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Through all the booms and busts and next-big-things of the aquaculture industry, one species has remained in steady supply -- rainbow trout. In most major markets, farm-raised rainbows are as reliable a daily staple as chicken, and they may appear in nearly as many forms.

Whole trout, still the most common form, range in size from single-serving fish of 8 ounces or less to 3-pounders perfect for the poaching pan. For those who want additional convenience, trout processors now offer a variety of other product forms, including boned or "butterflied" whole fish (the two fillets attached by skin along the backbone, with the center bone and ribs removed) and separate fillets. At least one processor now offers "guaranteed boneless" fillets, with all fins removed and the row of pin bones cut out of the middle of each fillet.

As with other domesticated animals, the breeding and culture practices that have made trout so readily available have also bred some of the wild flavor out of this fish. The challenge in serving trout is to preserve and enhance its delicate flavor. Too heavy a hand with seasonings and you overwhelm the flavor of the fish; too subtle and it can come out bland. The recipe given below, named after the city of Grenoble in the French Alps (an area noted for its trout), relies on the tart and salty tang of capers and lemon together with the nutty flavor of browned butter.

Native to western North America and northeast Asia, the rainbow trout (Oncorhynchus mykiss) may be the world's most widely transplanted fish species. Wild populations have been established, mainly for sport purposes, in many of the temperate parts of the world, from northern Europe to New Zealand, Tasmania and the temperate parts of South America and South Africa. It's proven just as adaptable as a farmed fish; in fact, most of the world's production comes not from the United States but from northern Europe, particularly France, Scandinavia, and the United Kingdom. The U.S. ranks second in world trout production at more than 27,000 metric tons per year (roughly 50 million fish). And approximately 80 percent of that comes from one state, Idaho. Most of Idaho's farms are located near the Snake River, where there is an abundant supply of clean, cool water. An extensive aquifer gathers runoff from snowmelt on the Snake River Plateau and the surrounding mountains, transporting it through the rock and releasing it in springs that feed into the river. Some of this water is diverted into ponds and raceways (flow-through channels) where the trout are raised from hatchery fingerlings to market size.

Trout farms are also found in many eastern states, especially West Virginia, home to the U.S. Trout Farmers Association. Most of these are smaller operations, often just a few tanks of well or spring water on a farm that also produces other crops or livestock. Some Appalachian trout farmers have found another ready source of cool, clean water in the region's many abandoned coal mines. While a mine is in operation, water that seeps in must be continually pumped out, but when pumping ceases, mine shafts become wells, gradually filling up with groundwater to form underground reservoirs. Pumped to the surface, this water turns out to be in the ideal temperature range for trout farming.

In addition to different forms, trout farmers offer fish in different colors. In the wild or on the farm, the color of a trout's flesh depends on its food supply. A naturally occurring pigment called astaxanthin, found in many crustaceans, accumulates in the flesh of salmon and trout that eat them, and this pigment is the source of the orange-red color typical of salmon. Wild rainbow trout in fresh water eat a mixture of insects and small crustaceans, which gives the meat a light pink color. Their seagoing cousins, salmon and steelhead (the latter a rainbow trout that has migrated to the ocean), eat a higher proportion of crustaceans, mainly small shrimp and their smaller relatives called krill, and have resultingly darker orange meat.

The vast majority of farmed rainbow trout get a diet based on grain and fish meal, and they have pale-colored meat that cooks up to an ivory color. But if you feed them salmon feed, which includes a synthetic form of astaxanthin, the meat takes on a typical salmon color, and to my taste, a slightly fuller, more salmonlike flavor as well. Most Western trout farms now produce at least part of their crop in the salmon-colored form, sometimes labeled "steelhead" even if they have never seen salt water.

Skin color, too, can vary. The most striking example is "golden" rainbow trout, raised by a few farmers. This is not the same as the wild golden trout, a separate species native to a few streams of California's Sierra Nevada, but simply a color strain of domestic rainbow trout. By crossing and back-crossing descendants of a single golden-colored mutant, breeders have created a true-breeding stock of trout with the characteristic lengthwise rainbow stripes of normally pigmented rainbow trout on a brilliant golden-yellow skin. The result is an especially pretty fish that stands out in a retail display (and usually sells for a bit more). All the examples I have tasted were also raised on pigmented feed, and compare in meat color and flavor with other red-meated trout.

Whatever the color, I always cook trout with the skin on; it helps hold the meat together, as well as providing flavor (most of which is in the thin fat layer just under the skin). I also like to eat the skin -- the tiny scales are entirely edible -- but some prefer to leave it behind on the plate.

Rainbow trout is usually identified in older books by the Latin name Salmo gairdneri, part of the same genus as Atlantic salmon and various Eastern trouts. Earlier in this century, rainbow trout was found to be nearly identical to the Kamchatka trout of far eastern Russia, then known as Salmo mykiss. More recent studies suggested that both of these fish, as well as other Pacific slope trouts, have important anatomical and behavioral traits more in common with the Pacific salmons (genus Oncorhynchus) than with other Salmo species. In 1989, ichthyologists Gerald R. Smith and Ralph F. Stearley proposed to reclassify both rainbow and Kamchatka trout as a single species, O. mykiss, and the American Fisheries Society has adopted that usage.

Sauteed Trout Grenobloise
serves 2
If cooking butterflied trout, 2 servings is about all you can manage in a skillet at one time; to double the recipe, you might try 4 fillet portions from larger fish.

2 8-ounce boneless trout, butterflied, or 2 trout fillets, about 6 ounces each
1 tablespoon oil or clarified butter
Pinch of salt and white pepper
2 tablespoons unsalted butter
Juice of 1/2 lemon
2 teaspoons capers, drained

Heat the oil in a large skillet over medium-high heat. Season the trout lightly with salt and pepper and add it to the pan, skin side up. Cook until lightly browned, about 3 minutes, turn, and continue cooking until the tail meat begins to flake. Remove to a serving platter or individual plates. Let the pan cool slightly and add the butter, lemon juice and capers. Brown the butter slightly and pour the sauce over the fish.

Jay Harlow is a Bay Area cookbook publisher and author of ten books including Once Upon a Bagel, The California Seafood cookbook & Beer Cuisine.