Pupping areas and mortality rates of young tiger sharks Galeocerdo cuvier in the western North Atlantic Ocean

Abstract

From 1995 through 2006, 2577 bottom longline sets were conducted in the western North Atlantic Ocean from ~36° N, 75° W to 26° N, 97° W, an area that encompasses both the coastal waters off the southeastern USA (Atlantic) and the northern Gulf of Mexico (Gulf). Over the course of the study, 335 young-of-the-year (YOY) and 219 juvenile tiger sharks Galeocerdo cuvier were cap- tured. In the Gulf, YOY catch per unit effort (CPUE) was highest from ~88 to 83° W. In the Atlantic, the area of highest YOY CPUE occurred from 31 to 33° N. Distribution of juvenile tiger sharks was more uniform than observed for YOY throughout the range of the study. Annual survival rates of YOY and Age 1+ tiger sharks were estimated to be 51 and 62% in the Gulf and 39 and 27% in the Atlantic. Total instantaneous mortality rates of YOY and Age 1+ tiger sharks were, respectively, estimated to be 0.67 and 0.47 in the Gulf and 0.93 and 1.32 in the Atlantic. In the Atlantic, age-specific natural mortality (M) estimates ranged from 0.12 to 0.50 and from 0.08 to 0.36 for YOY and juvenile tigers sharks, respectively. Estimates of age-specific M were more variable in the Gulf, with values ranging from 0.11 to 0.98 for YOY and from 0.08 to 0.57 for juveniles. In the absence of nursery areas, the pro- duction of numerous fast-growing offspring presumably helps the species survive in the face of high total mortality.