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Fishery Bulletin 91(3), 1993 



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Standard Length of Prey (mm) 



49 



Figure 3 



Relationship between size of winter flounder and predation by sevenspine 

 bay shrimp. Figure presents original data with sample sizes above each 

 bar (Al and predictions made by logistic regression (B). Bars on prob- 

 abilities reflect one standard error of the estimate. These probabilities 

 are based on the equation: Logit = 4.39-10.42 * prey size I. 



gest that small summer flounder are vulnerable to pre- 

 dation over a large fraction of this range, with a 50% 

 chance of mortality when encountering a shrimp of 

 45 mm TL (Fig. 1). 



Winter flounder also settle (April-May) in north- 

 eastern U.S. estuaries (Pearcy, 1962) including those 

 in New Jersey (pers. observ. ) when adult sevenspine 

 bay shrimp are abundant. Winter flounder settle at a 

 smaller size than summer flounder (7.8 mm SL for win- 

 ter flounder, Chambers and Leggett, 1987, vs. 13 mm 

 SL for summer flounder, Keefe and Able, In press); 

 however, our data indicate that the relationship be- 

 tween flounder size and vulnerability to predation is 

 similar for the two species. This suggests that winter 

 flounder must approximately double their size (i.e., 

 reach approximately 17mm) before they achieve a size 

 refuge from predation by large sevenspine bay shrimp 



(Fig. 3). Both winter and summer flounder ap- 

 pear to exhibit a pronounced decrease in vul- 

 nerability to predation between 9 and 20 mm 

 SL. 



In summary, the vulnerabilities of both spe- 

 cies of flounder were significantly affected by 

 small differences in prey size. The duration of 

 time spent within this size range (i.e., growth 

 rate) can be quite variable depending upon 

 habitat for winter flounder (Sogard, 1992) and 

 temperature effects for summer flounder ( Keefe 

 and Able, 1993). Thus slight variation in size 

 at settlement, or growth after settlement, may 

 have important effects upon survival for both 

 species. This scenario suggests that variabil- 

 ity in stage duration, not rate of mortality, 

 may be a critical determining factor of year- 

 class strength as has been suggested by a num- 

 ber of authors (Sissenwine, 1984; Chambers 

 and Leggett, 1987; Houde, 1987; Bailey and 

 Houde, 1989). 



Acknowledgments 



We are grateful to Matthew Pearson, Lynn 

 Wulff, and Roger Hoden for assistance in con- 

 ducting experiments, and R. Christopher 

 Chambers who provided a constructive review 

 of an earlier draft. This project was funded 

 through a fellowship grant from the Electric 

 Power Research Institute (EPRI); support was 

 also provided from a Leatham grant (Rutgers 

 University, Biological Sciences), Manasquan 

 Marlin and Tuna Club Scholarship Fund, The 

 New Jersey Sea Grant College Minigrant Pro- 

 gram, and the Institute of Marine and Coastal 

 Sciences <IMCS), Rutgers University. 



Literature cited 



Able, K. W., R. E. Matheson, W. W. Morse, M. P. Fahay, 

 G. Shepherd. 



1990. Patterns of summer flounder Paralichthys 

 dentatus early life history in the Mid-Atlantic Bight 

 and New Jersey estuaries. Fish. Bull. 88:1-12. 

 Bailey, K. M., and E. D. Houde. 



1989. Predation on eggs and larvae of marine fishes 

 and the recruitment problem. Adv. Mar. Biol. 25:1- 

 83. 

 Chambers, R. C, and W. C. Leggett. 



1987. Size and age at metamorphosis in marine fishes: 

 An analysis of laboratory-reared winter flounder 

 (Pseudopleuronectes americanus) with a review of 



