Unless a recent change in the New Zealand sea lion's status to the highest risk of extinction leads to ways to reduce it becoming bycatch at sea, the population will continue to decline, the New Zealand Sea Lion Trust says.
Sea lions were reclassified as nationally critical under the New Zealand Threat Classification system.
Bottlenose dolphins, mostly found in Doubtful Sound, Fiordland and Bay of Islands, also had their threat status uplifted to nationally endangered.
Research had shown sea-lion numbers were declining in the Auckland Islands but it was not known why, although sea lions were known to have suffered naturally-occurring bacterial epidemics and direct fishing-related mortality.
Sea Lion Trust chairman Shaun McConkey, of Dunedin, said the decline of nearly 50% was despite efforts to limit the number of sea lions caught in the squid fishery in sub-antarctic waters and attempts by the fishing industry to minimise its impact by attaching sea-lion exclusion devices.
"Good science has led to knowledge of the population decline but we also need good science to tell us how effective the sea-lion exclusion devices are," Mr McConkey said.
Unless the change in status produced a verifiable change in fishing methods that greatly reduce sea-lion bycatch, the trust was concerned that the population would continue to decline, he said.
"We feel all threatened species should have a population management plan.
"The Department of Conservation would then set the sea-lion bycatch limit, rather than the Ministry of Fisheries."
Forest and Bird Conservation advocate Nicola Vallance said greater steps were needed by the Government to protect sea lions.
"One sea-lion death in a squid net is too many for a species that is heading at breakneck speed towards extinction."
