Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


John E. Olney

Committee Member

John A. Musick


During spring 1990 and 1991, ichthyoplankton surveys were conducted in lower Chesapeake Bay to estimate seasonal egg production, population biomass and the impact of predation by gelatinous zooplankton on early life stages of black drum, Pogonias cromis. Rearing experiments indicated that at least three species of sciaenid (silver perch, Bairdiella chrysoura; weakfish, Cynoscion regalis and P. cromis) were spawning in the survey area during both years. Specific identification of eggs based on previously published ranges of outside egg diameter (OED) were not reliable due to considerable overlap in diameter distributions. Analysis of weekly OED frequency revealed the presence of three modes which differed in temporal occurrence, suggesting the products of three species. Genetic typing of eggs using RFLP analysis of mtDNA confirmed the presence of three species, but demonstrated that eggs of a certain size class represented two species. Results illustrate that reliance on previously published ranges of egg diameter for specific identification of spring spawning sciaenids may overestimate the spawning biomass of black drum by as much as 50% owing to the misidentification of weakfish eggs as those of black drum. Black drum enter Chesapeake Bay in early spring and spawn throughout the day over a discrete time period from late April through May in a spatially limited area off the city of Cape Charles, Virginia. Seasonal egg production is low compared to other sciaenid stocks and estimates of spawner biomass indicate that the population is comprised of relatively few individuals. During 1990 and 1991, peak egg production of black drum occurred in the lower portion of Chesapeake Bay where potential encounter with gelatinous zooplankton predators was least likely. These observations were consistent between years with vastly different gelatinous zooplankton communities. Analysis of small-scale patterns of spatial coincidence between eggs and predators reveal that chances of physical contact are least in areas of peak production.



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