Conservationists fear scores of endangered sea lions could be drowning in gillnets off the WA coast every year and have called for an urgent review of fishery rules.
It follows the release of data in State Parliament showing four sea lions drowned in the nets in the past three years, two in 2009 and two last year.
The figures also show the fishery caught an average of two dolphins, one fur seal and 60 protected grey nurse sharks each year, and in 2009 had caught a whale.
Oceans campaigner for the Conservation Council of WA, Tim Nicol said the deaths were likely to be grossly under-reported as there were no observers and no cameras on gillnet vessels working off WA.
He said placing observers on gillnet fishing vessels in South Australia had seen the number of recorded deaths of sea lions jump from less than 10 to more than 250 a year and had led to changes in the way that fishery was managed.
Mr Nicol said there were only 2000 sea lions left in WA and the creatures were the rarest sea lions in the world. He said some of WA's sea lion colonies had just 10 breeding females and even one death could be too many. "Deaths in fishing nets is the most likely reason that our sea lion populations are not recovering," Mr Nicol said.
Gillnets are nets up to 12km long strung vertically in the ocean for up to 24 hours. The nets are used to catch demersal scale fish and sharks. Mr Nicol said the nets could not be seen by sea lions and acted like "walls of death" to marine mammals.
A Department of Fisheries spokesman said there had been15 years of independent on-board observations in the commercial gillnet fisheries and studies had shown few interactions with sealions.