Szedlmayer et al Growth of age-0 Cynosaon regalis in Chesapeake Bay 



751 



studies of American shad Alosa sapidissima suggested 

 that year-class strength is estabHshed before the juve- 

 nile stage (Crecco et al. 1983, Crecco and Savoy 1984, 

 Crecco and Savoy 1985). Consequently, the importance 

 of critical periods during the juvenile stage may be 

 species-specific. Because several age-0 cohorts of 

 weakfish showed variable growth rates and distribu- 

 tion, survival of the juvenile stage of this species should 

 not be assumed to have a constant rate. 



Different population parameters among cohorts are 

 difficult to relate to salinity and/or temperature dif- 

 ferences observed among stations. First, juvenile weak- 

 fish are transient, as observed over the present study 

 area and in earlier studies (Harmic 1958, Massman 

 1963, Chao and Musick 1977, Shepherd and Grimes 

 1983), and until accurate residency times can be esti- 

 mated it may be ineffective to ascribe cohort differ- 

 ences to particular habitat parameters. Second, other 

 factors not measured in the present study, e.g., prey 

 abundance, turbidity, currents, and predation may also 

 be linked to cohort differences. 



In comparison with other juvenile fish, age-0 weak- 

 fish appear to grow at an average rate. Juvenile growth 

 rates derived from length frequencies for other sciaenid 

 fishes were similar to our estimates for weakfish: C. 

 arenarius (~1 mm/day, Shlossman and Chittenden 

 1981), C. nothus (0.8-1.3 mm/day, DeVries and Chit- 

 tenden 1982). Shenker and 011a (1986) provide esti- 

 mates of juvenile fish growth rates ranging from a low 

 of 0.26 mm/day {Sehastes melanops) to a high of 4.7 

 mm/day (Coryphaena hipporus). Other growth rate 

 estimates of juvenile fishes include: 1.5 mm/day for 

 Anoplopoma fimbria (Boehlert and Yoklavich 1985), 

 1.0-1.3 mm/day for Chanos chanos (Kumagai et al. 

 1985), and 1.1 mm/day for Alosa sapidissima (Crecco 

 and Savoy 1985). 



The ecological advantage of extended spawnings that 

 result in multiple cohorts within a single age-0 year- 

 class can be thought of as a "hedged bet" strategy that 

 spreads age-0 production over time to take advantage 

 of a variable environment (Lambert and Ware 1984). 

 However, distinct cohorts within age-0 fish can also 

 result from environmental factors acting on a single 

 spawning effort; for example, through variation in prey 

 availability (Timmons et al. 1980, Keast and Eadie 

 1985, Wicker and Johnson 1987), or a combination of 

 biotic and abiotic factors (Lambert 1984, Crecco and 

 Savoy 1985). However, previously published informa- 

 tion indicates that the multiple cohorts observed in the 

 present study probably resulted from multiple spawn- 

 ings. Shepherd and Grimes (1984) showed that large 

 weakfish "tiderunners" 55-80 cm enter the Delaware 

 Bay estuary in the spring and spawn. In the summer 

 these were replaced by 25-35 cm gravid weakfish. Har- 

 mic (1958) showed a repeating pattern of multiple 



spavsming over 3 years, where a peak of egg abundance 

 occurred in mid-June, followed by a conspicuous gap, 

 and another peak in mid-July. 



In summary, the present study showed that multi- 

 ple cohorts exist within the age-0 year-class of Chesa- 

 peake Bay- York River weakfish. These cohorts showed 

 significant differences in growth rates and appeared 

 to partition habitats. Consequently, population studies 

 directed at predicting year-class strength from juvenile 

 surveys need to consider the potential for age-0 cohort 

 variability. 



Acknowledgments 



We thank Steven Weiss and Susan Engels for their 

 help in mounting and reading scales, and Paul Gerdes, 

 Stephen O'Neil, and Robert Siegfried for their help in 

 field collections. Work was completed as part of EPA 

 grant number R810334, to Michael P. Weinstein at the 

 Virginia Institute of Marine Science, College of William 

 and Mary. 



Citations 



Boehlert. G.W., and M.M. Yoklavich 



1985 Larval and juvenile gi-owth of sablefish, .4 noplopoma fim- 

 bria, as determined from otolith increments. Fish. Bull.. U.S. 

 83:47.5-481. 

 Buchanan-WoUaston, H.J.. and W.C. Hodgson 



1929 A new method of treating frequency curves in fishery 

 statistics, with some results. J. Cons. Int. Explor. Mer 4: 

 207-22.S. 

 Campana, S.E.. and J.D. Neilson 



1985 Microstructure of fish otoliths. Can. J. Fish. Aquat. Sci. 

 42:1014-1032. 

 Chao, L.N., and J. A. Musick 



1977 Life history, feeding habits, and functional morphology 

 of juvenile sciaenid fishes in the York River Estuary, Virginia. 

 Fish. Bull., U.S. 7.5:657-702. 

 Conover, D.D., and M.R. Ross 



1982 Patterns in seasonal abundance, growth, and biomass of 

 the Atlantic silverside, Menidia menidia, in a New England 

 estuary. Estuaries 5:275-286. 



Cooper, G.P. 



1937 Food habits, rate of growth, and cannibalism of young 

 largemouth bass in stateoperated rearing ponds in Michigan 

 during 1935. Trans. Am. Fish. Soc. 66:242-266. 



Crecco, V.A., and T.F. Savoy 



1984 Effects of fluctuations in hydrographic conditions on year- 

 class strength of American shad (,4/<ist7 sapidiss^inw) in the Con- 

 necticut River. Can. J. Fish. Aquat. Sci. 41:1216-1223. 



1985 Effects of biotic and abiotic factors on growth and relative 

 survival of young American shad, Alosa sapidissima, in the 

 Connecticut River. Can. J. Fish. Aquat. Sci. 42:1640-1648. 



Crecco. V.A., T.F. Savoy, and L. Gunn 



1983 Daily mortality rates of larval and juvenile American shad 

 (Alosa sapidissima) in the Connecticut River with changes in 

 year-class strength. Can. J. Fish. Aquat. Sci. 40:1719-1728. 



