For the sushi-savoring diner who bites into a raw slice of bluefin tuna belly meat, the world may seem perfect for a moment, and at Zenbu Sushi Bar and Restaurant in La Jolla, patrons reverently devour three to five pounds of the rich, buttery flesh, or toro, per night.

What many diners probably don’t know is that for each pound of farm-fattened bluefin tuna that hits sashimi platters, roughly 25 pounds of wild sardines were devoured, a “fish-in-fish-out” ratio that environmentalists say is unsustainable. But in the close vicinity of Ensenada, Baja California, where a sizeable bluefin aquaculture industry has developed over the past 12 years, coastal communities have prospered with more and better jobs and boosted local cash flow, say industry advocates. They maintain that wild tuna stocks from which the Baja ranches acquire their fish are currently sustainable and healthy and that responsible fishery management will ensure a safe future for the species and for the industry.

Some marine biologists agree — but only, that is, if the Mexican federal government keeps the tuna-ranching industry from growing any bigger.

“We feel the present size of the industry is sustainable in terms of feeding the tuna, but we don’t believe it would be wise to cultivate increasingly more fish,” said José Zertuche-González, professor of marine botany at the Autonomous University of Baja California in Ensenada and a coauthor of a 96-page industry report released last year. “We’ve warned the government and stakeholders against growing this industry.”

In fact, the federal government has already placed a cap on the industry’s growth, though several of the tuna ranches in Baja have not yet reached their legal production limits, and annual tuna output is liable to continue growing. Already, the Baja bluefin business, hardly a decade old, produces about 10 percent of the world’s annual bluefin tuna consumption. The tuna are netted in the summer months by Mexican purse seiners as the fish, mostly 20- to 80-pound juveniles, migrate past the western coast of Baja California, usually within 100 miles of the shore. The tuna are not killed, however. Rather, they are carefully kept from harm — whether slight bruising or the jagged mouths of mako sharks — as crews usher them from the nets into floating corrals towed by powerful vessels. These boats proceed to transport the entire catch toward the coast at a rate of one knot before transferring them to specialized cages anchored several hundred yards from shore.

Here, the bluefins are fed and fattened for months on a diet of mostly sardines, eventually attaining the oil content that makes the belly meat of a well-fed bluefin the 24-karat gold of the seafood business. At slaughter time, carefully trained workers, including divers, gently remove the tuna one fish at a time and bring them onboard a platform vessel. The animals are deftly killed, gutted, bled, and placed in near-freezing water — all within 45 seconds of capture. Approximately 4000 tons of bluefin tuna undergo this process each year, and upwards of 90 percent go to Japan, where the fish may pull thousands of dollars each.

At such rates, those in and around the Baja industry are poised to profit — and, according to last year’s report, the industry has created more than 1000 jobs on Mexican vessels and in the Ensenada processing plants and as many as 3500 indirect jobs in nearby communities. As for cash, Ensenada alone has reportedly seen between $70 million and $100 million enter its economy annually in recent years.

But even those who believe in the industry’s merits acknowledge the disapproval of environmentalists.

“These tuna have a very high metabolism,” said Zertuche-González. “They need a lot of protein to survive compared to other fishes, and there is concern that we’re giving too much protein to a fish that will only be sold in gourmet restaurants.”

At Nozomi Noodle and Teriyaki in Kearny Mesa, many customers gladly pay between $9 and $14 per ounce of Baja bluefin toro, either raw or lightly seared. Owner and chef Chris Chung buys the flesh for $30 to $40 per pound from a local importer, and he reports that popularity among diners is growing. Chung says he serves more than he used to and now runs through several pounds of toro every night.

The executive sushi chef at La Jolla’s Café Japengo, Jerry Warner, prefers to use wild Atlantic bluefin; the oily toro from ranch-fattened fish tends to go rancid sooner, he says. But Matt Rimel, owner of Zenbu, feels the culinary quality of Baja bluefin is excellent. His customers pay as much as $18 for a slight morsel, and diners are clearly gaining a liking for the soft yet strong-tasting meat, says Rimel, who buys his tuna whole. Only about 20 percent of the meat can be classified as toro; the rest is bright red flank muscle and appears on Zenbu’s menu as “local bluefin.”

But Elena Rivellino, co-owner of Sea Rocket Bistro in North Park, isn’t entirely convinced of the sustainability of bluefin tuna aquaculture and has been reluctant to include bluefin on Sea Rocket’s menu. The restaurant does, however, serve local Pacific sardines.

“The further down on the food chain you eat, the more available the resource is and the more you can afford to take,” Rivellino said. “Sardines are great, and you can eat the whole thing. The bones are perfectly edible, and some people eat the head and tail. There’s almost no waste.”

Nor does Casson Trenor vouch for ranch-reared bluefin tuna. The author and fisheries-sustainability expert says the leading problem is the bluefin tuna’s rapacious metabolism and the industry’s demand for sardines. Since 2000, the Baja tuna ranches’ consumption of the sardines that are landed in Ensenada has increased sixfold to its current level of more than half the annual catch.

The tuna-ranching industry “is an incredible waste of protein,” said Trenor, whose new book — Sustainable Sushi: A Guide to Saving the Oceans One Bite at a Time — just hit shelves at Borders and at the UC San Diego bookstore. Page 16 discusses the bluefin ranching industry, which is expanding around the globe. “Sardines are high in omega 3s. They’re almost empty of mercury and PCBs. They grow very quickly and reproduce in large numbers. They’re an excellent, excellent resource, and to turn them into a luxury protein that only the very rich can afford is an insult to the ocean.”

But ranching advocates, including Ted Dunn, cofounder and manager of Maricultura del Norte, the largest of the bluefin-producing companies in Ensenada, point out that their tuna would be eating sardines whether in cages or in the wild. Meanwhile, the industry’s economic boon to Baja is undeniable. Maricultura del Norte produces some 1200 tons of whole bluefin tuna each year, says Dunn. The company owns a pair of 200-foot-long tuna purse seiners, leases other vessels, and manages 16 tuna cages while employing as many as 200 people at a time. Moreover, Maricultura del Norte pays sardine fishermen three to five times what they would otherwise receive from bait buyers, fishmeal processors, and restaurant wholesalers.

However, overfishing of Pacific bluefin tuna could become a serious issue, say some biologists. With the Atlantic bluefin tuna fishery on the verge of collapse, more and more nations are turning their attention toward Pacific stocks of the fish. University of Rhode Island biologist Barry Costa-Pierce, who helped lead the 2008 industry report with Professor Zertuche-González, acknowledges that Pacific bluefin tuna could be looking at the same dire fate that has met its Atlantic counterpart, the population of which has diminished to an all-time low.

“We have reason to believe that the Pacific bluefin is on the same trajectory as Atlantic bluefin tuna,” said Costa-Pierce.

For one thing, the average bluefin tuna caught near Baja seems to be smaller today than in years prior. Mexican purse-seiner and sportfishing catch records show a decline in average individual fish size, from 115 pounds in 1995 to just 30 pounds in 2005.

Pacific sardine populations are believed to be in better shape. Although estimated sardine biomass along the U.S. west coast has declined slowly since 2000, according to Kevin Hill, research fishery biologist with the National Oceanic and Atmospheric Administration’s La Jolla office, sardine abundance is known to wax and wane by natural oceanic cycles, and overfishing of Pacific sardines is not believed to be a concern. But with bluefin ranches as far away as Australia now purchasing up to two-thirds of the sardines landed along the United States’ west coast, availability issues could arise.

Maricultura’s Dunn would like to see a government-enforced cap placed on annual tuna harvest, as well as a minimum size limit, even though that wouldn’t prevent pre-spawners from entering the production line as they do now; the Baja bluefin industry subsists on sexually immature juveniles just one or two years old. Bluefin tuna become sexually viable at four or five. Dunn and his business partners would also like to see the federal government illegalize the canning of bluefin tuna, which still occurs on occasion, he says, when the tropical yellowfin tuna fleet strays northward into bluefin territory.

“When you have a fish that you can sell for $20 per pound, why put it in a can and make $1?” Dunn argues. “We want to make that illegal so that people can make more money by catching fewer tuna.”

That could benefit fishermen and help conserve tuna. Still, toro will continue to be served on platters by the ounce, and this is one fishery that will almost certainly never feed the poor. Just how long it can continue to feed the wealthy remains a matter unknown.