We spread it on sandwiches. Stuff it into hollowed tomatoes. Stir it into casseroles. It's the brown-bagger's reliable, the dieter's staple, the penny-pincher's ace-in-the-hole. And when all else fails and the cupboard is almost bare, it's the cook's salvation.

Americans reach for it so often that we down 700 million pounds a year--65 percent alone used for salads and sandwiches. That translates into 3.1 pounds of canned tuna for every man, woman and child in this country.

In some ways, we take tuna for granted--one can appears indistinguishable from the next. But there are small thought significant differences in those cans, and behind them a highly competitive industry which has experienced intermittent prosperity and slumps due to image problems and a fluctuating economy. Here is a look at America's favorite finned fare and the hard times on cannery row.

CANNED TUNA seems innocuous enough--not the type of food to cause family arguments or to strike fear into the heart of the Saturday morning shopper. But the tuna industry has suffered its share of public relations disasters. From boycotts to recalls, the industry has weathered a few storms in the past two decades.

Beginning in the early '70s, environmentalists and marine mammal advocates encouraged the boycott of light tuna--the cheap kind most commonly available at the supermarket. Beginning with the use of the purse-seine net in 1959, tuna fisherman had scooped up hundreds of thousands of dolphins and porpoises each year as they fished for the tuna. Tuna can usually be found running with the dolphins--which are naturally easy to spot as they surface to replenish their air.

Lots of bad publicity, a Marine Mammal Protection Act and a few court cases later, we have slipped into a boycott-free decade. Motivated by public outcry, the industry has discov-See ECONOMICS, E3, Col. 1 Against ---The Tide --ECONOMICS, From E1 ered ways to save the dolphins and porpoises, and the mortality has slipped to about 15,000 annually--the government-established quota allows no more than 20,500.

"The tuna industry has been doing quite well in coming in under the quota," said Sherrard Foster, marine issues specialist for the Defenders of Wildlife, adding that law requires the industry to continue a search for more effective ways to limit dolphin deaths. "There are many people who can't stand the thought of even one dolphin being killed." But, she added, there is little possibility of reaching zero mortality.

With the dolphin and porpoise problems behind it, the tuna industry now faces new concerns. In early June, a voluntary recall was announced by the FDA when the Bumble Bee tuna processing plant in Honolulu discovered tiny holes in the sides of several of its cans. The punctures, investigators found, were caused by broken wire baskets in the plant's pressure cooker. The recall, said FDA spokesman Jim Green, involves 40 million cans of tuna. So far, only five of the recalled cans have had similar holes, and no cases of illness or death have been reported.

Green predicts the recall will continue "for several months." It involves four brands: Bumble Bee (distributed nationwide), Cloverleaf (sold in the New York and Baltimore areas) and Mid Pacific and Coral (both sold in Hawaii). The cans range between 3 1/2 and 13 ounces in weight, and are stamped on the top with code numbers beginning with 21 or 22.

This recall comes on the heels of the botulism-related canned salmon recall, which, according to one industry spokesman, caused a drop in sales of all canned fish. The FDA is now working with the canned food industry to discover why recalls have increased recently (there have been 35 recalls in the last three years).

As if that's not enough, the industry has suffered a "glut" due to what U.S. Tuna Foundation spokesman David Burney refers to as a "backup in the system." Supermarkets affected by high interest rates have sought alternatives to stockpiling large inventories. Now, with the help of computers which constantly update inventory, grocers order only what they need. This has caused tuna to stack up at manufacturing plants. In turn, the industry has refused to buy the fish from the docks until it was sure of a market, which means that fishermen have had to cut back. As a result, said Burney, at least one canning operation has been forced to close.

The cost of harvesting tuna continues to climb, added Burney. Fuel costs increase frequently and the fishermen have to travel farther and farther to find the fish. Since nearby countries (such as Mexico) are enforcing their 200-mile limit, American tuna fisherman must fish the waters around Guam and American Samoa. Yet the price of canned tuna must remain competitive with hamburger and chicken or the consumer simply stops buying it.