The Australian Fisheries Management Authority yesterday took the rare step of shutting gill nets out of a swath of Southern Ocean, near Kangaroo Island.

The four-kilometre nets in the South Australian fishery, which supplies flake to Melbourne fish shops, first came under scrutiny earlier this year over sea lion deaths.

In a move to reduce an estimated 374 sea lion deaths each breeding cycle, the authority began observing all the fishery's boats in May.

Over the past four months, 20 dolphins were counted dead in the nets, compared with 25 in the previous eight months, according to the authority.

Even the authority had acknowledged the ''significant and serious under-reporting'' in this fishery on interactions with marine mammals, said Kathryn Warhurst, of the Conservation Council of South Australia.

Humane Society International's Alexia Wellbelove said the South Australian experience showed the importance of having a monitoring camera, or human observer, aboard each vessel in such contentious fisheries. ''I think what it shows is that the industry isn't always telling us about their encounters with seals and dolphins,'' Ms Wellbelove said.

But Kyri Toumazos, a spokesman for the shark gill netters who work the fishery, said the increased deaths came about when shark fishers were displaced from their usual fishing grounds to waters off the newly re-opened Murray River mouth.

''There have been enormous numbers of dolphins there recently attracted by the bait fish there,'' Mr Toumazos said.

He defended the fishery as transparent, and said he expected it to re-open after the authority's six-month ban.

''Meanwhile, it will reduce the supplies of flake to Melbourne in particular,'' he said.