Latitudinal variation in life-history traits of bonnethead sharks, <i>Sphyrna tiburo,</i> (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&amp;nbsp;mm and 1398&amp;nbsp;mm TL, respectively) and attained the largest estimated median size at maturity (830&amp;nbsp;mm and 944&amp;nbsp;mm, respectively) and the oldest estimated median age at maturity (3.0+&amp;nbsp;years and 4.0+&amp;nbsp;years, respectively). The largest near-term embryos (297&amp;nbsp;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).