Abstract:

Centrophorus zeehaani is one of at least 10 deep-sea shark species globally suffering major population declines attributable to expanding human resource use. Spatial closures have the potential to contribute to recovery of populations if home range and movements can be studied and understood. We implemented the first passive acoustic tracking study of sharks in the deep ocean (300–700 m depths) to evaluate the effectiveness of a large (~100 km long) fishery closure off southern Australia implemented to protect C. zeehaani. Using an array of 21 moored acoustic receivers, we passively tracked 71 tagged individuals over a 15-month period. Sixty-one sharks were detected repeatedly over an average duration of 408±153 days. The average along-slope range was 19.2±12.2 km and the maximum was 75 km – the full width of the array. Each month an average of 0.71 fewer males were detected; the number of females detected did not vary significantly between months. Individual males left the closure, but returned during the study period. Movement along-slope was influenced by month and release point, with shifts south and eastward occurring during austral winter – particularly by some males. Detection depth was strongly correlated with seafloor depth confirming that synchronous diel vertical migration (night time ascent) between population average depths of 640 m and 340 m occurred mainly on the seafloor. Different individuals occupied different depths on the seafloor. We conclude that the closure studied is effectively located to help conserve C. zeehaani because it has sufficient along-slope extent and depth range to encompass the home range of a high proportion of the individuals in the local population. Our work demonstrates the utility and uncertainties associated with acoustic tracking in the deep ocean, and the need to evaluate species movement and behaviour when relying on spatial closures to meet conservation objectives.

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