For all the tuna Americans consume, most of us will never eat anything but the deep, red fillet cut off the bone into a steak or finger-size slabs for sushi, or into chunks stuffed in a can. Flaky, mild, meaty, uniform, and inoffensive, tuna does indeed qualify as chicken of the sea.

Of course, there are other edible parts of the fish, popular elsewhere if not here. Southern Italians consume dried sacs of tuna sperm called lattume as well as the salt-cured roe sacs, called bottarga, neither of which I've ever seen served around here with both the supply and the demand for them presumably low.

Perhaps the most overlooked and the most delicious part of that fish is the collar, a popular dish in Japan. The collar is a horseshoe of bone, fat, and flesh just below the gills that separates the head from the prized fillet. The collar is often the part a fishmonger throws away with the tail, the head, and innards. But some consider the meat contained in the collar to be the most delicious and unique part of the tuna.

Few Japanese restaurants in Seattle serve collar, or kama. Shiro's in Belltown and Nishino in Madison Park serve the collar of yellowtail— often confused with tuna, it is actually a species of amberjack — for $12. Yellowtail collar is also served downtown at Wann for $8. Shiro's also serves salmon collar for $9.50. Both are smaller than tuna.

I found only one place, Issian in Wallingford, that serves tuna collar, a giant dinner-plate sized kama for a mere $7.80, which might just be the biggest seafood bargain in the city.

Issian's maguro no kama is wider than your head, lightly seasoned, and grilled on a hot stone slab, a cooking method called ishiyaki that is the specialty of Issian. The tuna collar I ate looked to be from a fish that weighed at least 50 pounds.

The meat was rich, dark, extremely moist, and some of it required careful extraction from the curved, narrow structure it was attached to. There is a surprising amount of meat from the collar, at least one that big. It was an amount of meat that qualified as two generous entrees. But it is the quality, not the quantity, of the meat that makes kama special. Fatty, and close to the bone, it has a texture and succulence more like stewed pork than fish.

The Japanese typically season and serve their fish simply. Issian's kama comes with two lemon wedges and a small pile of grated daikon doused with soy sauce. Dining on tuna collar also comes with a modest reward for your conscience, the knowledge that you have consumed a throw-away cut that might otherwise go to waste.

"If you're going to have a fish that has landed in the market already," said Ken Peterson, spokesman for the Monterey Bay Aquarium and its Seafood Watch program, "eating more of it once it's caught is the more responsible way to go because you're not wasting some portion of it."

Peterson gave the endorsement conditionally because tuna is a worrisome species of fish for Seafood Watch, which researches and advocates sustainable forms of fishing and seafood consumption. Some tuna fisheries are well managed, some are not, Peterson said. Some methods of harvesting tuna, such as trawl nets, are more damaging to the marine environment and to other species of fish than more selective forms of fishing like trolling.

As an alternative, aquaculture also is not perfect; it creates artificial demand for supplies like fish meal, which can decimate other fisheries. These are all factors Seafood Watch considers when it makes its recommendations to consumers and to the seafood and restaurant industry, grading certain species as "best choice" or "good alternative."

Some species of tuna, like the bluefin, are so overfished, Seafood Watch simply recommends people not eat them and avoid the high-end sushi restaurants that serve it. It is not a difficult type of tuna to spot on a menu. If a restaurant serves bluefin, it will be unusually expensive. Most of it is consumed in Japan; in the U.S., it, and to some extent Chilean seabass, have become the poster child of overfishing for marine conservationists.

Seafood Watch refers to bluefin as the Porsche of the ocean because it accelerates just as quickly (zero to 60 in five seconds), is just as expensive with larger fish fetching prices close to $100,000 at market, and is almost as big, weighing more than 1,000 pounds. The vast majority of tuna caught and consumed in the world is skipjack, albacore and yellowfin (not to be confused with yellowtail); Issian serves yellowfin tuna.

The calculus used to grade the suitability of tuna for conscientious consumption is particularly complex because there are so many species harvested in different ways, and in different waters. Seafood Watch is unequivocal about bluefin and similarly discourages consumption of tongol tuna with a few exceptions. Albacore, skipjack, yellowtail and bigeye tuna, are generally approved for dining by Seafood Watch, but each comes with a caveat.

Those species of tuna should be avoided, Seafood Watch says, if it was caught using a purse seine (a giant net that closes around a school of fish) or a longline, which is baited with many hooks (some stretch for miles and contain thousands of hooks). Some longline fisheries, however, like those in Hawaii or the Atlantic, are given a pass because of protective regulations and precautions fishermen there take against by-catch, the incidental capture or killing of unintended species like porpoise, sea turtles, or sea birds.

Also making the grading of the tuna fishery complex is the fact that tuna is caught and ranched all over the world, mostly in the equatorial region of the Pacific Ocean, but also in the Indian and Atlantic. The fish is a popular choice for canning and eaten by most cultures. The surging popularity of sushi has also significantly increased demand. While eating the lesser or leftover cuts of tuna is not the salvation of the fishery, it does help.

"If this fish was brought to you in a sustainable manner," said Peterson, "yeah it's nice if you're using every part of it. Not wasting is a nice to-do."

But using more of a particular fish is just part of blunting the impact of eating seafood. Another is eating small species like sardines, anchovies, mackerel, and shellfish.

"We talk a lot about not eating at the top of the food web all the time," Peterson said. "The carbon footprint of an oyster is many orders of magnitude less than the footprint of beef that goes into hamburger. Smaller fish, like herring, taste wonderful when the right chef is preparing them. If you're eating those fish, it's a more productive way of taking wildlife out of ocean than trying to catch swordfish… America likes the tuna steak, the big piece of salmon. That's something you need to think of as a special-occasion dish, not an everyday dish."

Eating all the lesser cuts and throw-away parts of fish is a philosophy the aquarium also wants to see extended to land animals because the use of terrestrial resources ultimately affects the health of oceans through run-off and erosion.