Age and Growth of the Sandbar Shark, Carcharhinus plumbeus, before and after Population Depletion

Abstract

Recent studies have shown that by 1991 the sandbar shark (Carcharhinus plumbeus) population along the Atlantic coast of the United States had declined in abundance to approximately 20% of its level in the late 1970s. This phenomenon allowed us to test the hypothesis that compensatory (density-dependent) growth occurred after severe population reduction. Age and growth of sandbar sharks were investigated by counting rings and back-calculating lengths at previous ages from vertebral samples collected in 1980-1981 and 1991-1992. The collections included 188 sharks from 1980-1981 and 412 sharks from 1990-1991 ranging in length from 51-172 cm precaudal length (PCL). All sharks were mature at lengths > 136 cm PCL. Minimum and maximum ring counts, which included a birth mark, were 1 and 25. Age at maturity was 15-16 years for both sample periods and both sexes. For sexes combined, the von Bertalanffy growth parameters were L,. = 199 cm PCL, K = 0.057, to = -4.9 years for the 1980-1981 sample and L,. = 164 cm PCL, K = 0.089, to = -3.8 years for the 1991-1992 sample. Statistical tests found significant differences between the two growth models. 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. However, age at maturity was unchanged between samples suggesting that any biological significance of a growth rate increase has not been realized.