The biology and ecology of the tiger shark (Galeocerdo cuvier) on the east coast of Australia

Abstract

The tiger shark (Galeocerdo cuvier) (Peron and Lesueur 1822) is the largest of the carcharhinids, with a circumglobal distribution in both tropical and warm temperate coastal and pelagic waters. In the western Pacific, G. cuvier movements are wide-ranging, encompassing the east coast of Australia and south Pacific Islands. Throughout the region, G. cuvier is exposed to a range of commercial, recreational, artisanal and illegal foreign fishery impacts, as both a target and by-product species. Listed as ‘near threatened’ on the International Union for Conservation of Nature (IUCN) Red List, suitable long term species-specific catch, catch rate and biological data are seldom available for large shark species like G. cuvier, particularly where historical commercial fishery logbook reporting has been poor. Shark control programs targeting large sharks along Australia’s east coast have been in operation for over 60 years, using relatively standardised fishing gear in nearshore waters all year round, with historical catch and effort data recorded by shark contractors. Historical catch, catch rate and biological data collected through the Queensland Shark Control Program (QSCP) since 1993 were investigated, which revealed significant declines (p < 0.05) in catch rates of G. cuvier at some tropical and all sub-tropical locations along the Queensland coast. Significant temporal declines in the average size of G. cuvier also occurred at four of the nine locations analysed (p < 0.05), which could be indicative of fishing reducing abundance in these areas. Inter-annual variability in catches at each location was considerable and warranted careful interpretation with respect to estimating population abundance from these data. Investigation into the spatial and temporal movements of tiger sharks, and how their abundances fluctuate in relation to environmental influences, provided a basis for determining the factors driving this variability. Tiger shark movements were recorded using PAT Mk-10 and SPOT5 electronic tags to investigate G. cuvier spatial dynamics, site fidelity and habitat use off the east coast of Australia. Of the 18 tags deployed, 15 recorded information on depth and/or temperature, and horizontal movements, with tracking times ranging between four and 408 days. Horizontal movements were characterised by combinations of resident and transient behaviour that coincided with seasonal changes in water temperature, with summer migrations from sub-tropical Queensland waters to the southern temperate waters of New South Wales (NSW) to exploit the seasonally warm and prey abundant waters. While the majority of movement activity was focused around the continental slope, large-scale migration was evident with one individual moving from offshore Sydney, Australia, to New Caledonia (≈ 1800 km) in 48 days. Periods of tiger shark residency outside of Australia’s fisheries management zones highlighted the potential vulnerability of the species to unregulated fisheries, and the importance of cross-jurisdictional arrangements for migratory species’ management and conservation. Understanding the life history strategies of commercially- and recreationally- exploited elasmobranchs is fundamental in implementing appropriate fisheries management regimes. Lengths-at-age and growth rates for G. cuvier captured on the east coast of Australia were estimated from vertebral growth band counts of 202 sagitally-sectioned centra from 112 females (71 – 430 cm total length (TL)) and 79 males (72 – 351 cm TL). Modelled growth coefficients for pre- and post-natal females (L∞ = 418.3, L0 = 101.5, k = 0.07) and males (L∞ = 350.5, L0 = 101.6, k = 0.12) were smaller than those previously reported for tiger shark populations in other regions of the world. The population sampled herein included older tiger sharks than previously aged elsewhere, with split-band and narrow banding patterns in large individuals being identified as potential sources of age underestimation. G. cuvier is the only carcharhinid with an aplacental viviparous reproductive mode. Its large size and semi-solitary nature has made the species’ reproductive parameters difficult to study, particularly for mature animals that exceed 300 cm total length (TL). I present the first analysis of the reproductive biology of G. cuvier from data and specimens obtained from the QSCP and from NSW game fishing tournaments. Pups (n = 112) from four pregnant sharks were used to assess the possibility of multiple paternity. Length at 50% maturity (L50) was 297 cm TL for males and 325 cm TL for females. Historical QSCP data for 83 litters indicate that parturition occurs when pups are 70 – 90 cm TL, and that considerable variation in pup size from the same litter does occur. Birth is in the austral summer after a gestation period of around 15 – 16 months. Female sharks that were pregnant, or had well-developed oocytes, or had small oocytes were found throughout the year, consistent with a triennial breeding cycle. There was no evidence of multiple paternity, based on the litters of the four sharks examined using nine microsatellite loci, which may have implications for the genetic diversity of this population.