Marks and Conover: Ontogenetic shift in diet of young-of-year Pomatomus saltatnx 



spring- and summer-spawned fish, respectively. Teleosts 

 were the next most-abundant prey type. Fish prey con- 

 stituted 37% by weight of the diet of spring-spawned 

 bluefish. Summer-spawned bluefish, by comparison, 

 consumed a much smaller amount (1.8%). The major- 

 ity of teleosts consumed were hakes ( Urophycis spp. ) 

 or other unidentifiable gadids. There were also limited 

 occurrences of engraulids and triglids iPrionotus spp.). 

 Based on number and weight, all other prey types 

 were relatively unimportant except crab larvae (zoea 

 and megalopae). A low proportion by weight was com- 

 prised of hydrated oocytes which were frequently 



present in stomachs. Rare prey included Caligus spp., 

 Loligo spp., a maldanid polychaete and a pteropod. 



Onset of piscivory 



We pooled data by size-class within cohorts to provide 

 an overall view of diet changes with body size. The 

 smallest size-class of summer-spawned P. saltatrix 

 (17-29.9 mm) fed exclusively on copepods and fish eggs 

 (Fig. 3). The 30.0-39.9 mm size-class expanded their 

 diet to include teleosts, amphipods, and miscellaneous 

 crustaceans. Bluefish in the 40.0-49.9 mm class also 



100 



Dry Weight of Prey 



80 - 



60 



40 



20 



100 



80 



60 



40 



20 



19-29.9 30-39.9 40-49.9 50-74 



Spring-spawned Bluefish Size (mm) 



17-29.9 30-39.9 40-49.9 50-64 



Summer-spawned Bluefish Size (mm) 



l- :: "l Copepod9 £$$! Fish Eggs I I Ostraeods 



E^Sy Fish Eggs 

 I I Misc. 



Amphipods 



Crab Larvae 



Figure 3 



Trophic ontogeny of four bluefish Pomatomus saltatnx size-classes for spring- and summer- 

 spawned cohorts expressed as the 9i dry weight of prey. (Misc. category includes ostraeods, 

 fish eggs, misc. crustaceans, and amphipods). Sample sizes for each size-class (smallest to 

 largest) were: summer-spawned fish, n - 25, 47, 25, 3; spring-spawned fish, 72 = 10, 19, 12, 18. 



