Buckel et al : Foraging habits of Pomatomus saltatrix 



773 



For our estimates of bay anchovy consumption by 

 the YOY bluefish population during their Septem- 

 ber migration, it was assumed that the entire YOY 

 bluefish population occurred on the shelf ft'om Cape 

 Hatteras, NC, to Montauk, NY. Support for this 

 comes from sharp declines in YOY bluefish CPUE in 

 early autumn estuarine beach seine surveys (Nyman 

 and Conover, 1988; McBride and Conover, 1991) and 

 an abrupt decline in average bluefish size on the shelf 

 during early autumn (Munch, 1997). Our estimates 

 show that given a 30-d migration period in Septem- 

 ber, the population of YOY bluefish could consume 

 from 6.0 to 6.8 billion bay anchovies in 1994 and from 

 2.2 to 5.3 billion in 1995. 



Summer-spawned bluefish consumed two orders 

 of magnitude less bay anchovy per day in 1995 

 (5 million) compared with 1994 (130 million). This 

 was partly due to the low abundance of this cohort 

 in 1995 (4.2 million ) compared with that in 1994 ( 16.8 

 million). The amount of bay anchovy in the diet of 

 summer-spawned bluefish (80'7f in 1994 vs. 60^^ in 

 1995 ) and the absolute size of bay anchovy prey taken 

 by the two cohorts (0.27 g in 1994 vs. 0.54 g in 1995) 

 explained the remainder. Interannual variation in 

 the size structure of the summer-spawned cohort and 

 their predominant piscine prey (bay anchovy) can 

 have dramatic effects on the predatory impact of this 

 bluefish cohort on bay anchovy populations. Although 

 the importance of size-dependent processes in fresh- 

 water fish predator-prey interactions is well de- 

 scribed (Kerfoot and Sih, 1987; Ebenman and 

 Persson, 1988), their importance for piscivore-prey 

 interactions in marine systems is just beginning to 

 be recognized (Juanes and Conover, 1995). 



There are no estimates of coastwide bay anchovy 

 abundance for 1994 and 1995; hence, no direct ex- 

 amination of the impact of this estimated bay an- 

 chovy loss to the continental shelf bay anchovy popu- 

 lation was made. There are bay anchovy biomass 

 density estimates for estuaries on the east coast of 

 the U.S. Vouglitois et al. (1987) estimated that the 

 bay anchovy standing crop in Barnegat Bay, NJ, 

 ranged from 830 to 4830 kg/km^. In Chesapeake Bay, 

 Luo and Brandt's (1993) bay anchovy biomass den- 

 sity estimate for September was -8000 kg/km-. We 

 calculated the standing stock of bay anchovy on the 

 shelf by multiplying the estimated ranges of estua- 

 rine bay anchovy densities by the area ( km- ) in which 

 bluefish were collected on the shelf This area (km-) 

 estimate was calculated by summing all the NEFSC 

 stratum areas in which YOY bluefish were captured 

 in the autumn groundfish survey in 1994 and 1995. 



During the month of September, bluefish (spring- 



"and summer-spawned combined) consumption of bay 



anchovy could account from ~2 to 22% of bay anchovy 



standing stock in 1994 and from 1% to 24% of the 

 bay anchovy standing stock in 1995. Bay anchovy 

 are probably less dense on the shelf than in the estu- 

 ary and our estimates of bluefish impact may be un- 

 derestimates. However, the latest bluefish stock as- 

 sessment (Mid-Atlantic Fishery Management Coun- 

 cil') has found that bluefish abundance may be lower 

 than that reported in the VPA ( NEFSC i). This could 

 mean that our impact estimates are overestimates. 

 At the upper range of the impact estimates, it is clear 

 that the YOY bluefish population on the continental 

 shelf consume a significant quantity of bay anchovy 

 biomass. The effect of this loss on the population dy- 

 namics of bay anchovy or the fish community on the 

 continental shelf is unknown. 



Acknowledgments 



We thank T. Hurst, S. Munch, personnel of the North- 

 east Fisheries Science Center of the National Ma- 

 rine Fisheries Service, and the crew of the KV Alba- 

 tross IV for aiding in the collection of bluefish used 

 in this study. We also thank A. Green for assistance 

 with stomach contents analysis. S. Morgan, T. 

 Rotunno, J. Galbraith, and A. Matthews aided in prey 

 identification. S. Munch, J. Galbraith, P. Kostovik, 

 and R. Rountree were instrumental in obtaining data 

 from the NMFS database. M. Terceiro kindly pro- 

 vided bluefish stock assessment information. We 

 thank R. Cerrato, R. Cowen, G. Lopez, and two anony- 

 mous reviewers for critical reviews. This research 

 was funded by the National Oceanic and Atmospheric 

 Administration (NOAA) Coastal Ocean Program and 

 by NOAA award nos. NA90AA-D-SG078 and 

 NA46RG0090 to the Research Foundation of SUNY 

 for the New York Sea Grant Institute. This work was 

 prepared for publication while J.A.B. held a National 

 Research Council-NOAA Research Associateship. 



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