104 



Fishery Bulletin 91(1). 1993 



There was a marked increase in the maximum size 

 of prey consumed as spring-spawned bluefish size in- 

 creased beyond 30mm (Fig. 3). This is in agreement 

 with the generalization that prey size increases with 

 increasing predator size in fishes (Brooks & Dodson 

 1965, Tyler 1972, Ross 1977 and 1978, Hunter & Kim- 

 brell 1980, Roberts et al. 1981, Smale 1984, Ryer & 

 Orth 1987, Wetterer 1989, Persson 1990). There was, 

 however, only a moderate increase in the maximum 

 prey sizes consumed by summer-spawned bluefish. The 

 reasons for this discrepancy are not clear, but may be 

 a function of prey availability and differences in size 

 at inshore migration. Summer-spawned bluefish ap- 

 pear inshore at smaller sizes than do spring-spawned 

 bluefish (McBride & Conover 1991). The shift to larger 

 prey items in summer-spawned fish may occur after 

 inshore migration. 



The onset of piscivory in the 30-70 mm size-range is 

 similar to that reported for other teleosts: 40-60 mm 

 for characids and 60-80 mm for pimelodids (Winemiller 

 1989), 23mm for Micropterus salmoides (Keast 1985), 

 and 60-100 mm for Gaclus morhua (Bowman & 

 Michaels 1984). 



Piscivory in larval stages has been reported in some 

 fishes, but apparently does not occur in bluefish. Hunter 

 & Kimbrell (1980) found Scomber japonicus to be pis- 

 civorous at 8 mmSL. This could perhaps be attributed 

 to the large mouth and strong, well-developed jaws 

 that are characteristic of larval scombrids (Fahay 1983). 

 Hunter (1980) also reported Sphyraena argentea to be 

 piscivorous at 4.4mm. Houde (1972) found evidence 

 for piscivory in S. borealis at 9 mm. Despite having 

 well-developed dentition at 4.3mm (Fahay 1983), lar- 

 val and postlarval P. saltatrix may lack either the jaw 

 structure, visual acuity, or swimming speed necessary 

 to feed on other fishes. 



Pomatomus saltatrix are known to be visual preda- 

 tors (Olla & Marchioni 1968, Olla et al. 1970, Van der 

 Elst 1976). It follows that reduced light levels after 

 the evening crepuscular period should reduce feeding 

 efficiency. Olla & Marchioni (1968) documented that 

 P. saltatrix detect and attack prey visually, so it is not 

 surprising that feeding appears to be correlated with 

 daylight periods. Kjelson et al. (1975) also reported 

 finding the lowest proportion of food in the gut of post- 

 larval fishes during the evening hours. 



Fish are considered "gape-limited" predators and are 

 ultimately restricted by mouth size (Hartman 1958, 

 Ross 1978, Hunter 1980, Roberts et al. 1981). Ontoge- 

 netic shifts in diet may be related to morphological 

 changes in mouth size during development that allow 

 for consumption of larger prey (Ross 1978, Grossman 

 1980, Roberts et al. 1981). Mouth width in P. saltatrix, 

 however, appears to increase isometrically with body 

 size. The inclusion offish in the diet at a size of 30 mm 



may be attributed to changes in feeding behavior with 

 growth, or simply a result of the mouth reaching a 

 size that permits fish ingestion. 



The size at which teleost prey constitute a substan- 

 tial portion of the diet is about 40-70 mm. This is also 

 the size-range in which P. saltatrix juveniles recruit to 

 the inshore waters of the MAB (Nyman & Conover 

 1988, McBride & Conover 1991). Hence, the dietary 

 shift is largely coincident with a habitat shift. This is 

 further supported by the observation that virtually all 

 piscivorous spring-spawned P. saltatrix were captured 

 close to shore. The limited occurrences of piscivory in 

 the summer-spawned cohort were more evenly dis- 

 persed across the shelf. However, summer-spawned 

 bluefish migrate inshore at a smaller size than do 

 spring-spawned fish (McBride & Conover 1991) and 

 may do so largely before the onset of piscivory. Our 

 results suggest that the overall impact of predation by 

 young bluefish on the abundance of other fishes is prob- 

 ably focused more on inshore rather than offshore 

 species. 



Acknowledgments 



We thank Robert Cowen, Jonathan Hare, and numer- 

 ous members of their laboratory for sharing samples, 

 ship time, and information. Special thanks to Francis 

 Juanes for his insight. We also acknowledge the help- 

 ful comments offered by Linda Jones, Ronald Hardy, 

 and two anonymous reviewers. An earlier version of 

 this manuscript was submitted by R.E. Marks to the 

 Graduate School of the State University of New York 

 at Stony Brook in partial fulfillment of the require- 

 ments for a Master of Science degree in Marine Envi- 

 ronmental Sciences. This work was supported under 

 grant NA86AA-D-SG045 to the New York Sea Grant 

 Institute. 



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