BATON ROUGE, La. (AP) _ The growing taste for Cajun cooking has contributed to an alarming decrease in Louisiana's supply of redfish and speckled trout, according to a conservation group that will ask the Legislature to try to prevent a shortage.

''There is tremendous interest worldwide and especially across the nation in Cajun cuisine, and one of the cornerstones of Cajun cooking is blackened redfish,'' said Mark Hilzim, executive director of the Louisiana chapter of the Gulf Coast Conservation Association.

In addition, a state official said, measures taken by Texas and Alabama to protect their supplies of the fish have sent commerical fishermen into Louisiana waters.

Hilzim said the out-of state fishing interests are taking large numbers of fish and that, combined with enivronmental factors such as pollution and the loss of wetlands, has increased the strain on the state's supplies.

According to a 1984 study, Louisiana's recreational saltwater fishing industry is valued at more than $640 million, Hilzim said. Speckled trout and redfish account for most of that, he said.

''We are developing a legislative package for this session which would attempt to close up some of the things we see as problems,'' Hilzim said.

Hilzim would not discuss the details of the proposal, saying the group wants to contact legislators first. The session convenes in April.

Pete Arnow, who heads the environmental division for the Louisiana attorney general's office, said he hadn't seen the plan, but, ''Texas has legislation in place that has made redfish a game fish, which means commercial fishermen can't sell them. ... Making it a game fish means you can't take them with a net, only with a rod and reel.''

However, Arnow said Texas' legislation has put more pressure on Louisiana's redfish supplies.

Hilzim said Alabama also has passed such legislation, resulting in even more pressure on Louisiana's industry.

The conservation group has chapters in 12 Louisiana cities, about 4,500 members locally and more than 20,000 members nationwide, he said.