Between the southern tip of mainland Florida and the curve of the keys just below it is a huge, shallow bay in which temperate and tropical zones meet, saline water mixes with the fresh variety and where the sport fishing is, or at least used to be, spectacular.

In the spring, people from every part of the world come to Florida Bay to cast for 150-pound tarpon that look like sea monsters speeding through three feet of glass-clear water.

During the rest of the year, there are bonefish, redfish, trout, snook and jack to fish for in more than 300,000 acres of water and mangrove islands that make up the southern quarter of Everglades National Park.

"Fish have always been thick as mosquitoes here," said Eddie Wightman, a professional guide on the bay for 23 years. "Twenty years ago, it was so absolutely unbelievable, nobody had a thought that it would ever run out."

But about eight years ago, the unthinkable began to happen. The bay took sick. Fishing stocks declined drastically; a few species almost disappeared. To people here, it was an unmerciful shock, like paradise being repossessed.

"There's no other place in the world where everything comes together like this," said Don Miller, executive director of the Everglades Protection Association. "Fishing here is a wildlife experience that is absolutely unique."

The association is one of the primary organizations that has lobbied the state and Congress to take measures to reverse that decline. Aided by the political clout of rich and influential fishermen who have come here since the days of Zane Grey and Ernest Hemingway, the effort has been a limited success.

"In the last three years, the situation has turned around 180 degrees," said Miller, his thick white hair blowing in the breeze as we skimmed over the pale green surface of the bay in his fishing boat. "But there is a long way to go. In some areas of the bay, we are void of fish, not just a species but all fish."

Like most wildlife problems in Florida, the bay's poor health was caused by a lack of fresh water. The north shore of the bay, where the sawgrass of the Everglades meets the waters of the Gulf, the Caribbean and the South Atlantic, serves as a nursery for most of the bay's fish.

That nursery depends on a mixture of fresh and salt water. But the fresh water that historically seeped down from the Kissimmee Valley and Lake Okeechobee through the Everglades has been diverted by water management programs to farmlands and cities on both coasts.

Without the flow of sweet water from the north, the delicate balance between fresh and salt water ecosystems has been upset.

The struggle between conservationists and sport fishermen on one side and the water control people on the other has pinched a third group--the commercial fishermen.

Most of them live on the gulf coast in towns like Everglades City and Chokoloskee, a 130-acre island of seashells created by Florida's indigenous Indians hundreds of years ago.

The sport fishermen claim that the commercial netters are making a severely stressed aquatic system more critical. They persuaded the Department of the Interior to set catch limits on fish and to propose rules to eliminate all commercial fishing in the national park by Dec. 31, 1985.

"There has been no evidence provided by the national park or any other group showing commercial fishing is harmful to any fish in the park," complained Jerry Sansom, a wildlife biologist and the exective director of Organized Fishermen of Florida, which represents 1,600 commercial fishermen in the state.

Sansom says the commercial netters, some of them descendants of families that fished the bay 100 years ago, are being used as blue-collar scapegoats by the sport fishermen, a group he refers to as "the silk stocking crowd."

It is almost too distracting here to concentrate on fish. The breeze is warm and constant, the air clear. Brown pelicans, cormorants and gulls dive into the shallow water for food. Pink flamingos and white egrets stand above the water in concentration.

Don Miller has taken us as close as his outboard can to a mangrove island. Now he is standing on a platform above the boat's stern using a 15-foot pole that looks like a shuffleboard stick to push us into shallower flats.

"See how clear the water is," said Miller, a short, amiable man with a tattoo of an eagle on one forearm and a peacock on the other. "You can see the fish you're stalking here. It's more like a hunt."

We have spent the day exploring the bay within the national park. There is now only about half an hour to fish before an approaching storm hits. We know it is coming because the birds have flown to shelter on the protected sides of nearby islands.

"You'll have to come back again," said Miller after he has caught and released a four-pound redfish. "This bay will be here. It's coming back."