Latitudinal variation in life-history traits of bonnethead sharks, Sphyrna tiburo, (Carcharhiniformes : Sphyrnidae) from the eastern Gulf of Mexico

Abstract

Life-history traits (size at age, growth rates, size and age at maturity, size of near-term embryos and litter sizes) of bonnetheads, Sphyrna tiburo, were analysed to test for latitudinal differences by comparing data collected from three areas along Florida's Gulf of Mexico coastline between March 1998 and September 2000. A total of 539 sharks were collected during the study: 207 in north-west Florida (latitude ~30°N), 176 in Tampa Bay (~28°N) and 156 in Florida Bay (~25°N). Male and female bonnetheads in north-west Florida had the largest predicted asymptotic sizes (1007 mm and 1398 mm TL, respectively) and attained the largest estimated median size at maturity (830 mm and 944 mm, respectively) and the oldest estimated median age at maturity (3.0+ years and 4.0+ years, respectively). The largest near-term embryos (297 mm TL) were also collected at the highest latitude, but no latitudinal difference in litter size was found. These differences in life-history traits provide supporting evidence that a pattern of latitudinal variation exists. Male and female bonnetheads in north-west Florida also had the fastest growth rate compared with the other locations, supporting the hypothesis that growth rate is inversely related to the length of the growing season (i.e. a pattern of countergradient variation exists).