SHEPHERD AND GRIMES: GROWTH VARIATIONS IN WEAKFISH 



migration. Presumably, seasonal migrations to take 

 advantage of ephemeral resources (Morse 1980) 

 must be completed in a limited time, and longer 

 migrations would require greater energy reserves. 

 Because swimming speed is a function of body size 

 (Marshall 1966), larger (faster growing) fish would be 

 better able to bear the high energy cost and complete 

 the migration, and would thus be favored by selec- 

 tion. However, the penalty imposed by increased 

 somatic growth to cope with migration would be de- 

 creased annual gonad growth and fewer gametes pro- 

 duced annually. Therefore, the life history strategy of 

 migratory fish might be to assure survival by varying 

 the energetic commitment between somatic and 

 gonad growth according to migration cost. 



These factors may explain much of the geographic 

 variation in weakfish growth. The seasonal move- 

 ments of weakfish follow a northward and inshore 

 route in spring to spawning grounds, and feeding con- 

 tinues while inshore during summer and early fall 

 before migrating offshore in mid to late fall to 

 southern overwintering areas. The energy required 

 for migration of northern fish to southern waters 

 would be greatest due to the distance traveled 

 Therefore, northern fish emphasize somatic growth 

 and longevity to maximize chances of surviving mi- 

 gration and producing gametes. Increased chances of 

 survival for larger individuals and greater longevity 

 result in an increased number of spawning oppor- 

 tunities, to offset the losses in annual gonad produc- 

 tion. By increasing lifetime spawning frequency, this 

 strategy has the added benefit of increasing chances 

 for survival of gametes introduced into less environ- 

 mentally predictable northern estuaries. In contrast, 

 southern fish have little distance to migrate, so the 

 energetic requirements for the journey are propor- 

 tionately less. Because a smaller size is less of a 

 handicap in migration, the growth strategy is shifted 

 to increased gonad growth. This is indicated in the 

 greater fecundities at length for southern fish 

 (Shepherd and Grimes 5 ). Greater emphasis on re- 

 production may increase adult mortality (Gerking 

 1959), causing the decreased longevity we observed 

 in southern weakfish This tradeoff between gonadal 

 and somatic growth should not become effective until 

 the onset of maturity. This is, in fact, the case in weak- 

 fish, as growth differences between regions become 

 evident only after age 1 , which is the approximate age 

 at maturity (Merriner 1973). It should be noted that 



5 Shepherd, G., and C. B. Grimes. In prep. Reproduction of weak- 

 fish, Cynoscion regalis, in the New York Bight and evidence for 

 geographically specific life history characteristics. Unpubl. 

 manuscr. Rutgers University. New Brunswick, NJ 08903. 



the cumulative gamete production for these two life 

 history strategies is approximately the same 

 (Shepherd and Grimes footnote 5), thus the size dif- 

 ferences between northern and southern weakfish do 

 not appreciably alter the reproductive potential of 

 the species. 



Growth of southern- origin weakfish may also be 

 limited by the availability of certain prey items. Jones 

 and Johnston (1977) wrote that fishes pass through a 

 series of food niches during a lifetime, and the upper 

 limits of growth are determined in part by the optimal 

 size for exploiting the final food niche available in a 

 given environment. Prey availability may limit max- 

 imum size of most southern weakfish to 40-50 cm. 

 Food habit studies by Welsh and Breder (1923) and 

 Merriner (1975) have shown that weakfish shift prey 

 preference to menhaden at about 35-40 cm. This size 

 range approaches the maximum sizes of weakfish for 

 southern waters {L x = 40 cm). Stratification of 

 menhaden by age-size with latitude has been 

 documented along the east coast, with 1-yr-olds pre- 

 dominant in the south and older, larger fish further 

 north (June and Reintjes 1959). The implication is 

 that net energy for southern weakfish feeding on 

 small menhaden is insufficient for growth beyond 

 —40 cm. Thus maximum size limitation of weakfish is 

 imposed by the energy available in the final food 

 niche. The energy saved from short migrations may 

 be utilized to maximize reproduction before reaching 

 the size limitations imposed by feeding. Northern 

 fish, on the other hand, may migrate north and take 

 advantage of a final food niche that allows superior 

 growth. 



The variations in growth may result from differ- 

 ences between genetically distinct groups. Our find- 

 ings of three or more or less distinct body- scale rela- 

 tionships in Regions I, II, and III may indicate dif- 

 ferent stocks (Rojo 1977). Similar stock separations 

 have been suggested by Perlmuter et al. (1956) and 

 Seguin (1960). However, the body-scale relations in 

 our study varied clinally, and such morphological 

 characters have been shown to display clinal varia- 

 tion with no apparent genetic discontinuity (Katz et 

 al. 1983). The genetic basis of these growth differ- 

 ences remains a topic for future research. 



Historic Variations in Growth 



Weakfish populations have fluctuated widely over 

 the last several decades, and growth rates have 

 varied similarly, but most noticeably since the pop- 

 ulation decline of the 1960's. For example, in weak- 

 fish from the New York Bight, age 4 females in 1929 

 were 34 cm compared with 48 cm in 1952 and 58 cm 



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