Study raises fear for sea lions

Unless measures are taken to mitigate [by-catch], the outlook for our sea lions is bleak - Dr Nic Rawlence. photo / Supplied

Academics fear for New Zealand's falling sea lion population after discovering a breed of sea lion was "rapidly" driven to extinction after human contact on the Chatham Islands in the 17th century.

An international team led by Otago University scientists has proved a distinct type of sea lion existed 650km east of New Zealand's mainland, but was wiped out 200 years after Polynesian settlement in about 1650.

The study indicated that any hunting rate greater than one sea lion killed per person per year was enough to wipe out an entire population in 200 years, adding to evidence that undetected by-catch may be behind falling sea lion levels in New Zealand.

"Sea lions were not able to withstand even low levels of sustained hunting pressure," said Otago archaeologist Dr Justin Maxwell, while associate professor Bruce Robertson, an Otago sea lion biologist, said: "Undetected by-catch may still be driving the decline - something the Government's recently released sea lion threat management plan (TMP) dismisses."

However, the Department of Conversation (DoC), in a document published last month, said although 100 sea lions a year were being caught by commercial trawlers 15 years ago, that number had dropped significantly.

This decline came from area-based measures, codes of best practice, individual vessel management plans and, most importantly, sea lion exclusion devices (Sled).

The Sled is designed to allow small species such as squid to become trapped in the net, but allows sea lions to break free. The Ministry for Primary Industries believes about 82 per cent of sea lions survive an encounter with a fishing vessel.

But Dr Nic Rawlence, who carried out the study, said: "What our research shows is that human harvesting and sea lions do not mix."

Since 1998, sea lion births have fallen 50 per cent, and with about 10,000 sea lions left, the population is in serious decline.

The New Zealand sea lion is the only native sea lion. While once it could be found all over the country, breeding sites are now limited to the Auckland Islands, Campbell Island and smaller sites in Otago and Stewart Island.

The researchers used ancient DNA analysis, radiocarbon dating and computational modelling to analyse the extinct population and to understand why it suddenly died out.

"Unless measures are taken to mitigate ... by-catch levels, the outlook for our sea lions is bleak," said Rawlence.