Date of Award

1989

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

First Advisor

Charles A. Wilson

Abstract

The objectives of this study were to obtain age estimates and describe growth for the two sciaenid fish species, red drum Sciaenops ocellatus and black drum Pogonias cromis, in the northern Gulf of Mexico. Age estimates were made using annulus counts in transverse sections of otoliths (inner ear bones). Aging techniques were validated for juvenile red drum by imparting oxytetracycline into otoliths as a time marker. Recaptures validated that annuli were deposited in otoliths yearly up to 2.5 years age, with the first annulus formed in the second winter at 1-1.5 years age. Age estimates for remaining age classes of red drum and all age classes of black drum were validated by marginal increment analysis, following the progression of annulus formation throughout the year. Analyses indicated that annuli were formed yearly during winter-spring months for red drum and black drum at all ages. There was low variability in age estimates made independently by two or three readers for each fish. Age estimates ranged from one to thirty-seven years for red drum, and one to forty-three years for black drum. Both species grew rapidly until approximate age at maturity, followed by a significant decrease in growth rates. Red drum adult females exhibited faster growth and attained larger sizes than males, while differences in black drum growth between sexes were not apparent. The majority of red drum and black drum from schools of primarily mature fish were greater than nine to ten years old. Relative year-class strengths were determined based on year-of-birth distributions back-calculated from age estimates. Dominant year classes for red drum were born 1971-1974, and the 1975-1977 year classes were relatively reduced in abundance. There was a significant negative correlation of red drum year-class strength with summer air temperature during juvenile stages, and a possible positive correlation with fall-winter sea levels. Great variability was observed in strengths of adjacent black drum year classes. Several strong year classes were each separated by three to four relatively weak year classes, and no correlation was observed with environmental variables.

Pages

175

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