The ultimate objective of this project was to assess the effects of the recent rapid expansion of targeted fishing on the sustainability of the Western Australian sandbar shark (Carcharhinus plumbeus) population. To do so, it was first necessary to examine the distribution and population structure of C. plumbeus in Western Australian waters and to determine the magnitude and size composition of catches in all target and bycatch fisheries around the State. This was achieved through on-board observation of commercial fishing operations between the Lacepede Islands in the Kimberley and Cape Leeuwin in the south-west of the State, over a three-year period.
Secondly, the reproductive rates, age and growth characteristics and age-specific levels of natural and fishing mortality in the population were quantified. Reproductive frequency, embryonic sex ratios, litter sizes and sizes at birth and maturity were determined through examination of several thousand C. plumbeus caught by commercial shark fishers and during fishery-independent surveys. Age and growth was studied by microscopy of vertebral cross-sections and analysis of the growth increments of tagged sharks. The annual periodicity of vertebral growth bands was validated by injecting tagged sharks with a fluorescing dye (calcein). Thanks to the considerable co-operation of the WA shark fishing industry, this was one of the most comprehensive age validation studies ever conducted in wild sharks. Natural mortality estimates were obtained from several indirect relationships with life-history correlates and age-specific estimates of fishing mortality rates were derived from recaptures of tagged sharks.
The population’s ability to withstand the estimated rates of fishing mortality was assessed using a modelling technique known as demographic analysis. To account for variability and uncertainty in the empirical biological data, stochastic methods were used to incorporate the model’s input parameters. As population growth rates were determined to be negative (ie. the population was being depleted) in all three years for which tagging data were available, demographic modelling was also used to investigate the effects of a wide range of alternative harvest strategies. The results from this study have been used for developing new management arrangements for the Western Australian target-shark fisheries and for the management of shark by catch in all WA-managed commercial fisheries.
Associate Professor Glenn Hyndes
Department of Fisheries, Western Australia, Stephen Newman
Mote Marine Laboratory, United States, Colin Simpfendorfer