Date Awarded


Document Type

Dissertation -- Access Restricted On-Campus Only

Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


John A. Musick


Juvenile weakfish Cynoscion regalis, life history was studied in the York River estuary, Virginia. to verify daily aging methods of juvenile fish, both male and female adults were induced to spawn by injection of 200 IU Human chorionic gonadotropin/Kg wet weight. Subsequent larval and juvenile fish were reared up to 275 days with wild plankton and a daily rotating diet of squid, liver, Anchoa mitchilli, and Menidia menidia. Otoliths and scales were examined for daily microincrements patterns. Otolith ring counts were highly variable (31% varied by &>& 15% among 3 counts). Two problems were evident: (1) Microincrements frequently split to form two increments; and (2) Otoliths from a size series of fish (6.12-13.1 mm) indicated that weakfish otoliths grew by bud formation rather than concentric deposition. Scale circuli showed little variation between counts (99.5% of 2 counts from an individual scale were the same). Daily scale deposition was suggested by rearing up to 100 days, after which ring deposition was less than daily, however further research is needed because only one fish was reared past 25 days. The advantages of scale circuli counting over otolith increment counting were increased precision and ease of preparation. Field samples were collected weekly from the York River channel, at night using a 4.9 m, 1.5 mm cod end, trawl, during the weakfish nursery period (Aug-Oct 1983). The new technique of daily aging by scales, was applied to 845 of 922 weakfish collected. Counts ranged from 3 to 100 circuli/scale. Three cohorts were defined from the 1983 0-age fish. Growth rates estimated from scales (0.76-1.13 mm/d) were similar to those from length frequencies (1.0-1.2 mm/d). Analysis of covariance showed a significant difference (0.05 level) in growth rates among cohorts and among stations, but third order interactions (station, cohort, growth rate) were not detected. Mortality/migration rates estimated from decline in mean catch were significantly different between cohorts 1 and 2 (0.05 level, t-test). Weakfish were first abundant as new recruits at the river mouth, and moved upriver as they grew. In the fall a reverse migration occurred. Birthdate frequency by station and date indicated that different cohorts used different areas of the York River.



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