The life histories of endangered hammerhead sharks (Carcharhiniformes, Sphyrnidae) from the east coast of Australia

Abstract

The life histories of two globally endangered hammerhead sharks, Sphyrna lewini and Sphyrna mokarran, were examined using samples collected from a range of commercial fisheries operating along the east coast of Australia. The catch of S. lewini was heavily biased towards males, and there were significant differences in von Bertalanffy growth parameters (L(∞) and k) and maturity [stretched total length (L(ST)) and age (A) at which 50% are mature, L(ST50) and A(50)] between those caught in the tropics (L(∞) = 2119 mm, k = 0·163, L(ST50) = 1471 mm, A(50) = 5·7 years) and those caught in temperate waters (L(∞) = 3199 mm, k = 0·093, L(ST50) = 2043 mm, A(50) = 8·9 years). The best-fit estimates for a three-parameter von Bertalanffy growth curve fit to both sexes were L(∞) = 3312 mm, L(0) = 584 mm and k = 0·076. Males attained a maximum age of 21 years and grew to at least 2898 mm L(ST). The longevity, maximum length and maturity of females could not be estimated as mature animals could not be sourced from any fishery. Length at birth inferred from neonates with open umbilical scars was 465-563 mm L(ST). There was no significant difference in length and age at maturity of male and female S. mokarran, which reached 50% maturity at 2279 mm L(ST) and 8·3 years. Sphyrna mokarran grew at a similar rate to S. lewini and the best-fit estimates for a two-parameter von Bertalanffy equation fit to length-at-age data for sexes combined with an assumed mean length-at-birth of 700 mm were L(∞) = 4027 mm and k = 0·079. Females attained a maximum age of 39·1 years and grew to at least 4391 mm L(ST). The oldest male S. mokarran was 31·7 years old and 3691 mm L(ST). Validation of annual growth-band deposition in S. mokarran was achieved through a mark, tag and recapture study.

Life-history traits of a small-bodied coastal shark

“…Annual band formation was validated from a flurochrome-labelled shark, recaptured after almost 1 year at liberty, and estimated to be 1.3 years at the time of recapture. Such annual band validation is consistent with reports from other coastal carcharhiniform sharks (Simpfendorfer et al 2002;McAuley et al 2006;Harry et al 2011a) and inshore elasmobranchs (Pierce and Bennett 2009). Annual band formation was further verified across all ages from centrum-edge analysis, with opaque bands forming during the Austral summer.…”

“…The values of APE (17.1%) and CV (24.2%) were high in comparison with those in other ageing studies of elasmobranchs (e.g. McAuley et al 2006;Jacobsen and Bennett 2010;Harry et al 2011a), thus indicating that there was a bias associated with assigning age estimates. The secondary reader generally produced higher age estimates, with the highest variability between readers occurring on older specimens (Fig.…”

“…In all the aforementioned cases, ages-at-maturity represent a small proportion of the maximum reported ages. Such early maturation contrasts with that of many large-bodied shark species that typically undergo maturation at much older ages, and at a greater proportion of their maximum age (Simpfendorfer et al 2002;Bishop et al 2006;Harry et al 2011a). Thus, the results of the present study indicate that for L. macrorhinus, the time to reach theoretical maximum length and the early maturation contrasts with that of large-bodied species.…”

“…Akaike's information criterion (AIC) was determined using the small-sample, bias-adjusted AIC (AIC c ) (Harry et al 2011a) and calculated as where K is the number of parameters þ1, n is the sample size, and…”

“…In the present study, TL was substituted for mass, similarly to previous ageing studies of elasmobranchs (e.g. Carlson and Baremore 2005;Jacobsen and Bennett 2010;Harry et al 2011a) Model Equation…”