President Carter ate his bluefin tuna and that is just fine. Any outdoorsman who has to wait six months between hunting and fishing trips - roughly the time between Carter's Christmas Day quail hunt and his June 29 tuna foray - deserves to sink his teeth into his quarry.

Fred Rushin, however, was hoping that we wouldn't choose that alternative as we trolled off the coast of Virginia Beach recently Ruskin likes to release most of the tuna captured on boats he skippers. Don Liverman, whose 42-foot Good News Rushin piloted out of Lynnhaven Inlet under bright skies and light winds, feels the same way.

The weather was clear for our trip, but it was only one day after Interior Secretary Cevil Andrus had been blown out of a planned tuna expedition by gale-force winds and forced to settle for 25 bluefish colser to shore.

So we weren't even sure we would see a tuna, let alone release one.

If it were possible to catch a glimpse of the irridescent bluefins, however, we felt confident Rushin was the one to put us on them. A thin wisp of a man, Rushin moved to Virgina Beach from California 12 years ago and has established himself as one of the top tuna skippers working Virginia's offshore waters. An ex-charterboat captain and a past president of the Cape Henry Billfish Club, Rushin now confines himself to guiding private craft on time to guiding private craft on time borrowed from his duties as a real-estate salesman.

This doesn't deter Rushin's magic with the tuna. Fleets of charterboats have been known to follow him to his favorite tuna waters.

Rushin knows so much about tuna because he studied them with the commitment of a scientist, poring over all data available on this compact, foot-ball-shaped fish. He contributes to that data himself by tagging and releasing fish for Woods Hole Oceanographic Institute in Massachusetts.

"I started tagging bluefins in 1969 and have probably tagged somewhere around 200 tunas from six to 135 pounds in that time," Rushin said.

His efforts have brought some interesting results. One of the tagged tuna, a 50-pounder, was caught a month later off the coast of Trinidad, having traveled 100 miles a day to get there. The fish swam south when it should, according to current theories, have been heading north.

That is part of the reason why Rushin tags tuna - because there is precious little known about the fish. But he also releases fish - if he has his druthers - because he thinks the tuna is "almost an endangered species," and every fish counts when stocks are so low.

The vulnerability of bluefins is evident both in their recapture rate and in the dwindling catches of sport fishermen.

"A good 25 percent of tagged fish get caught very quickly," said Rushin. Almost 90 percent of returned tags come from commercial seiners; sport fishermen account for about 10 of 100 returns.

That's pretty much the ratio for tuna catches in the Atlantic, too. Commercial fishermen take approximately 50,000 tuan (bluefins) in the 32 to 35 pound range to meet their quota under the 200-mile limit.

Sport fishermen catch about 5,000.

"Since 1970 the tuna stocks have drastically dropped, hence the need for management programs and laws to protect and rebuild the stocks," Rushin said.

"A good day's fishing four years ago was 40 to 50 school tunas. Three years ago it was 30. Two years ago 20. One year ago 10 to 15. This year 1 to 10 tuna has been good fishing."

Four years ago tuna sportfishing was restricted by law. The limit is four fish per angler for the day, with only one fish less than 14 pounds or more than 115 pounds permitted.

But four fish per angler can still mean a lot of tuna blood spilt on the decks. Rushin, for one, would prefer to see the fish dart back into the depths with a tiny piece of plastic sticking out of its back.

By midafternoon on our trip, we had released dozens of bluefish and caught two albacore that were kept for marlin bait. There was one tuna in the fish box - a 25-pounder that zapped a cedar plug and fought with uncanny strength against the 30-pound-test outfit.

But we were determined to do our part as good sportsmen and release any of the tuna we caught. At 3 p.m. the chance arrived.

A patch of ripped water from the dorsal fins of "pushing" tuna was spotted portside. Ten minutes later as we trolled the area, Ed Neal of San Francisco parked himself into the fishing chair and pumped another school bluefin to the boat.

Liverman's son Donny deftly lifted the glowing blue and green fish abroard. In seconds Rushin had inserted the tag and the tuna was carefully plunked back into the Atlantic.

With a flash of cobalt blue, the big fish vanished - alive and kicking.