Effects of body size on probability of 

 predation for juvenile summer and 

 winter flounder based on laboratory 

 experiments 



David A. Witting 

 Kenneth W. Able 



Rutgers University, Institute of Marine and Coastal Sciences, 

 Marine Field Station, Great Bay Blvd, 

 PO. Box 278, Tuckerton, Ml. 08087 



Predation by brown shrimp [Crang- 

 on crangon ) has been hypothesized 

 to impose significant mortality on 

 settling juvenile plaice (Pleuro- 

 nectes platessa) in European wa- 

 ters (Edwards and Steele, 1968; 

 Lockwood, 1980; Veer and Berg- 

 man, 1987; Pihl, 1990). Laboratory 

 experiments found that predation 

 rate was dependent upon prey den- 

 sity and that predation, estimated 

 by gut content analysis, could be 

 the source of previously unex- 

 plained density-dependent mortal- 

 ity of early juvenile plaice in the 

 Wadden Sea (Veer, 1986; Veer and 

 Bergman, 1987). An important 

 finding of this work is that preda- 

 tor and prey body size may have 

 profound effects upon rate of pre- 

 dation and that recently settled 

 plaice (under 30mm) are much 

 more vulnerable to predation than 

 are larger fish. We were prompted 

 to investigate the role of the 

 sevenspine bay shrimp (Crangon 

 septemspinosa) as a predator on 

 metamorphosing and small juvenile 

 summer {Paralichthys dentatus) 

 and winter (Pleuronectes ameri- 

 canus) flounder because 1) all of 

 these species are abundant compo- 

 nents of New Jersey estuaries, 2) 

 they all co-occur temporally, 3) the 

 sevenspine bay shrimp is morpho- 

 logically similar to the European 



brown shrimp, and 4) fish scales 

 were present in the guts of this 

 shrimp (Wilcox and Jeffries, 1974). 

 We conducted laboratory experi- 

 ments to determine the size range 

 over which juveniles of these fish 

 species may be susceptible to pre- 

 dation by sevenspine bay shrimp. 



Methods 



We used small cylindrical glass 

 bowls, 197 mm diameter, containing 

 2cm of washed sand (sufficient for 

 both predators and prey to burrow 

 in), filled with seawater to a depth 

 of 50 mm. We conducted all experi- 

 ments at prevailing ambient water 

 temperatures (Table 1) by immers- 

 ing the bowls in a flow-through sea- 

 water bath. 



We collected transforming and 

 juvenile winter flounder in May- 

 July, 1990 and summer flounder 

 in January-February 1990, with 

 nightlights, plankton nets, and 

 seines. Adult sevenspine bay 

 shrimp were collected (January- 

 July) by using a seine. All collec- 

 tions were made in the Great 

 Bay-Little Egg Harbor estuarine 

 system (New Jersey). In the labo- 

 ratory, we fed experimental ani- 

 mals in excess (shrimp-chopped 

 fish and shrimp; flounder-live 

 brine shrimp nauplii), and then 



Contribution No. 93-33 of the Institute of Marine and Coastal Sciences, Rutgers University. 



starved them for 24 hours before 

 the start of each experiment. 

 After anesthetization (25mg-l _1 

 MS-222) we measured body sizes 

 (Table 1) of flounder (in mm, stan- 

 dard length [SL]) and shrimp (in 

 mm, total length [TLJ, from the 

 tip of the antennal scale to the end 

 of the uropod, Price, 1962). 



At the start of each trial, we 

 placed one flounder into each con- 

 tainer and allowed it to acclimate 

 for 24 hours. We then introduced 

 one shrimp into each container at 

 approximately 1500 hours. All con- 

 tainers were covered with perfo- 

 rated clear plastic sheets and were 

 left undisturbed for 18 hours. We 

 ran the experiments under a natu- 

 ral light cycle (11 hours dark, 13 

 hours light, lights out at 1800) us- 

 ing fluorescent lighting. Because 

 sevenspine bay shrimp are unlikely 

 to forage during the day (Haefner, 

 1979), we introduced the shrimp 

 three hours before darkness in or- 

 der to reduce the likelihood of pre- 

 dation immediately following their 

 introduction. We recorded water 

 temperature at the start and end of 

 each trial. For each experiment, we 

 set up several control containers 

 (prey present, predator absent) and 

 observed no flounder mortality in 

 these controls (rc-15 for summer 

 flounder, n = 12 for winter flounder). 

 At the termination of each trial we 

 scored predation as plus or minus 

 based on the presence or absence of 

 a live flounder. 



We repeated trials for winter and 

 summer flounder and varied shrimp 

 and flounder size (Table 1). Because 

 settlement in summer flounder oc- 

 curs earlier than winter flounder, 

 experiments involving summer 

 flounder were conducted during late 

 winter and winter flounder experi- 

 ments were conducted in the spring, 

 resulting in different ambient wa- 

 ter temperatures (Table 1). In tri- 

 als involving winter flounder, we 



Manuscript accepted 19 March 1993. 

 Fishery Bulletin: 91:577-581 (19931. 



577 



