absence of fishing pressure (Rothschild, 1986). The importance of processes occurring during the early life history of fishes for influencing future population dynamics has long been appreciated (Hjort, 1914), though the underlying mechanisms are still debated today (Miller et al., 1988). With the notable exception of studies on small pelagic fish larvae (Lasker, 1975, 1978; Hewitt et al., 1985; Theilacker, 1986), the evidence that starvation is a direct cause of mortality in fish larvae is sparse (Anderson, 1988; Bailey and Houde, 1989). The process of predation has received increasing attention and support as a primary source of mortality in larval and juvenile fish (Sissenwine, 1984; Houde, 1987). For instance, the mortality rates of embryos and yolk-sac larvae, which are often high, cannot be explained by starvation (Bailey and Houde, 1989). Laboratory studies have indicated that, in general, the responsiveness of larvae to predatory attacks increases with increasing larval size, while the probability of capture decreases with increasing size (Webb, 198 1; Folkvord and Hunter, 1986; Purcell et al., 1987; Butler and Pickett, 1988; Fuiman, 1989; Margulies, 1990). These results provided support for the concept that faster larval growth leads to increased foraging opportunities and reduced predation pressure (Werner and Gilliam, 1984; Bailey and Houde, 1989; Beyer, 1989; Cushing, 1990). Some investigators have proposed that early life survival is a function of growth rate (Houde, 1987). Anderson (1988) suggested that the hypothesis that survival is a direct function of growth represents a rational theoretical framework for recruitment research. However, some studies have shown that faster growth and larger size do not always lead to reduced predation pressure on fish larvae. In a mesocosm study, Fuiman (1989) found that overall vulnerability of herring larvae to predation by older herring increased with increasing larval size. He suggested that larger size may have made larvae more conspicuous to larger visual predators. In their critique of the ‘bigger is better’ hypothesis, Litvak and Leggett (1992) presented experimental evidence that adult sticklebacks preyed selectively on larger, rather than smaller, capelin larvae. Optimality theory would predict that a predator would select prey with the greatest net gain of energy per unit of effort. Therefore, it would not be surprising to find that predators would select prey sizes other than the smallest available. In the earlier predation studies which had been used to support the ‘bigger is better’ hypothesis the predator had been presented with prey of only one size at a time, thereby eliminating the component of predator choice (Folkvord and Hunter, 1986; Purcell et al., 1987; Butler and Pickett, 1988; Fuiman, 1989; Margulies, 1990). What these studies measured was the ability of the prey to escape capture by a particular predator which increased with increasing size, not whether a predator would actually select that prey given a choice of prey sizes. The inland silverside, Menidia beryllina Cope, is an annual, zooplanktivorous, estuarine species ranging from Cape Cod, MA, USA, to Vera Cruz, Mexico (Robbins, 1969; Chernoff et al., 198 1). Eggs are spawned on submerged aquatic vegetation in late spring in the Pettaquamscutt River, RI, USA (Huber et al., data not shown). YOY spend the summer in the upper estuary where they reach 35-45 mm standard length by the end of September and spawn the following spring (Huber et al., data not shown).

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Ecol. 199 (1996) 165-177

161

Because US Environmental Protection Agency (EPA) uses M. beryllina as a standard species for bioassays (Weber et al., 1988), we are attempting to determine the potential population-level consequences of sublethal toxicity test endpoints, such as reduced growth rate, by investigating the importance of size for survival of YOY M. beryllina. The present study addresses growth of larvae in the Pettaquamscutt River estuary as well as the influence of larval size on predation mortality. The growth and survival of M. beryllina larvae in the absence of predation was assessed in two experiments using in situ mesocosms. The size-specific vulnerability of YOY M. beryllina to YOY of four species of potential predators, juvenile striped bass, Morune suxutilis Walbaum, white perch, Morone americana Gmelin, bluefish, Pomatomus saltatrix Linnaeus and crevalle jack, Caranx hippos Linneaus, was assessed in a series of experiments using in situ mesocosms and laboratory aquaria. Of the four predator species only YOY striped bass are not commonly found in this estuary; however, their range does overlap substantially with that of M. beryllina and they are known to prey on silversides (Matthews et al., 1988, 1992). Finally gut contents of field collected potential predators were examined to determine if the results of the predation experiments were consistent with actual prey consumption in the wild.

2. Materials

2.1. Mesocosm

and methods growth and survival

The M. beryllina larvae used in these experiments were obtained by spawning laboratory stock cultures collected from the Pettaquamscutt River in 1991, using the methods of Middaugh et al. (1986). Larvae were fed rotifers, Brachionus plicatilis, for 5-7 days post hatch and were given Artemia nauplii (Reference Artemia II strain; Bengtson et al., 1985) beginning at 5 days post hatch. Menidia beryllina larvae were placed in in situ mesocosms in the Pettaquamscutt River to assess growth and survival in the absence of predation. Two separate 7-day growth and survival studies were conducted. The experiments were conducted using larvae of the same age (7-14 days post hatch (DPH)) as required for the standard EPA toxicity test. In the first experiment five mesocosms were stocked with 45 larvae (90 mm3) in the first week of June. A second experiment was conducted in an identical manner 2 weeks later, with the exception that 3 mesocosms were stocked with 45 larvae and 3 mesocosms were stocked with 135 larvae (270 mp3). The mesocosms were 1 m diameter X 1 m high cylinders, constructed of 500 pm nylon mesh. The diameter of the mesocosms (1 m) was the same as that recommended by de Lafontaine and Leggett (1987) and the mesh size was identical to that used by Laurence et al. (1979) in this same estuary. The mesh size was chosen to retain the larvae, exclude any predators and to permit zooplankton from the surrounding waters to enter freely. The top of the mesocosm was open, the bottom was a cone funnelling down to a 10 cm canvas sleeve. A plastic jar attached to the canvas sleeve allowed for the collection of the larvae at the conclusion of the experiment. Each mesocosm was secured

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I J. Exp. Mar. Biol. Ecol. 199 (1996) 165-177

within a wooden frame. Once attached to the frame the mesocosm units were deployed in a sheltered shallow cove (50 cm depth mean low water) in the upper basin of the Pettaquamscutt River where M. beryllina larvae are found. The cove was shallow and the tidal range was small (25-30 cm). The mesocosms were distributed throughout the cove and were deployed by sinking the legs of the wooden frames into the sediment to a depth of 60 cm, leaving the bottom of the mesh cylinder flush with the bottom of the cove. The depth and the volume of the mesocosms varied from 50-80 cm and 0.4-0.6 m3, respectively, throughout the tidal cycle. The stocking densities were based on the average volume of the mesocosms (0.5m3). Larvae were collected at the conclusion of an experiment by raising the mesocosm and rinsing the larvae into the plastic jar at the bottom. Live larvae were transported to the laboratory where they were anesthetized in MS-222, measured, enumerated, rinsed in ammonium formate and dried at 60°C for 48 h. Average initial standard lengths (estimated to the nearest 0.1 mm with an ocular micrometer and a dissecting microscope) and average initial dry weights .(to the nearest 0.001 mg with a Cahn C-30 microbalance) of the larvae were determined from a subsample of 50 (experiment 1) or 44 (experiment 2) fish taken from the rearing aquarium. The survival and dry weight of the larvae were measured in each mesocosm. Instantaneous growth rates (G),

G=

ln(W,) - in(K) t, -t,

where W, equals average initial dry weight at time t, and W, is the final dry weight at time t, , were calculated for each fish (experiment 1) and for a subsample of 20 fish per enclosure (experiment 2). A one-way ANOVA (PcO.05) was used to determine statistically significant differences in G among the five mesocosms in experiment 1. In the second experiment, a nested ANOVA (PcO.05) was used to determine if differences in G could be attributed to stocking density.

2.2. Predation

experiments

Striped bass larvae obtained from Chesapeake Bay spawning stock were held for two months at 10%0 and 25°C and were fed Artemia nauplii daily. To eliminate the problem of confronting laboratory-reared striped bass with novel prey during the predation experiments, striped bass were fed larval M. beryllina of a variety of sizes a few days prior to the experiments. Juvenile white perch, bluefish and crevalle jack were collected from the Pettaquamscutt River as they became available and were held for up to 7 days at 10%0 and 25°C. Seven predation experiments on the relative vulnerability of different size classes of M. beryllina were conducted (Table 1). Initially, the size classes of silversides used were representative of the first few weeks of age. As the summer progressed larger sized larvae and juveniles were exposed to various predators. Because they could be obtained

5 6 7

Striped Bass Striped Bass White Perch White Perch Striped Bass Bluefish Striped Bass Crevalle Jack

1 2 3 4

28.42 2.9 26.9+ 3.1 38.6% 1.6 37.5* 0.4 48.35 2.4 114.42 14.4 30.1? 5.8 51.4+ 9.1

Mean pred size SL (mm) + SD

Reps

10 10 10 3 4 5 5 and 5 5

Date

613 615 7/16 7/18 7/18 812 8124 9/10

Medium

17.321.2 10.7t0.7 15.1+1.0

Small 8.5kO.4 9.620.5 9.920.8 10.8?0.4 10.8+0.4 12.020.6 8.5kO.7 10.1+1.3

Mean prey size SL (mm)%SD

experiments

12.1-cl.l 12.820.8 16.9k1.3 20.6+2.0 20.622.0 23.9k2.0 14.9kl.O 19.2%1.7

Large

species and their sizes as well as the size ranges of Menidia beryllina used in the seven predation

Predator

of the predator

Experiment

Table 1 Summary

Y Y

Y Y Y Y Y

Lab 76 1

Y Y

$

Z ?

5 e

t

5 h 3

b a B 3 00 Field 500 1 2

170

T.R. Gleason,

D.A. Bcngtson

I J. Exp. Mar.

Biol. Ecol.

199 (1996)

165-177

in large quantities from hatcheries and were relatively easy to maintain in the laboratory, YOY striped bass served as a generic predator. The other three predator species were used as they became available in the field. When field collected predators were used, laboratory-reared silversides which were within the size range present in the estuary at that time were used as prey. However, the final experiment using crevalle jack as predators was conducted late in the season and the M. beryllina used were significantly smaller than those then present in the field. Two types of experimental chambers were used: in the laboratory 76-l rectangular glass aquaria were used; the previously described mesocosms were used for field predation experiments. In each laboratory experiment one or two groupings of six 76-1 aquaria were used. One aquarium per grouping served as a no-predator control, while the other aquaria were considered as replicates. Similarly, in the field experiments one mesocosm was randomly selected as a no-predator control and the other 5 mesocosms were considered replicates. To ensure a hungry predator and a uniform level of hunger, the predator was added to the experimental unit on the afternoon prior to the trial and received no food before the experiment was conducted. At the conclusion of an experiment, the predator was removed from the experimental chamber and the surviving silversides were retrieved. The duration of the predation experiments was chosen to allow the predator enough time to consume somewhat less than half of the available prey. In the first three laboratory experiments 10 M. beryllina larvae from each of two size classes (8 and 12 mm) were exposed to predation by striped bass or white perch for 3 h. Experiments 4, 6 (laboratory portion) and 7 were terminated in 1 hr as larger predators consumed more larvae in less time. In the field studies (experiment 5 and the field component of experiment 6) due to the larger volume of the mesocosms, larger numbers of prey were used and the experiments were conducted over a 24-h period. At the conclusion of each experiment the total number of larvae consumed from each size class was determined by subtracting the number remaining from the initial number. The no-predator control allowed us to account for handling mortality and verify our ability to retrieve all of the live larvae. A paired t-test ((u =0.05; Sokal and Rohlf, 1969) was used to determine if the predators preyed selectively on a particular size class. Later experiments (5-7) used equal numbers of three size classes of prey. Selectivity for a particular size-class was indicated if the proportion consumed, of prey of any prey size, was significantly different from 33% using a paired t-test (a=0.05; Sokal and Rohlf, 1969). In one final experiment, striped bass were used in a lab-field comparison where 5 replicates and one control were set up simultaneously in the 76-1 aquaria and the 500-l mesocosms. The density of prey per unit volume was approximately 0.3 fish l- ’ in both containers.

2.3. Gut content analysis In addition to the predation experiments, potential predators were periodically collected during routine beach seining operations from the Pettaquamscutt River for gut content analysis. Presence or absence of silversides in the gut was noted. This allowed for a qualitative estimation of the importance of M. beryllina to the diet of these fish in the wild.

T.R. Gleason, D.A. Bengtson / J. Exp. Mar. Biol. Ecol. 199 (1996)

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77

171

3. Results

3.1. Mesocosm

growth and survival

Survival in individual mesocosms averaged &O-89.4%, with a range of 77-95% in the two experiments and was not affected by stocking density (Table 2). Mean instantaneous growth rates ranged from 0.122 to 0.135 day-’ in the two experiments. There were no significant differences in G among fish in the five enclosures in experiment 1 and no significant differences in G between fish at the two stocking densities in experiment 2.

3.2. Predation

experiments

Laboratory-reared striped bass juveniles (Experiments 1, 2, 4 and 6) consistently demonstrated significant selectivity for the smallest M. beryllina larvae available (Tables 1,3 and 4) both in the laboratory and field experiments. Field-collected juvenile white perch and crevalle jacks also exhibited significant selectivity for the smallest M. beryllina larvae available (Tables 1,3 and 4). Only field-collected juvenile bluefish demonstrated significant selectivity for the largest size class of M. beryllina available (Tables 1 and 4). Table 2 Mean percent survival and mean instantaneous mesocosms from 7 to 14 days post hatch Experiment

Mesocosm

1 2 2

5 3 3

Table 3 The mean numbertSE Exp No

n

Larval density

(m-‘)

of Menidia beryllina consumed

Predator

M. beryllina classes

G+SD

per predator

0.12250.065 0.128-tO.070 0.135 kO.080

by size class experiments

l-4

size

Striped bass

1.920.5

0.6kO.3

2

Striped bass

(+) 3.4rfro.7

(-) 1.620.3

3

White perch

(+) 6.320.7

(-) 0.620.2

4

White perch

(+) 5.Ok2.0

(-) 0.720.7

4

Striped bass

(+) 6.520.3

(-) 2.320.5

(+)

(-)

was based on significance

(%)

88.0+7.6 88.923.1 89.423.0

Large

Selectivity

Survival+SD

90 90 270

Small 1

rate for larval Menidia beryllina held in in situ

growth

of the paired f-test (+),

positive

selectivity;

(-),

negative

selectivity.

172

T.R. Gleason,

Table 4 The mean number+SE Exp No

D.A. Bengtson

I J. Exp. Mar. Biol. Ecol.

of Menidia beryllina

consumed

M. beryllina

Predator

per predator

6 6 7

165-177

by size class experiments

Medium

Large

33%

8.4Z1.2

8.820.6

7.2kl.O

Striped bass (lab)

(-) 4.250.6

(0) 2.820.4

(+) 1.0?0.6

2.720.3

Striped bass (field)

(+) 21.624.0

(0) 5.6? 1.9

(-) 4.42 1.3

Crevalle

(+) 4.421.0

(0) 1.620.5

(-) 2.4-cO.8

(+)

(-)

(0)

4.4-t 1.4

Bluefish

Jack

5-7

size classes

Small 5

199 (1996)

10.522.3 2.8kO.4

Selectivity was based on significance of the paired t-test comparing the number consumed within a size class to the number equivalent to 33% of the total prey consumed per individual predator. ( + ) = number consumed was significantly more than 33% of total (positive selectivity), (o)=number consumed was not significantly different than 33% of total (no selectivity), (-)=number consumed was significantly less than 33% of total (negative selectivity).

Table 5 Percent occurrence River

of various prey items for three potential

Predator

%

White Perch (n = 82)

71

Amphipod

18 11 44 41

Empty Unidentifiable

15

Menidia

sp.

67

Menidia

sp.

17

Alosa sp.

16

Empty

Crevalle Jack (n = 32)

Bluefish (n = 6)

predators

of M. betyllina

in the Pettaquamscutt

remains

Empty Fundulus

sp.

3.3. Gut content analysis Juvenile white perch (n = 82) were never found to have larval M. beryllina in their gut; 77% had amphipod remains, 18% were empty and 11% had unidentifiable remains (Table 5). Crevalle jack (n =32) were found to prey on fish larvae, but primarily on Fundulus sp. (41%) rather than Menidia sp. (15%). Bluefish (n = 6) were found to prey on Menidia sp. (67%), often with 2-4 juveniles in their gut simultaneously.

4. Discussion The high rates of survival observed in the mesocosms indicate that, in the absence of predation, M. beryllina survival is high during this 7-day period in their early-life-

T.R. Gleason, D.A. Bengtson I J. Exp. Mar. Biol. Ecol. 199 (1996) 165-177

173

history. Previous enclosure studies conducted with other species have also demonstrated high larval survival and growth rates in the absence of predation (Laurence et al., 1979; (diestad, 1982; de Lafontaine and Leggett, 1987). The G values observed in this experiment (0.122-0.135) were comparable to the G values (0.087-0.176) obtained by Letcher and Bengtson (1993) for this population in a laboratory growth study at moderate to high rations of Artemia nauplii. The high rates of growth observed in the mesocosms at larval concentrations of 90-270 mm3 suggests that adequate food was available in this environment for larval development and that starvation was unlikely to be an important factor influencing larval survival in the Pettaquamscutt River. The differences in size selectivity of the four predator species may well be due simply to their marked differences in size. We are not suggesting that comparison of bluefish with a mean fork length of 114 mm to crevalle jack with a mean fork length of 51 mm would be indicative of species-specific preferences in prey selection. However, the sizes of the field-collected predators used in these experiments were representative of the sizes of potential predators encountered by M. beryllina in this system. The influence of size, within species, on size-selective predation warrants further research. Three out of the four predator species (white perch, striped bass and crevalle jack) preyed selectively on the smallest size-class of M. beryllina available. This would appear to support the hypothesis that smaller larvae are more vulnerable to predation. However, based on our gut content analyses, white perch and crevalle jack did not demonstrate a propensity to prey on M. beryllina in the wild. In addition crevalle jack typically appear late in the summer and may not encounter larval silversides. Since YOY striped bass are not commonly found in this system, their selectivity in the wild could not be assessed. As a result it would appear that those species which demonstrated selective predation on the smallest size classes of M. beryllina in the laboratory may not represent significant sources of predation mortality in their natural habitat, Pettaquamscutt River. Juvenile bluefish, on the other hand, demonstrated selectivity for the largest size class of M. beryllina in mesocosm experiments. Juanes et al. (1993) reported a significant linear relationship between juvenile bluefish length and prey length. According to their model, bluefish of the size used in our predation experiments (mean fork length = 114 mm) would be expected to selectively prey on juvenile fish whose length was similar to that of the largest size class (mean standard length = 23.9 mm) offered to the bluefish in our predation experiment. Therefore, it was not surprising that juvenile bluefish preyed selectively on the largest size class available in the experiment. Though our sample size for gut content analysis was small (n = 6) juvenile bluefish do prey on silversides in the wild. We were unable to collect juvenile bluefish after mid July, suggesting that 1992 was a year of low bluefish recruitment to this system. Friedland et al. (1988) and Juanes and Conover (1994) have indicated that Menidiu menidia Linnaeus comprised a significant portion of diet of juvenile bluefish. When abundant, juvenile bluefish might represent a significant source of predation mortality on the largest members of the M. beryllina yearclass in the Pettaquamscutt River. While the actual source of the mortality was not determined, otolith back-calculation analyses have indicated that size-selective mortality was directed at the larger members of the M. betyllina cohort, in the Pettaquamscutt River, during periods of each of two summers (Gleason, 1995; Gleason and Bengtson, submitted). These findings are counter to conventional wisdom which

174

T.R. Gleason, D.A. Bengtson I J. Exp. Mar. Biol. Ecol. I99 (1996) 165- 177

asserts that larger larvae are less vulnerable to predation, but supports Litvak and Leggett (1992) who found that sticklebacks preyed selectively on larger capelin larvae. At first glance, our findings of bluefish selectively preying on the largest members of the M. berylha cohort, might appear to conflict with the findings of Juanes et al. (1993). They reported that bluefish preyed on the smallest members of the M. menidia yearclass. We believe that rather than conflicting, our results are complementary. It is conceivable that YOY bluefish could simultaneously prey on the smallest M. menidia and the largest M. beryllina. Menidiu menidiu initiate spawning approximately one month earlier and have inherently higher growth capacity than M. beryllinu (Bengtson, 1984). When the two species co-occur M. menidiu are typically larger, though there can some overlap in their respective size distributions. The size frequency distributions for the two species from a July 31, 1990 sample collection (Gleason, 1995) illustrate this point (Fig. 1). We have superimposed on this size-frequency distribution the estimated size preference (using the regression equation of Juanes et al., 1993) of five bluefish collected on 31 July 1992 (Fig. 1). We are not suggesting that this figure portrays the actual size relationships between prey and predator, since we are comparing fish collected two years apart and we have a limited sample of bluefish (n = 5) from that date. However, this figure does illustrate how YOY bluefish could potentially prey on small M. menidiu and large M. beryllinu simultaneously.

Fig. 1. Size frequency distribution of YOY Menidia menidia and M. beryllina collected Pettaquamscutt River on 31 July 1990 (Gleason, 1995). The estimated bluefish prey size preference on five fish collected on 31 July 1992 (note different years) using the bluefish prey size preference of Juanes et al. (1993).

from the was based regression

T.R. Gleason, D.A. Bengtson I J. Exp. Mar. Biol. Ecol. 199 (1996) 165-177

175

The substantial growth observed in the two mesocosm growth experiments indicated that food was not limiting to larval M. beryllina in June 1992 in the Pettaquamscutt River. The high rates of survival observed in the mesocosm experiments indirectly suggests that predation may be a significant source of YOY mortality. The predation experiments indicated that size-selective predation mortality can be directed at the largest members of a population of larval and juvenile fish rather than only the smallest as many previous studies had suggested. The importance of providing predators with a choice when conducting predation experiments should not be overlooked. Additionally, following laboratory predation studies with field verification of prey consumption is necessary to determine the relevance of the laboratory findings. Since M. beryllina are found in a variety of habitats, from freshwater to estuarine, our results suggest that size-specific survival of YOY M. beryllina may vary geographically, spatially and temporally depending on the particular suite of predators encountered by individual populations or cohorts. In the Pettaquamscutt River predation pressure may potentially be directed towards the larger members of the M. beryllina year class.

Acknowledgments The authors thank the late W.E.R.E. Lafarge and Marty Lafarge for permitting us to access our study site through their property. Gustav0 Bisbal, Jim Kinney, Marina Huber, Karen Salomon and Bob Briggs provided valuable field support. Walter Berry, Michael Fogarty, Marina Huber, William H. Krueger, Grace Klein-MacPhee, Richard Voyer and three anonymous reviewers provided valuable comments on an earlier version of this manuscript. James Heltshe provided valuable statistical advice. The research was supported by Cooperative Agreement CR-819601 between US EPA/Environmental Research Laboratory, Narragansett, RI, and the University of Rhode Island. This paper is AED Contribution number X227.

References Anderson, J.T., 1988. A review of size dependent survival during pre-recruit stages of fishes in relation to recruitment. /. Northw. Atl. Fish. Sci., Vol. 8, pp. 55-66. Bailey, K.M. and E.D. Houde, 1989. Predation on eggs and larvae of marine fishes and the recruitment problem. Adv. Mar. Biol., Vol. 25, pp. l-83. Bengtson, D.A., 1984. Resource partitioning by Menidia menidia and Menidia beryllina (Osteichthyes: Atherinidae). Mar. Ecol. Prog. Ser., Vol. 18, pp. 21-30. Bengtson, D.A., A.D. Beck and K.L. Simpson, 1985. Standardization of the nutrition of fish in aquatic toxicological testing. In, Nutrition and feeding in fish, edited by C.B. Cowey, A.M. Mackie and J.G. Bell, Academic Press, New York, NY, pp. 431-446. Beyer, J.E., 1989. Recruitment stability and survival - simple size-specific theory with examples from the early life dynamics of marine fish. Dana, Vol. 7, pp. 45-147. Butler, J.L. and D. Picket& 1988. Age-specific vulnerability of Pacific sardine, Sardinops sagax, larvae to predation by northern anchovy, Engraulis mordax. Fish. Bull., Vol. 86, pp. 163. Chemoff, B., J.V. Conner and CF. Bryan, 1981. Systematics of the Menidia beryllina complex (Pisces: Atherinidae) from the Gulf of Mexico and its tributaries. Copeia, pp. 319-336.

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