Buckel el al.: Foraging habits of Pomatomus saltatnx 



761 



fish (Buckel and Conover, 1996). Brieny, YOY blue- 

 fish were acclimated to experimental tanks for two 

 weeks, acclimated to 15°C for 3 days, and then held 

 at IS'C for a 12-48 h starvation period. They were 

 then fed a single meal of previously frozen and thawed 

 adult bay anchovy (bluefish sizes were: mean bluefish 

 TL=122 |range=99-146|, mean wet weight=15.79 g, 

 prey mean wet weight=0.88 g, mean prey wt/predator 

 wt=5.8%). After a predetermined period of time, indi- 

 \idual bluefish were sacrificed and their stomach con- 

 tents removed, blotted dry, and weighed (±0.01 g). 



The exponential GER model (see Buckel and 

 Conover, 1996) was fitted to the proportion of meal 

 remaining versus time by using nonlinear regi'ession 

 analysis. The GER estimate from this experiment 

 was used along with estimates from Buckel and 

 Conover ( 1996) to develop an empirical function de- 

 scribing YOY bluefish GER from 15° to 30"C. This 

 function was used to estimate bluefish GER from 

 water temperatures at which bluefish were collected 

 on the continental shelf. Bluefish daily ration was 

 estimated for each cohort by region in 1994 and 1995 

 with the Eggers ( 1979) equation: 



D = F-R2A, 



YOY bluefish abundance in 1994 and 1995 were used 

 to calculate the daily consumption of bay anchovy 

 by the YOY bluefish population in the following steps. 

 First, the biomass of the spring- and summer- 

 spawned cohorts on the shelf was calculated. This 

 was done by determining the numbers in each spawn- 

 ing cohort (this was necessary because the VPA was 

 for all YOY combined) based on relative abundance 

 estimated from the NMFS autumn bottom trawl sur- 

 vey and then multiplying by the average individual 

 fish weight in each cohort. Secondly, this cohort bio- 

 mass was multiplied by the estimates of daily ration 

 (lowest and highest) to determine the daily biomass 

 of prey consumed by each cohort. To determine the 

 amount of bay anchovy consumed daily, each cohort's 

 biomass was multiplied by the mean proportion of 

 bay anchovy by weight in each cohort's diet. Finally, 

 a daily estimate of the numbers of bay anchovy con- 

 sumed by the YOY bluefish population was calcu- 

 lated by dividing the biomass of bay anchovy con- 

 sumed by the average bay anchovy weight ingested 

 by each cohort. Bay anchovy weight was calculated 

 from mean bay anchovy length ingested with regres- 

 sions from Hartman and Brandt ( 1995a >. 



where D = is bluefish daily ration; 



F, = the mean gut fullness over 24 hour; and 

 R^, = the exponential gastric evacuation rate 

 (see above). 



Mean gut fullness was calculated by taking the av- 

 erage of the individual time period (/) gut fullness 

 means from the feeding chronology analysis (see 

 above). The mean of the means was used to give each 

 time period equal weight even though sample sizes 

 for given time intervals varied over the diel cycle. 

 Daily ration was calculated for those geographical 

 regions and cohorts that had a sufficient diel record 

 (e.g. gut fullness estimates throughout the diel cycle). 

 The standard eiTor of the daily ration estimate was 

 approximated by using the delta method ( Seber, 1973 ). 

 Estimates of the biomass of prey consumed by the 

 YOY bluefish population during their southward 

 migration requires estimates of the numbers of YOY 

 bluefish in the population. The NEFSC, NMFS, 

 Woods Hole, MA, has used virtual population analy- 

 sis (VPA) to estimate the abundance of the east coast 

 bluefish population (NEFSC). These estimates of 



NEFSC. 1997. Report of the 23rd Northeast Regional Stock 

 Assessment Workshop (23rd SAW): Stock Assessment Review 

 Committee (SARC) consensus summary of assessments. North- 

 east Fisheries Sci Cent. Ref. Doc. 97-05, 191 p. I Available from 

 National Marine Fisheries Service, 166 Water Street, Woods 

 Hole, MA 02543-1026.1 



Results 



Dietary analyses 



The stomach contents of 989 young-of-the-year (626 

 spring- and 363 summer-spawned) and 275 adult 

 bluefish were examined. All YOY bluefish were cap- 

 tured between 12 and 29 September 1994 and 7 and 

 23 September 1995. Continental shelf bluefish diets 

 were dominated by teleost fish and squid prey (see 

 Table 1 for common and scientific names of bluefish 

 prey). 



In both 1994 and 1995, the dominant fish prey of 

 spring-spawned bluefish in all three geographical 

 regions was bay anchovy (Table 2). Other spring- 

 spawned bluefish prey included long-finned squid, 

 striped anchovy, butterfish, and round herring. But- 

 terfish were slightly more important in 1994 than in 

 1995. Also in 1994, amphipods made up a substan- 

 tial portion of spring-spawned bluefish diets in the 

 SNE region. In 1995, channeled whelk were a rela- 

 tively important prey of bluefish in the C-D region; 

 the foot and operculum were found in bluefish col- 

 lected over a large geographical area. 



The dominant fish prey of summer-spawned blue- 

 fish across all geographical regions was bay anchovy 

 in both 1994 and 1995 (Table 3). The incidence of 

 long-finned squid in summer-spawned bluefish di- 

 ets was low; however, other invertebrates such as 



