The average size of fish off the West Australian coast is getting smaller as recreational and commercial fishers continually take big breeding fish, according to new research.

Research by the University of WA using underwater cameras to record fish species has found that many of the State's most valued species are rare and smaller than recorded historically.

The cameras have recorded 6500 hours of data over the past five years, measuring the number of fish swimming past the camera and their size.

For species such as dhufish, breaksea cod and western blue groper the average size seen was approximately half the maximum size attainable.

Western blue groper, which can grow to 160cm, did not exceed 120cm. Only 1.8 per cent of the 111 dhufish seen exceeded 92cm, below the maximum size of 1.2m.

The Centre of Marine Futures director, Professor Jessica Meeuwig, said size was a critical issue in protecting fish stocks and big females were the engine room for stock recovery.

"This means the size of fish in WA waters are actually quite small and the amount and quality of eggs a female fish will produce is directly relative to her size," she said.

"It is the big old fat females we need to protect. The only way to protect them is in no-take spatial closures coupled with restrictions on bag limits and size limits."

Professor Meeuwig said stocks of the "vulnerable five", identified by the Department of Fisheries as dhufish, pink snapper, red snapper, baldchin groper and breaksea cod, had been over-fished.

"Fish such as dhufish, which used to be called 'schoolies' because they were found schooling in areas like Geographe Bay, are now so rare we see them typically once every 17 hours," she said.

Recfishwest chairman Craig Leatt-Hayter said "no fish areas" was not a solution because some species were not territorial and they moved around.

"A combination of factors including maximum-size catches needs to be considered for all fish," he said.

The Department of Fisheries implemented seasonal restrictions for recreational demersal scale fishing in the West Coast Bioregion by imposing a two-month ban on some of WA's severely low stocked fish.