Estimates of Fishing and Natural Mortality for Subadult Red Drum in South Carolina Waters

Abstract

Subadult red drum Sciaenops ocellatus were sampled in Charleston, South Carolina, from 1991 to 1999. Tagged individuals were subjected to either live recapture and release by research biologists or harvest and subsequent tag recovery by recreational anglers. Tag recovery data aggregated into 4-month periods were analyzed using Brownie models that were parameterized in terms of fishing effort and instantaneous rates of fishing (F) and natural (M) mortality. Within-year estimates of fishing effort were calculated from the U.S. National Marine Fisheries Service's Marine Recreational Fisheries Statistics Survey and adjusted to reflect a 4-month harvesting period. The level of annual effort appeared stable over all years and was held constant for all analyses; however, the within-year seasonal pattern of fishing effort varied substantially. Estimates of F and M depended heavily on the values used for the tag-reporting rate (λ) and the tag-retention and immediate survival rate (φ). For age-1 fish, values of φ·λ ranging from 0.8 to 0.3 produced F values between 0.27 and 0.71 and M values between 0.88 and 0.44, respectively. For age-2 fish, similar values for φ·λ yielded F values of 0.35–0.92 and M values of 1.37–0.83, respectively. The natural mortality estimates for age-2 fish also reflect emigration from the bay and estuarine systems to the coastal ocean.