FISHERY BULLETIN VOL 77. NO 1 



1972; Chittenden 1976; Sholar'^). Within this 

 temperature regime, southern populations begin 

 reaching home estuaries at the higher tempera- 

 tures, while northern populations do so at the 

 lower temperatures. Peak numbers of shad enter 

 the St. Johns River, Fla., in mid-January when 

 water temperatures are at an annual low of 15°C; 

 the peak in juvenile emigration occurs simultane- 

 ously (Leggett and Whitney 1972; Williams and 

 Bruger 1972). Shad first enter the Connecticut 

 River in late March-early April when water tem- 

 peratures are approximately 4°C and peak in 

 abundance at 13°C (Leggett and Whitney 1972). In 

 general, most shad populations north ofCapeHat- 

 teras begin entering rivers at approximately 4°C, 

 and the peak in upstream migration occurs at 

 temperatures between 10° and 15°C (Leggett and 

 Whitney 1972). 



The lower thermal tolerance of juvenile shad in 

 freshwater was near 2.2°C in a short-term 

 laboratory .*udy (Chittenden 1972) and roughly 

 3°-4°C in small ^jutdoor ponds (Blair'^). This lower 

 thermal limit agrees closely with the lowest tem- 

 perature at which subadult shad were collected 

 during NMFS offshore surveys (3°C). Chittenden 

 (1972) also reported that juveniles ceased feeding 

 when water temperatures dropped below 4.4°C. 

 However, we collected 17 juvenile and subadult 

 shad (9-32 cm FL) during a NMFS coastal survey 

 in January 1978, at stations with bottom tempera- 

 tures between 2.8° and 4.3°C. All but one stomach 

 were filled with mysids and copepods, indicating 

 active feeding at these temperatures in saltwater. 



Further evidence to support our bottom temper- 

 ature regime for predicting the coastal movements 

 of shad is provided by North Carolina's anadro- 

 mous fishery research program. Their annual sur- 

 veys on river herring since 1971 show that shad 

 occur off the North Carolina coast from January to 

 April, at bottom temperatures between 6° and 

 12°C and at depths <26 m (Johnson et al."*). Shad 

 catches decline substantially when water temper- 

 atures exceed 12°C, coinciding with entry into es- 

 tuaries or possibly, northward migration. This 



'^Sholar, T. M. 1977. Anadromous fisheries research pro- 

 gram. Cape Fear River System, phase 1. N.C. Proj. AFCS 12.6.3 

 p. 



'^Blair, A. B. 1977. American shad culture and distribu- 

 tion studies at Harrison Lake National Fish Hatchery. Proc. 

 Workshop American Shad, Amherst, Mass., Dec. 1976, 10 p. 



">Johnson. H. B.. B. F. Holland, Jr., and S G 

 Keefe. 1977. Anadromous fisheries research program, north- 

 em coastal area. Section II. N.C. Proj. AFCS 11-2, 41 p. 



temperature range concurs with offshore bottom 

 temperatures having the most frequent shad 

 catches during NMFS bottom trawl surveys (7°- 

 13°C). The shallow depths traveled by coastal 

 migrants during the winter and spring would ac- 

 count for their unavailability to offshore sam- 

 pling. 



Critical data on the oceanic phase of most anad- 

 romous fishes are lacking (Harden-Jones 1968), 

 and our general description of shad movements 

 must await additional research at sea to corrobo- 

 rate or correct the proposed migratory cycle. It 

 would seem energetically wasteful for North At- 

 lantic populations to follow the same shoreward 

 route as do Middle and South Atlantic shad. The 

 return of all populations to this region may have 

 historical significance, since shad are believed to 

 have been most abundant in the mid-Atlantic por- 

 tion of their coastal range (Leim 1924). Variations 

 in life history patterns among populations are 

 generally considered to be adaptive responses 

 (Cole 1954; Murphy 1968; Gadgil and Bossert 

 1970), and differences in life history characteris- 

 tics among shad populations in rivers ( Carscadden 

 and Leggett 1975b) may also exist at sea. 

 Endocrine-induced differences in the timing of 

 migratory behavior and gonadal maturation may 

 be life history strategies of adaptive significance, 

 considering the species' wide geographical range 

 (21° of latitude). The lengthy period of migration 

 toward the mid-Atlantic coast from offshore by 

 prespawning adults may stem from population- 

 specific responses to photoperiod or temperature 

 cues. Further study on the sensory systems and 

 environmental cues involved in migration is re- 

 quired before a more comprehensive explanation 

 for the migratory cycle of shad is available. 



ACKNOWLEDGMENTS 



We are indebted to the Resource Surveys Inves- 

 tigation section and other staff members at 

 NMFS, Woods Hole, for their cooperation in this 

 endeavor. Special thanks go to Ralph Mayo for 

 sharing his computer expertise and to Bill 

 Leggett, Emory Anderson, Jon Gibson, Brad 

 Brown, and an anonymous reviewer for reviewing 

 the manuscript. The Massachusetts and Virginia 

 Cooperative Fishery Research Units provided 

 financial support. We dedicate this paper to R. J. 

 Reed for his contributions to the study of Ameri- 

 can shad biology. 



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