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Decline in whale shark size and abundance at Ningaloo Reef over the past decade: The world’s largest fish is getting smaller
Authors:Corey J.A. Bradshaw  Ben M. Fitzpatrick  Craig C. Steinberg
Affiliation:a School for Environmental Research, Charles Darwin University, Darwin, Northern Territory 0909, Australia
b Research Institute for Climate Change and Sustainability, School of Earth and Environmental Sciences, University of Adelaide, South Australia 5005, Australia
c South Australian Research and Development Institute, P.O. Box 120, Henley Beach, South Australia 5022, Australia
d School of Plant Biology, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia
e Australian Institute of Marine Science, PMB 3 Townsville MC, Queensland 4810, Australia
f Australian Institute of Marine Science, P.O. Box 40197, Casuarina MC, Northern Territory 0811, Australia
Abstract:Over-exploitation of whale sharks threatens the future of these wide-ranging pelagic fish. A long-term continuous record (4436 sightings) from a large aggregation (300-500 resident individuals) of whale sharks at Ningaloo Reef, Western Australia shows that mean shark length declined linearly by nearly 2.0 m and relative abundance measured from ecotourism sightings (corrected for variation in search effort and environmental stochasticity) has fallen by approximately 40% over the last decade. This population-level result confirms previous predictions of population decline based on projection models parameterised using mark-recapture estimates of survival. The majority of these changes are driven by reductions in the number of large individuals in the population. Phenomenological time series models support a deterministic (extrinsic) decline in large females, although there was some evidence for density dependence in large males. These reductions have occurred despite the total protection of whale sharks in Australian waters. As this species is highly migratory, the rapid change in population composition over a decade (<1 whale shark generation) supports the hypothesis of unsustainable mortality in other parts of their range (e.g., overfishing), rather than the alternative of long-term abiotic or biotic shifts in the environment. As such, effective conservation of whale sharks will require international protection, and collaborative tagging studies to identify and monitor migratory pathways.
Keywords:Australia   Decline   Exploitation   Rhincodon typus   Size reduction   Whale shark
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