Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


John A. Musick


The objectives of my research were to test the hypothesis that compensatory (density-dependent) growth of sandbar shark (Carcharhinus plumbeus) occurred after severe population reduction, to describe the juvenile sandbar shark fauna present in the Chesapeake Bay during 1980-81 and 1990-93, and to perform demographic analyses to examine potential population growth. Age and growth of sandbar sharks were investigated by counting rings in vertebral samples collected in 1980-81 and 1991-92. Age at maturity was 15-16 years for both sample periods and both sexes. For sexes combined, the von Bertalanffy growth parameters were L&\sb{lcub}\infty{rcub}& = 199 cm precaudal length (PCL), K = 0.057, t&\sb{lcub}\rm o{rcub}& = &-&4.9 years for the 1980-81 sample and L&\sb{lcub}\infty{rcub}& = 164 cm PCL, K = 0.089, t&\sb{lcub}\rm o{rcub}& = &-&3.8 years for the 1991-92 sample. Significant differences in size at age and annual incremental growth of juveniles suggest a small increase in juvenile sandbar shark growth rate between the two sampling periods. Annual catches of sharks &>&105 cm PCL declined substantially between survey periods. Males and females were present in a 1:1 ratio. During 1980-81 juveniles ranged in age from 0-7 yr, but in 1990-93 few sandbar sharks over age 4 were taken. Based on the best estimate of fishing mortality the population ranged from 10,087 to 8509 sharks from 1989-1993. Annual year-class size was variable but all estimates were within one order of magnitude. Juvenile sandbar sharks declined in abundance by approximately 15% between 1989 and 1993. The annual population growth rate was highest under a scenario of natural mortality (M) = 0.05 and maximum age of 30 yr, but was only 11.9%/yr. at higher juvenile mortality rates and adult M fixed at 0.10, the best estimate of M for sandbar sharks, population growth rate was only 2.6%/yr. Adding fishing mortality (F) at immature ages caused the population to decline unless F levels were &<&0.10 and 0.05 at maximum age = 30 and 60 yr, respectively. It is apparent that sandbar shark populations will decline under any substantial fishing mortality on immature ages, and mature fish can only be exploited at very low levels of fishing mortality.



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