Abstract.- Age-0 weakfish Cifno- 

 scion 7-egalis, were sampled from the 

 Chesapeake Bay- York River estuary 

 in 1983 and 1984 to test for the ex- 

 istence of multiple cohorts and com- 

 pare growth rates. In both years 

 juveniles were first collected in the 

 estuary in late July, were significant- 

 ly more abundant from August to 

 mid-October, but were uncommon 

 by the end of October. Significantly 

 more weakfish were collected in 

 1983 than in 1984. Daily ages were 

 determined based on scale circuli. 

 Cohorts were separated by local min- 

 ima in birthdate distributions: cohort 



1 hatched before 16 July 1983, cohort 



2 (16 July- 15 August 1983), cohort 



3 (15 August-9 September 1983), 

 cohort 4 (5 May-20 June 1984), and 

 cohort 5 (5 July-1 September 1984). 

 Compared with other cohorts, groups 

 1 and 4 had significantly slower 

 gi-owth rates (0.83 and 0.84 mm/day, 

 respectively), while cohort 5 had a 

 significantly faster growth rate (1.09 

 mm/day), and cohorts 2 and 3 were 

 intermediate (both at 0.96 mm/day). 

 Weakfish juveniles were transient 

 and migrated up the estuary as they 

 grew, as indicated by cohort and age 

 distributions over stations and dates. 

 Also, cohorts appeared to partition 

 the estuary; for example, cohort 1 

 was more common up the estuary 

 compared with other cohorts. The 

 existence of multiple cohorts with 

 significantly different growth rates 

 and apparent differential habitat use 

 suggests that the juvenile stage of 

 weakfish may show high variability 

 in survival, and the presence of mul- 

 tiple cohorts within a single year- 

 class must be considered in the esti- 

 mation of juvenile recraitment to the 

 adult population. 



Differential Growth 

 Among Cohorts of Age-0 

 Weal<fish Cynoscion regalis 

 in Chesapeal<e Bay* 



Stephen T. Szedlmayer 



Marine Field Station. Rutgers University. Tuckerton, New Jersey 08087 

 Present address: Auburn Marine Research and Extension Center 

 Auburn University, 4170 Commanders Drive, Mobile, Alabama 36615 



Michael P. Weinstein 



Envirosphere Company, Ebasco Services Incorporated 

 160 Chubb Avenue, Lyndhurst, New Jersey 07071 



John A. Musick 



Virginia Institute of Marine Science, College of William and Mary 

 Gloucester Point, Virginia 23062 



A common assumption of early-life- 

 history studies of fishes has been that 

 the most critical period for survival 

 occurs during egg and larval stages, 

 thereafter juvenile fish show fairly 

 constant growth and mortality rates 

 (Gushing 1975, Williams 1983, Hew- 

 itt et al. 1985, Victor 1986). How- 

 ever, postmetamorphic survival may 

 be highly variable and we may need 

 to extend the critical survival concept 

 to include the juvenile stage of fishes 

 to better understand year-class vari- 

 ability (Walline 1985", Eckert 1987, 

 West and Larkin 1987). The exis- 

 tence of multiple cohorts within age-0 

 fishes may help explain variable 

 annual survival of juvenile fishes 

 (Buchanan-Wollaston and Hodgson 

 1929, Cooper 1937, Lambert and 

 Ware 1984). Survival is difficult to 

 estimate without information on resi- 

 dency time, but growth rate can be 

 used as an indicator of potential sur- 

 vival because it is an integrator of 

 most other environmental factors. 

 For example, those individuals that 

 grow the fastest will increase their 

 chances of survival because they 



Manuscript accepted 12 June 1990. 

 Fishery Bulletin, U.S. 88:745-752. 



'Contribution no. 1604, Virginia Institute of 

 Marine Science. College of William and Mary. 



grow through the size range vulner- 

 able to predators at a faster rate 

 (Tonn and Paszkowski 1986, Post 

 and Prankevicius 1987, Post and 

 Evans 1989). Faster growth also im- 

 plies that they are more physically 

 fit, e.g., larger individuals may be 

 able to survive overwinter stress bet- 

 ter than smaller individuals (Conover 

 and Ross 1982, Takita et al. 1989). 

 The key to evaluating variation in 

 growth rates among age-0 fishes is 

 the ability to age young fish. Larval 

 and juvenile fishes have been assigned 

 daily ages by length-frequency anal- 

 ysis, a somewhat imprecise method 

 (MacDonald 1987), or by counting 

 microincrements in otoliths (see re- 

 views: Campana and Neilson 1985, 

 Jones 1986). Daily aging of juvenile 

 weakfish Cynoscioyi regalis, by oto- 

 lith microincrements was unreliable 

 (Szedlmayer 1988), but a new method 

 of counting circuli in scales appears 

 promising (Szedlmayer et al. In press). 

 The present study uses this new 

 method to test for multiple cohorts 

 within age-0 weakfish, and to esti- 

 mate growth rates and differential 

 habitat use by fish collected in the 

 Chesapeake Bay- York River estu- 

 ary, Virginia. 



745 



