AbStraCt.-Along the U.S. east 

 coast, the bluefish Pomatomus 

 saltatrix spawns in offshore conti- 

 nental shelf waters during at least 

 two distinct periods: spring and 

 summer. Juveniles migrate to in- 

 shore nurseries where they complete 

 the first growing season. Previous 

 studies have shown that diet during 

 the oceanic larval stage consists of 

 copepods, while older juveniles cap- 

 tured inshore feed largely on teleost 

 prey. To determine timing of the on- 

 togenetic shift in diet to piscivory, 

 we examined the feeding habits of 

 189 early-juvenile bluefish (18- 

 74mmTL). Samples were collected 

 from continental shelf waters of the 

 Middle Atlantic Bight (MAB) during 

 spring and summer of 1988 and 

 1989. Spring- and summer-spawned 

 P. saltatrix differed in body size, prey 

 size, and in the proportions of prey 

 types consumed. Copepods were the 

 most common prey type in fish 

 <60mm. Teleost prey appeared ini- 

 tially in the diet of 30 mm in- 

 dividuals and became the major di- 

 etary item in spring-spawned fish 

 >40mmTL. Gut fullness and inci- 

 dence of piscivory peaked in late af- 

 ternoon and were positively corre- 

 lated with daylight hours. There was 

 no evidence of an abrupt increase in 

 mouth width associated with this on- 

 togenetic shift in diet. Because juve- 

 nile bluefish migrate inshore soon 

 after becoming piscivores, their im- 

 pact as predators on the abundance 

 of other young fishes is probably fo- 

 cused on inshore/estuarine, rather 

 than offshore species. 



Ontogenetic shift in the diet of 

 young-of-year bluefish Pomatomus 

 saltatrix during the oceanic phase 

 of the early life history* 



Rick E. Marks 



Marine Sciences Research Center, State University of New York 



Stony Brook. New York 1 1 794-5000 



Present address: National Fisheries Institute, Inc , 



1 525 Wilson Boulevard. Suite 500, Arlington. Virginia 22209 



David O. Conover 



Marine Sciences Research Center. State University of New York 

 Stony Brook, New York 1 I 794-5000 



The bluefish, Pomatomus saltatrix 

 (Linnaeus), occurs along the Atlantic 

 coast of North America from Florida 

 to the Gulf of Maine. Throughout its 

 range the species is found from shal- 

 low coastal waters to the outer, con- 

 tinental shelf at various times of the 

 year (Bigelow & Schroeder 1953, 

 Lund & Maltezos 1970, Kendall & 

 Walford 1979, Nyman & Conover 

 1988). Pomatomus saltatrix is an im- 

 portant component of the recreational 

 fishery along the east coast of North 

 America (NMFS 1991). 



Two major spawning concentra- 

 tions exist in the western Atlantic. 

 Spring spawning occurs in the South 

 Atlantic Bight (SAB) during March- 

 May, with a peak in April. Summer 

 spawning occurs in the Middle 

 Atlantic Bight (MAB) during the 

 months of June-September, with a 

 peak in July (Kendall & Walford 

 1979, Nyman & Conover 1988, 

 McBride & Conover 1991). 



The spring-spawned larvae move 

 northeastward in waters associated 

 with the Gulf Stream. Juveniles cross 

 shelf waters at an age of 40-70 d, 

 and enter bays and estuaries of the 



Manuscript accepted 4 November 1992. 

 Fishery Bulletin, U.S. 91:97-106 ( 1993). 



■"Contribution 866 of the Marine Sciences Re- 

 search Center, State University of New York 

 at Stony Brook. 



mid-Atlantic coast in late spring 

 (Kendall & Walford 1979, Nyman & 

 Conover 1988). Summer-spawned lar- 

 vae may either spend most of the 

 summer at sea or inhabit the inshore 

 nursery areas of the MAB for a brief 

 period before the onset of autumn, 

 when both cohorts move southward 

 to wintering grounds in the SAB 

 (Kendall & Walford 1979, Nyman & 

 Conover 1988). 



Once inshore, P. saltatrix feed al- 

 most exclusively on piscine prey 

 (Lassiter 1962, Richards 1976, 

 Naughton & Saloman 1978, McDer- 

 mott 1983, Smale & Kok 1983, Smale 

 1984, Olla et al. 1985, Friedland et 

 al. 1988). Very little is known, how- 

 ever, about the diet of young P. 

 saltatrix at sea. In the only published 

 account, the diet of newly-hatched 

 P. saltatrix was found to consist 

 mainly of copepods (Kendall & Nalpin 

 1981). Hence, at some point during 

 ontogeny there must be a shift in diet 

 from zooplankton to fish. If the diet 

 shift occurs early in development, 

 then juvenile P. saltatrix may be im- 

 portant predators of larval fishes on 

 the shelf. On the other hand, the 

 shift from a zooplankton to a fish- 

 dominated diet may occur coinciden- 

 tally with the habitat shift to estua- 

 rine waters. 



97 



