Marks and Conover. Ontogenetic shift in diet ofyoung-of-year Pomatomus saltatrix 



99 



Laboratory analysis 



Preserved P. saltatrix were wet weighed (±0.001 g) and 

 measured (total length, TL, to ±0.1 mm). Fish in the 

 18-74 mm size-range were examined for diet compari- 

 sons. The minimum size represents the stage at which 

 metamorphosis into the juvenile, phase is completed 

 (Norcross 1974). The maximum fish size represents 

 the largest specimen collected. 



Stomachs were removed from the pharynx and an- 

 terior to the intestine, cut longitudinally, and the con- 

 tents transferred to a petri dish. The gut cavity was 

 then washed to remove any adhering particles. Prey 

 items were identified to species or lowest taxon pos- 

 sible, enumerated, and measured for total length 

 using a dissecting scope equipped with an ocular 

 micrometer. 



Weighted mean prey length was calculated for 

 each stomach to provide an accurate representation of 

 prey length consumed. Individual bluefish often con- 

 sumed several small prey (e.g., copepod) and one 



large prey (e.g., teleost). Computation of the weighted 

 mean is equivalent to adding up all the original mea- 

 surements and dividing the sum by the total number 

 of measurements. Hence, the most common prey 

 type will influence the weighted mean length in pro- 

 portion to its numerical occurrence (see Sokal & Rohlf 

 1981). 



Protocol for determining various prey length was 

 defined as follows: (1) largest diameter for hydrated 

 oocytes, (2) metasome + cephalosome for copepods, 

 (3) anterior edge of head to anterior edge of caudal 

 rami for fish lice {Caligus spp.), (4) anterior edge of 

 eye to anterior edge of uropod for amphipods, (5) base 

 of rostrum to anterior edge of telson for other Crusta- 

 cea, and (6) total length for polychaetes, ostracods, 

 pteropods, squid (Loligo spp. ), and teleost prey. 



Fish scales were present in <5% of the guts and 

 were not regarded as a prey type, but were used to 

 indicate the occurrence of piscine prey. The presence 

 of any teleost part (e.g., otolith, spine, fin ray) was 

 recorded as piscine prey. 



