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• 3 H : The proportions of winter flounder and sand shrimp consumed by searobins
are not different from the proportions of the two prey at the start of experiments. Finally, we report on video observations of the search and attack strategies of
searobins feeding on juvenile winter flounder.
2. Materials and methods
2.1. Field collections and stomach content analysis Striped searobins were collected in the Navesink River Sandy Hook Bay Estuarine
System NSHES located at the apex of the mid-Atlantic Bight in central New Jersey Fig. 1. The Navesink River has one primary freshwater source that produces a
downstream salinity gradient 10–30‰ extending from the head of the river to the north end of Sandy Hook where Sandy Hook and Raritan Bays meet the Atlantic Ocean.
Average depth also increases along the downstream axis of the study area, from ¯ 1 to 15 m below mean low water bMLW. Tides are semidiurnal in the system with a range
of approximately 1.4 m.
Thirteen stations with bottom depths 3 m bMLW were established for gillnet sampling throughout the NSHES Fig. 1. Stations in the river generally had finer
grained sediments, and more vegetation Ulva lactuca, Zostera marina, Ceramium spp. than those located in the bay, which were adjacent to sandy beaches.
Gillnets were fished at the stations biweekly, for 2 h during daytime high tides, from late May through the beginning of November 1998. The total lengths TLs, mm of
searobins collected in gillnets were measured and their stomachs excised for dietary analysis. Diet items were identified to species when possible, enumerated, and weighed
to the nearest 0.1 g to estimate contributions of each prey to the total prey in searobin stomachs. The TLs of sand shrimp rostrum to end of telson and standard lengths SLs
of winter flounder in stomachs were measured to examine predator–prey size relation- ships.
2.2. Laboratory experiments 2.2.1. Collection and maintenance of experimental animals
Striped searobins were collected with an otter trawl and winter flounder and sand shrimp were collected with a haul seine in the NSHES. Animals were transported to the
Howard Marine Sciences Laboratory, Highlands, NJ, USA and maintained in aquaria with a continuous flow of ambient seawater pumped from Sandy Hook Bay
temperature 5 15.6–25.48C; salinity 5 20.5–25.5‰. A 15:9 h light:dark cycle day 5 0500–2000 EST was maintained in all laboratories. Aquaria were provided with 2–3
cm of washed sand substratum 0.5 mm, 0.82 f, sorting coefficient 5 0.33 except
¯ where noted below. Searobins ranging from 212 to 320 mm TL n 5 23, x wt 5 229 g,
range 5 112–320 g were fed live killifish Fundulus heteroclitus ad libitum between
214
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. Manderson
et al
. J
. Exp
. Mar
. Biol
. Ecol
. 242
1999 211
– 231
Fig. 1. Stations where searobins were collected in gillnets in the Navesink River Sandy Hook Bay Estuarine System. Crosses indicate stations where fish were absent. Pie diagrams show average diet composition by weight in a June and b July–August, 1998. n 5number of searobins collected. Inset shows the location of the
study area in the Mid-Atlantic Bight.
J .P. Manderson et al. J. Exp. Mar. Biol. Ecol. 242 1999 211 –231
215
experiments. Prey were also fed ad libitum; flounder with live Artemia and chopped clam and shrimp with frozen fish.
Predators were starved for 24 h before they were used in experiments. All of the prey were weighed g, measured winter flounder: TL and SL; sand shrimp TL and fed not
more than 3 h before they were exposed to predators.
2.2.2. Winter flounder size selection Searobin selectivity for sizes of flounder prey was examined in circular tanks 1.8 m
diam. 3 0.5 m deep with and without sand substratum Table 1. Starved searobins n 5 12, 212–309 mm TL were acclimated to tanks for 20 h. One hour before predators
were exposed to prey, searobins were isolated in arenas within opaque PVC cylinders 0.7 m diam. 3 0.6 m deep. Fourteen prey, which included two individuals in 10-mm
size classes ranging from 30 to . 90 mm TL, were introduced to the area outside the cylinders. Predators were released after 1 h and allowed to feed for 2 h 0900–1100
EST. Experiments were terminated by removing predators and draining the arenas. The sand substratum was searched with garden rakes to recover surviving flounder.
Gape sizes of freshly killed searobins n 5 13, 227–320 mm TL were measured to determine the morphological constraint on maximum prey size. Mouth width was
measured with Vernier calipers as the distance between the maxillary bones in the mouth interior. To measure esophagus width, predators were decapitated at the cleithrum and
calipers were inserted into the esophagus which was stretched by applying consistent pressure to the calipers. Total length and body depth BD mm, maximum dorso-ventral
distance were measured for flounder n 5 346, 28–100 mm TL to determine the relationship between prey body depth and predator gape size.
Table 1 Design of laboratory experiments performed to examine relationship between striped searobin predators and
winter flounder and sand shrimp prey; effective replicates were those in which prey were consumed Experiment
Replicates No. of
Temperature treatment
Effective replicates prey
8C
a
Flounder size selection Sand present
15 11 14
18-22 Sand absent
14 12 14
18-22
b
Prey selection Prey ratio, period no. flounder: no. sand shrimp
10:10, Night 20 15
20 16-18
10:10, Day 20 17
20 16-18
5:15, Day 20 9
20 16-18
15:5, Day 20 10
20 16-18
b
Behavioral observations Searobin-winter flounder
10 5 10
18-22
a
Flounder ranged in size from 30 to 120 mm TL.
b
Flounder ranged in size from 40 to 60 mm TL mean 50.7964.26 while sand shrimp ranged in size from 30 mm to 50 mm TL mean 37.562.9 mm.
216 J
.P. Manderson et al. J. Exp. Mar. Biol. Ecol. 242 1999 211 –231
2.2.3. Day–night prey selection Diurnal variation in prey selectivity was determined by offering equal numbers of
flounder n 5 10 and shrimp n 5 10 to individual searobins n 5 20, 212–309 mm TL, 112–320 g in 3 h day 1000–1300 EST and night 2000–2300 EST experiments
Table 1. Twenty predators were exposed to each treatment. The flounder used were within the size range of fish 40–60 mm TL most frequently consumed in the size
selection experiments. Shrimp were sized 30–50 mm TL to match flounder as closely as possible. Predators were introduced to tanks 2.5 m diam. 3 0.5 m deep with sand
substratum and fed ad libitum for the first 24 h of a 48-h acclimation period. Searobins were isolated within PVC cylinders and prey were introduced to the tanks in the same
manner as in the size selection experiments. Experiments were terminated by removing predators, draining arenas, and sieving the substratum through 3-mm mesh to recover
surviving prey.
2.2.4. Prey switching The effect of relative prey density on searobin prey selection was examined by
presenting three prey ratios no. flounder:no.shrimp; 5:15, 10:10, 15:5 to individual predators n 5 20; 212–309 mm TL in 3 h experiments Table 1. Switching
experiments were performed during the day 1000–1300 EST because time of day did not significantly P 5 0.09 influence predation rate or prey choice in the day–night
selection experiments see Results. Equal numbers of searobins n 5 5 were exposed to the 5:15 and 15:5 prey ratios in four trials producing 20 replicates per treatment. We
used the daytime treatment of the day–night prey selection experiment as the 10:10 treatment in the switching experiment because switching experiments immediately
followed the day night selection experiment and experimental protocols were identical.
2.2.5. Behavioral observations Videotaped observations of individual searobins n 5 10, 228–309 mm TL feeding
on flounder 40–60 mm TL were made in glass fronted rectangular tanks 2.5 m long 3 0.8 m wide 3 0.5 m deep during 2-h daytime experiments 0800–1000 EST;
Table 1. Predators were isolated within the tanks in PVC cylinders and ten flounder were allowed to acclimate to the areas outside the cylinders for 1 h. Predators were then
released and exposed to prey for 2 h. At the end of trials, the predator was removed and all surviving prey were collected.
Videotapes were analyzed to quantify: 1 the percent of time searobins spent ‘walking’ on or probing the substratum with modified pectoral fin rays and potentially
searching for prey, 2 the number of attacks and 3 the location bottom or water column where attacks occurred.
2.3. Statistical analysis 2.3.1. Predator–prey body size relationships
Predator–prey body size relationships were explored using the quantile regression technique described by Scharf et al. 1998, and implemented with
STATA
statistical software Statacorp, 1995. The method permits statistical evaluation of changes in
J .P. Manderson et al. J. Exp. Mar. Biol. Ecol. 242 1999 211 –231
217
minimum and maximum prey size with changes in predator size using least absolute values regression with bootstrapped estimates of coefficient standard errors. We used the
rule n . 10 q
to determine quantiles q reflecting trends in minimum and maximum prey size given sample size n Scharf et al. 1998. Total length mm was the size parameter used in all
regressions. Total lengths of flounder consumed by searobins in the field were estimated from measured standard lengths using the regression equation published in Able and
Fahay 1998
TL 5 1.213 SL 2 0.447 2.3.2. Winter flounder size selection
The nonlinear relationship between prey size and mortality prevented the use of a linear logistic model to test for the effects of prey size and substratum type on prey
selectivity. Therefore we used logistic generalized additive models GAMs with spline smoothers S-plus 4.5, 1997 to nonparametrically model the effects of prey size on the
mortality of flounder in the presence and absence of sand. Logistic GAMs use the ratio of response frequencies logits and scatterplot smoothers to fit data-defined and
unspecified functions to the relationship between response and predictor variables Hastie, 1993. Since strong predator–prey body size relationships were not evident see
Results, replicates in which prey were consumed were pooled within treatments. Individual fish were scored as live or dead and these response frequencies were used in
