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Fishery Bulletin 91(1). 1993 



Prey dry weights were obtained either from the lit- 

 erature or measured directly after oven-drying for 24 h 

 at 60° C. Where individual prey weights were very low 

 or existed as exoskeleton material, dry weights were 

 obtained by drying a known number of fresh prey items 

 to obtain an average weight per prey item (see 

 Grossman 1980, Ryer & Orth 1987). 



Dry weights of 49 bluefish (17-74 mm) with empty 

 guts were determined by oven-drying at 60° C until 

 recording constant weight. The time required for a 

 constant measure was 24-48 h, depending on individual 

 fish size. The regression equation (Log Y=3.128 x Log 

 X-6.059) was used to predict the dry weights of all 

 bluefish in this study (Marks 1991). Gut fullness was 

 measured using a ratio of prey dry weight to indi- 

 vidual P. saltatrix dry weight. 



Mouth width measurements were taken to detect mor- 

 phological changes during ontogeny. Mouth width was 

 measured as the width at the posterior tip of the 

 maxillaries using digital calipers (±0.01 mm) (see 

 Hartman 1958, Ross 1978, Hunter 1980, Hunter & 

 Kimbrell 1980). Additionally, body depth of prey fish 

 was measured at the widest location. 



Diet analysis 



Diet was analyzed using the methods 

 outlined by Hyslop (1980) (see also 

 Lassiter 1962, Naughton & Saloman 



1978, Friedland et al. 1988): Total num- 

 ber of stomachs in which a food item 

 occurred divided by the total number of 

 stomachs (%F); total number of individu- 

 als of a taxon divided by the total num- 

 ber of food items (%N); and total dry 

 weight of a taxon divided by the total 

 dry weight of all food items (%W). 



Bluefish were grouped by spring- and 

 summer-spawned cohorts based on size 

 and date of capture (Kendall & Walford 



1979, Nyman & Conover 1988, McBride 

 & Conover 1991). Trophic ontogeny was 

 examined by arbitrarily splitting blue- 

 fish into size groupings and using per- 

 cent dry weight of prey categories to as- 

 sess dietary importance. 



Results 



General diet description 



A total of 189 P. saltatrix were exam- 

 ined for gut content (Table 1). Approxi- 

 mately 84% had food present in the stom- 

 achs. Spring-spawned bluefish consumed 



an average of 31 prey items per individual compared 

 with 85 prey items for summer-spawned fish. 



Spring-spawned fish were found to be significantly 

 larger (Table 1; ^-test for means with unequal vari- 

 ances, ^=5.24, df=156, P<0.001,) and had a greater 

 mouth width (0.01<P<0.05, r,=2.39) than did summer- 

 spawned fish. 



Overall, copepods were the dominant prey, account- 

 ing for 50% and 94% of the diet by weight (Table 2) of 



