Stark warning from Chathams sea lion's prehistoric demise

Rawlence N J, Collins C J, Anderson C N, Maxwell J J, Smith I W, Robertson B C, Knapp M, Horsburgh K, Stanton J A L and Scofield R P 2016. Human‐mediated extirpation of the unique Chatham Islands sea lion and implications for the conservation management of remaining New Zealand sea lion populations. Molecular Ecology. DOI: 10.1111/mec.13726. 

An international research team led by University of Otago scientists has shown that New Zealand’s Chatham Islands were once home to a unique population of sea lion, that was driven to extinction soon after first human settlement.

Sealion2

Male New Zealand sea lion, Sandfly Bay, Otago Peninsula. Copyright: David Waters

The researchers used ancient-DNA analysis, radiocarbon dating and computational modelling to reveal the relationships of the unique prehistoric population, and also to understand the reasons for its sudden extinction a few hundred years ago.

The team found a previously undiscovered lineage of sea lion on the isolated Chatham Islands, 650 km east of mainland New Zealand. The unique prehistoric Chathams sea lion was genetically clearly distinct from the modern population that persists in the Auckland and Campbell Islands, and mainland New Zealand today.

The Chathams supported a large, genetically diverse population of this unique sea lion, which went rapidly extinct around 1650 AD, following Polynesian settlement of the islands only 200 years earlier. The regional New Zealand sea lion population now contains only a fraction of the genetic diversity it once had. 

The Otago team used computational modelling to determine the level of human hunting likely to have caused the Chathams sea lion extinction. Modelling indicated that hunting rates greater than one sea lion/person/year resulted in the extinction of native populations within 200 years of first human settlement. Sea lions were not able to withstand even low levels of sustained hunting pressure.

The findings may also have important implications for the continued survival of New Zealand’s modern sea lions. The same modelling approach was used to estimate the survival prospects for the modern population under different mortality rates.

Sea lions are listed as endangered by the IUCN Red List. With only around 10,000 individuals remaining, the population is in serious decline (50% decline in pup births since 1998), with fisheries bycatch and resource competition the likely culprits.

The team’s computer models suggest that current reported and unreported bycatch levels may be unsustainable for the long–term survival of the species. The study adds to the growing evidence that undetected sea lion bycatch may still be driving the decline of the species, something the government’s recently released sea lion threat management plan (TMP) dismisses.

The Marsden and Allan Wilson Centre funded research included team members from the Universities of Otago, California, and Southern Methodist University, as well as the Museum of New Zealand Te Papa Tongarewa and Canterbury Museum.

See the paper here.