Assessing the effectiveness of the great australian bight marine park in protecting the endangered australian sea lion Neophoca cinerea from bycatch mortality in shark gillnets

Abstract

The Endangered Australian sea lion Neophoca cinerea occurs in low numbers, exhibits low fecundity, extreme philopatry and substantial population genetic structure at the breeding colony level. These traits may increase susceptibility to population decline, with additional mortality as bycatch in shark gillnets being a possible threat. The Great Australian Bight Marine Park (GABMP) was established, in part, to protect the small and remote Bunda Cliffs population from anthropogenic impacts such as commercial fishing. This study investigated the effectiveness of the GABMP in reducing spatial overlap between Australian sea lions and gillnets and in preventing bycatch. An independent fishery observer program reported a mortality rate of 0.0206 individuals (ind.) km-1 of gillnet set within the GABMP, amounting to between 4 and 15 (confidence bounds of standard error of the estimate) ind. killed there during the most recent breeding cycle. A mortality rate of 0.0093 ind. km-1 of gillnet set was recorded across the broader GAB region, amounting to between 14 and 33 ind. killed each breeding cycle during recent times, and between 128 and 177 over the 10 yr since the GABMP was established in the mid-1990s. These reported bycatch levels are unlikely to be sustainable and may represent minimum estimates, because drowned individuals may drop out of the gillnet and go unobserved. A tracking program involving 9 females (5.6% of the estimated female population) demonstrated that they spent only 27.7% of their time inside the GABMP. Four of them regularly travelled more than 180 km from home, or 9 times further than the southern boundary of the GABMP. These results indicate that the level of protection afforded by the GABMP to Australian sea lions residing at Bunda Cliffs is unlikely to reduce bycatch to below the levels that would reduce the risk of decline in this small population. Suggested improvements to the GABMP include a year-round closure to gillnetting, low bycatch limits and extension of the southern boundary further south. Additional regulatory mechanisms may be needed in the gillnet fishery to minimise its impact on this and other small Australian sea lion populations. © Inter-Research 2011.