Temporal and latitudinal comparisons of reproductive parameters in a heavily exploited shark, the bonnethead, Sphyrna tiburo (L. 1758), in the southern Gulf of Mexico.

Abstract

In the southern Gulf of Mexico, the bonnethead shark, Sphyrna tiburo, is one of the most frequently captured species in landings of small-scale fisheries. Based on the analysis of two fishery-dependent sampling periods (1993-1994 and 2007-2014), this study aimed to determine reproductive parameters and identify temporal differences between the two time periods. In the first sampling period, 776 males and 352 females with a size range of 28.0-120.0 cm total stretched length (LT ) were analysed, and in the second sampling period, 387 males and 432 females with a size range of 28.0-122.0 cm LT were analysed. The size at 50% maturity in the second sampling period was significantly different between sexes, 82.6 cm LT for females and 73.8 cm LT for males (no estimation was possible for the first sampling period). The size at 50% maternity was not different between sampling periods, 97.3 cm LT for the first sampling period and 99.0 cm LT for the second sampling period. Litter size varied from 3 to 19 embryos and the average was not statistically different in both periods, 10.1 (S.D. = 3.8) for the first sampling period and 11.3 (S.D. = 3.5) for the second sampling period. The female reproductive cycle is asynchronous, and it seems to be annual, with a gestation period of 5-6 months, and a consecutive ovarian cycle and gestation period. Temporal (between sampling periods) and latitudinal (southern Gulf versus northern regions) variations occur in the synchronicity of the reproductive cycle, temporal variation in the relationship between maternal length and litter size, and latitudinal variation in average size of mature sharks.