Fishery biology and demography of the Atlantic sharpnose shark, Rhizoprionodon terraenovae, in the southern Gulf of Mexico

Abstract

The Atlantic sharpnose shark, Rhizoprionodon terraenovae, is the most abundant shark in the Gulf of Mexico. This species is captured with different fishing gears and is available throughout the year because of its coastal habits. A demographic analysis incorporating biological data on reproduction, age at maturity ( i mat), fecundity, maximum age ( i max), sex ratio (1:1) and natural mortality at specific ages ( M) was carried out. Previously described age and growth data of individuals from the northern Gulf of Mexico were used. The variability of the environment and the high level of fishing pressure prompted us to assume that M decreased because of potential density-dependent effects. A first run under non-exploitation conditions, incorporating the input parameters i mat=4, i max=10, and M iterating randomly (uniform) from 0.42 to 0.378, yielded a reproductive rate ( R 0) from 1.12 to 1.44, a generation time ( G) from 5.8 to 5.9 years and an intrinsic rate of increase ( r) between 0.021 and 0.062. The final run was with estimates of fishing mortality added (based on F=0.46 from a Leslie–DeLury analysis, which considered catch, effort and catch per effort data and a constant catchability coefficient ( q) from an assumed relatively homogeneous artisanal fleet). Trial results by varying F and age at first capture ( i fc) showed that the combination of i fc=4 and F=0.1 yielded a positive R 0 value of 1.07, which indicates that R. terraenovae is very vulnerable to fishing pressure. Thus, the empirical knowledge of the fishery and current F and i fc situation appears consistent with an eventually declining population, and the fishery may be at risk of future collapse. These first results will allow us to establish the strategies necessary to manage this fishery.