ICES Journal of Marine Science: Journal du Conseil Advance Access published online on September 20, 2007
ICES Journal of Marine Science: Journal du Conseil, doi:10.1093/icesjms/fsm146
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A method for evaluating the impacts of fishing mortality and stochastic influences on the demography of two long-lived shark stocks
1 Department of Fisheries, Government of Western Australia, Western Australian Fisheries and Marine Research Laboratories, PO Box 20, North Beach, WA 6920, Australia and Centre for Ecosystem Management, School of Natural Sciences, Edith Cowan University, 100 Joondalup Drive, Joondalup, WA 6027, Australia
2 School of Biological Sciences and Biotechnology, Murdoch University, Murdoch, Western Australia 6150, Australia
School of Earth and Environmental Sciences, James Cook University, Townsville, QLD 4811, Australia
Correspondence to R. B. McAuley: tel: +61 8 9203 0210; fax: +61 8 9203 0111; e-mail: rory.mcauley{at}fish.wa.gov.au
McAuley, R. B., Simpfendorfer, C. A., and Hall, N. G. 2007. A method for evaluating the impacts of fishing mortality and stochastic influences on the demography of two long-lived shark stocks. — ICES Journal of Marine Science, 64.
Stochastic demographic models were developed for Carcharhinus obscurus and C. plumbeus populations off the west coast of Australia by resampling the input parameters for life tables from empirical biological data collected from commercial target fisheries and fishery-independent surveys. The models were used to examine the effects of multiple scenarios of age-specific survival, derived from the fishing mortality rates estimated from a tagging study on sharks and indirect estimates of natural mortality. In the absence of fishing, median estimates of the rates of intrinsic population increase (r) were 0.025 for both species. Inclusion of the age-specific fishing mortality rates estimated for C. obscurus recruits born in 1994 and 1995 resulted in the median estimates of r declining to 0.007 and 0.012, respectively, suggesting that recent harvest levels of mainly neonates by the target fishery were probably sustainable. However, the model also suggested that the population was more susceptible to exploitation of older sharks than was previously believed. The C. plumbeus model indicated that fishing mortality between 2001 and 2004 was probably unsustainable. The increasingly negative trend in median r estimates (from –0.032 to –0.049), and the population’s apparently limited capacity for density-dependent compensation through changes in fecundity, somatic growth and longevity, suggests that management intervention is necessary to prevent continued stock depletion.
Keywords: Carcharhinus obscurus, Carcharhinus plumbeus, demographic analysis, fishing mortality, gauntlet fishery, stochastic
Received 16 February 2007; accepted 22 August 2007.
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