Life histories of commercially important tropical sharks from the Great Barrier Reef World Heritage Area

Elasmobranchs (sharks and rays) are captured by a number of coastal fisheries operating across tropical northern Australia. As they are typically not targeted by these fisheries, accurate data on their biology and composition in the catch is often lacking, impeding sustainable use and management. Effective fisheries management is particularly important for elasmobranch populations as they often have biological characteristics that make them susceptible to overfishing and slow to recover once overfishing has occurred.\ud \ud The largest extractive fishery for sharks in Queensland waters is the East Coast Inshore Finfish Fishery (ECIFF). In 2011 the commercial gillnet sector of this fishery had a total allowable catch (TAC) of 600 t, although catches rates were as high as 400 t.yr-1 in 2004, prior to the introduction of a TAC. The large geographic area, relatively low value, and disparate nature of the ECIFF mean monitoring of the catch is difficult. Yet as the fishery occurs predominantly within the Great Barrier Reef World Heritage Area (GBRWHA), it is closely scrutinised by the general public and stakeholder groups (e.g. marine park management, tourism, conservation groups). Effective and defensible science-based management is therefore especially important for the ECIFF.\ud \ud Between 2006 and 2009 an onboard vessel observer survey program was undertaken on the ECIFF with the goal of obtaining biological information that could be used to help manage, in particular, the shark component of the fishery. The observer survey was the most extensive ever undertaken on the fishery and covered the three major habitats in which the fishery operates; river (estuarine), intertidal (0 – 2 m depth) and inshore coastal (2 – 25 m depth). At least 38 species of elasmobranchs were found to occur within the fishery, however the catch was dominated by Carcharhiniformes; 95% of individuals were from 25 species of the families Carcharhinidae, Hemigaleidae and Sphyrnidae. The main carcharhinform taxa could be qualitatively categorised into four groups based on similar catch characteristics and life history traits: small coastal species (<1000 mm) were captured primarily as adults, moderate sized coastal species (1000–2000 mm) were captured at all sizes, large coastal semi-pelagic species (>2000 mm) were captured primarily as neonates or juveniles, and hammerheads were captured at all sizes.\ud \ud The life history characteristics of five species occurring in the fishery were investigated in detail using biological samples collected during the observer program, from fishery-independent sampling, and from purchase or donation from commercial fishers. The milk shark, Rhizoprionodon acutus was the fourth largest component of the elasmobranch catch by number in the ECIFF, making up 7.8 % of all carcharhiniform sharks caught. Growth was rapid in this species; von Bertalanffy growth parameters for males were L∞ = 821 mm, k = 0.94 and L0 = 424 mm and for females were L∞ = 859 mm, k = 0.63 and L0 = 423 mm. Females and males attained a maximum age of 8.1 and 4.5 years, respectively. The size at which 50% of females and males were mature was 780 and 742 mm, respectively. The age at which 50% of females and males were mature was 1.8 and 1.1 years of age, respectively. Despite being widely distributed globally and heavily exploited throughout its range, these are the first comprehensive estimates of age, growth and maturity for this species.\ud \ud The life histories of two globally endangered hammerhead sharks captured by the ECIFF were also examined in detail. The scalloped hammerhead, Sphyrna lewini and the great hammerhead, S. mokarran, were the fourth and third largest components of the elasmobranch catch by weight in the ECIFF. The catch of S. lewini was heavily biased towards males and significant differences in growth and maturity characteristics were found between those occurring within the GBRWHA and individuals sampled from temperate waters off northern New South Wales. The life history of females was difficult to establish as adults could not be sourced from any fishery. The best-fit estimates for a three-parameter von Bertalanffy growth curve fit to both sexes were L∞ = 3312 mm, L0 = 584 mm and k = 0.076. Males attained a maximum age of at least 21 years while the longevity of females could not be determined. For S. mokarran, the best-fit growth parameters for a two parameter von Bertalanffy growth curve fit to both sexes and assuming a fixed size at birth (L0) of 700 mm, were L∞ = 4027 mm, and k = 0.079. Females lived to at least 39.1 years and males to at least 31.7 years. Length and age at 50% maturity was not significantly different between sexes and occurred at 2279 mm and 8.3 years.\ud \ud The spot-tail shark, C. sorrah, is the second most important component of the elasmobranch catch in the ECIFF both by number and weight. For C. sorrah the best-fit growth parameters for a two parameter von Bertalanffy growth function with a fixed length at birth (L0) of 550 mm were L∞ = 1085 mm, and k = 0 5513 for males and L∞ = 1265 mm, and k = 0 3389 for females. Growth was not sexually dimorphic prior to reaching maturity and as such 50% maturity occurred at 933 mm and 2.3 years in both sexes. Fifty percent maternity occurred at 1029 mm and 3.4 years indicating females began reproducing 1–2 years after reaching maturity. Males attained a maximum age of at least 9 years and females at least 14 years. Females had an annual, synchronous reproductive cycle with ovulation occurring in March and parturition in early December after a gestation period of approximately 9 months. Females gave birth to between 1 and 6 pups of approximately 550 mm in length, and there was an increasing relationship between maternal length and fecundity.\ud \ud The largest component of the elasmobranch catch in the ECIFF was the Australian blacktip shark, C. tilstoni. However, analysis of the life history of this species was confounded by the presence of the morphologically similar common blacktip shark, C. limbatus. Genetic methods were used to distinguish between these species, however a mismatch was found between identification using genetics and identification by vertebral counts. This mismatch was thought to be due to hybridisation between the two species. As there was no clear way to distinguish between the two species, a multi-faceted approach to species identification was developed.\ud \ud Following this, best fit growth parameters for C. tilstoni using a two parameter von Bertalanffy growth function with a fixed length at birth (L0) of 619 mm were L∞ = 1748 mm, and k = 0.137 for males and L∞ = 2138 mm, and k = 0.099 for females. However, growth was more accurately described by a two-phase variant of the von Bertalanffy growth function that suggested a cessation in growth occurs around 4.1–4.5 years of age. Like C. sorrah, growth was not sexually dimorphic prior to maturity and 50% maturity occurred at 1208 mm and 5.5 years in both sexes. Fifty percent maternity occurred at 1374 mm and 7.5 years indicating that females began reproducing approximately 2 years after maturity. Carcharhinus tilstoni has an annual, synchronous reproductive cycle, with ovulation occurring in March and parturition in early December after a gestation period of approximately 9 months. Females gave birth to between 1 and 7 pups with a mean size at birth of 621 mm.\ud \ud The carcharhiniform sharks captured by the Queensland ECIFF display a range of life history characteristics ranging from small (<1000 mm) and rapidly growing (k = 0.94) species such as R. acutus to large (>4000 mm) and slow growing (k = 0.079) species such as S. mokarran. An appreciation of these life history characteristics is essential in data deficient fisheries such as the ECIFF where the productivity of captured species differs greatly and some species may be more vulnerable to overexploitation than others. This highlights the importance of ongoing studies on life history of sharks. The new life history data from the research should be used to help improve the management of the ECIFF and can help provide a sounder biological basis for decision making