Rig under threat

percent between the periods 1971-74 and 2001-2004.

Dr Bray said the study was unusual because the same net and same location had been used for its duration; most other long-term studies involved a change in technique and or location.

Twenty-three fish species were identified but 11 comprised most of that catch. They were rig, yellow-eyed mullet, snapper, barracouta, kahawai, warehou, spotty, flounder, blue mackerel, grey mullet and dab.

Dr Bray said the reasons for the decline in numbers and sizes was not known, but probably included commercial and recreational fishing pressures, deterioration of the seafloor community by trawling and dredging, and consumption of zooplankton and phytoplankton by mussel farms.

The results reflected anecdotal evidence from recreation fishermen who had fished in the sounds for years and reported a decline in fish numbers, he said.

The situation was not yet dire and a recovery forecast showed the fish populations were likely to bounce back if conditions were right.

"There's hope if we get a reasonable sustainable yield management programme, none of the fish is anywhere near extinction and that's not true in some other parts of the world," Dr Bray said. However, he did not expect the study to spark major changes.

"Fisheries are declining all over the world, other studies have shown that and we don't think our study is gong to stop that."

Dr Bray and Dr Struik have doctorates in plant and animal ecology from the University of Wisconsin. They moved to New Zealand in 1963 and live in Nelson and Duncan Bay. As well as their study of estuarine fish, they have worked on forest regeneration, possum populations.