760 



Fishery Bulletin 97(4), 1999 



Prey-type selectivity 



The feeding selectivity of spring-spawned bluefish 

 on the continental shelf was determined from the 

 relative abundance of prey in individual bluefish guts 

 and the relative abundance of prey in trawl catches. 

 Four prey categories were examined in 1994 (bay 

 anchovy, butterfish, squid, and "other") and three 

 prey categories in 1995 (bay anchovy, squid, and 

 "other"). The "other" category included any teleost 

 fish that did not fall into the previously mentioned 

 groups. The relative abundance of these prey in the 

 field was calculated by including only those prey that 

 were of a size that bluefish could theoretically con- 

 sume (prey FL<80'^ of bluefish PL; determined from 

 an independent study, Scharf et al., 1997). Addition- 

 ally, the index was calculated for stations where at 

 least ten spring-spawned bluefish were captured. 

 Chesson's (1978) index was used to determine blue- 

 fish prey preference as 



a. 



r, /P, 





i=L 



,m, 



where a, 



P, = 



m 



the selectivity for prey type / for an in- 

 dividual bluefish; 



the relative abundance of prey type / in 

 an individual bluefish stomach; 

 the relative abundance of prey type / in 

 the environment; and 

 the number of prey types available. 



Values of a, were averaged for each year. Random 

 feeding occurs when mean a= llm (1994=0.25 and 

 1995=0.33); values of a^ > llm or a^ < llm represent 

 "selection" and "avoidance" of prey, respectively. Ran- 

 dom feeding was tested by using a ^-test to compare 

 mean a, with 1//?; for each prey type within each year 

 (Chesson, 1983). 



Prey-size relationships and size selectivities 



Prey sizes of all bluefish prey wei'e measured directly 

 or determined indirectly from regression equations. 

 The relationship between ingested prey size and blue- 

 fish length was determined for YOY and adult blue- 

 fish in both 1994 and 1995. In order to compare the 

 relative prey size ingested by spring- and summer- 

 spawned bluefish cohorts in 1994 and 1995, the fre- 

 quency distributions of bay arichovy FL to bluefish 

 FL ratios were examined. Ratios were calculated for 

 all bay anchovy prey that were measured ( 1994, 

 n=388; 1995, ;i=202). 



Size-selective feeding on bay anchovy prey was 

 examined by comparing the sizes of bay anchovy in- 

 gested by bluefish at a specific station with sizes of 

 bay anchovy captured in the trawl at that station. 

 Stations from 1994 and 1995 that had «>15 bay an- 

 chovy length measurements for each bluefish cohort 

 were used in the analysis. Bay anchovy lengths at 

 each station were obtained from archived data at 

 NEFSC-NMFS, Woods Hole, MA. 



Selectivity for bay anchovy prey size categories was 

 measured by using Chesson's ( 1978 ) index ( described 

 above). Bay anchovy lengths ingested by bluefish and 

 collected at each station were partitioned into four 

 bay anchovy FL categories: 25-34, 35-44, 45-54, and 

 55-64 mm. Values of a, were calculated from indi- 

 vidual bluefish for each size bin at each station sepa- 

 rately; these were then averaged across stations. As 

 above, values of a, > 1/m or a^K llm represent "selec- 

 tion" and "avoidance" of size categories, respectively. 

 Estimated values of mean a, were compared with 

 llm by using a Mest ( 1994 spring-spawned, lA?i=0.25, 

 and summer-spawned, l/«z=0.33; 1995 spring- 

 spawned, l/m=0.33). 



Feeding chronology, daily ration estimates, and 

 impacts on bay anchovy 



Values of gut fullness were used to examine feeding 

 chronology of YOY bluefish. Gut fullness values (F) 

 were calculated as 



F=GIW, 



where G = prey wet weight; and 



W = bluefish wet weight (total weight minus 

 prey wet weight). 



Gut fullness values for individual bluefish were 

 pooled over eight 3-h time periods and averaged. This 

 analysis was performed for both spring- and sum- 

 mer-spawned bluefish by geographic region. 



In order to estimate feeding rates of bluefish on 

 the shelf, bluefish gastric evacuation rate (GER) es- 

 timates were needed. Previous work on YOY blue- 

 fish GER showed that prey type and bluefish body 

 size did not have a significant effect on bluefish GER; 

 of the factors examined, temperature had the only sig- 

 nificant effect on bluefish GER (Buckel and Conover, 

 1996). These laboratory experiments described blue- 

 fi.sh GER from 21 to 30C. In our continental shelf 

 collections, YOY bluefish were found in water tempera- 

 tures as low as 15^C (mean of surface and bottom tem- 

 perature). Therefore, a laboratory experiment to mea- 

 sure YOY bluefish GER at 15''C was performed. 



The experiment was conducted in an identical 

 manner to previous GER experiments on YOY blue- 



