Differential Growth Among Cohorts Of Age-0 Weakfish Cynoscion-Regalis In Chesapeake Bay

Age-0 weakfish Cynoscion regalis, were sampled from the Chesapeake Bay-York River estuary in 1983 and 1984 to test for the existence of multiple cohorts and compare growth rates. In both years juveniles were first collected in the estuary in late July, were significantly 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 minima 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 growth 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 multiple cohorts within a single year-class must be considered in the estimation of juvenile recruitment to the adult population