Stock Assessment of Scalloped Hammerheads in the Western North Atlantic Ocean and Gulf of Mexico

Abstract

The status of the western North Atlantic Ocean population of scalloped hammerheads Sphyrna lewini (Sphyrnidae [hammerhead sharks]) was assessed from 1981 through 2005 by using Schaefer (logistic) and Fox surplus-production models. The population declined rapidly before 1996 but began rebuilding in the late 1990s as fishing pressure decreased. The Akaike information criterion for small sample sizes—a test of goodness of fit for statistical models—indicated that the Fox model provided a slightly better fit to the data. Bootstrapped parameter values showed that in 2005 the probability of the scalloped hammerhead's being overfished was greater than 95% (the population was estimated to be 45% of that which would produce the maximum sustainable yield [MSY]) and a 73% probability that overfishing was occurring (fishing mortality was approximately 129% of that associated with the MSY). The size of this population was estimated to be 17% of what it had been in 1981, that is, it has been depleted by about 83% from the virgin stock size. Monte Carlo simulation predicted that the population had a 58% probability of rebuilding in 10 years if the 2005 catch level (4,135 individuals) were maintained and an 85% probability of rebuilding if the 2005 total catch were halved. Sensitivity analyses showed that the stock assessment results were most sensitive to removing the University of North Carolina longline survey index of relative abundance, the method of weighting indices, and excluding fishery-dependent indices of relative abundance.