Bluefin Tuna, Thunnus thynnus.
Orphaned image [larger image].
The western Atlantic Bluefin Tuna fishery in the Gulf of Maine is in danger of collapse, according to University of New Hampshire (UNH) researchers. Further, the team found that the number and quality of the captured fish has declined markedly in recent years.
Using notes collected by veteran tuna grader Robert Campbell from the Yankee Fisherman’s Co-op in Seakbrook, New Hampshire, Walter Golet led a team of marine biologists that analyzed the quality of 3,082 Atlantic bluefin tuna, Thunnus thynnus.
“In a drawer, he had two or three notebooks with every fish he graded in the last 14 years, from 1991 through 2004,” said Golet, who is a PhD candidate in UNH’s Large Pelagics Research Lab.
Tuna are awarded different grades, with A being the highest grade possible, based on their fat and oil content and on their overall body shape.
According the team’s analysis of fat and oil content, the probability of landing a C+ fish in 1991 was 16 percent and 9 percent for August and September, respectively. However, by 2004, the probability of tuna being graded in the C+ category for August and September increased to 68 percent and 76 percent, respectively (Figure 1, below).
“Fat content is in high demand for the market, because that’s what makes the meat taste good,” stated Golet, noting that fish with well-marbled tail meat, fat in their mid-section muscle and belly, and a rotund shape can command upwards of $50 per pound on the sushi market.
The team also found that bluefin tuna are leaner when they arrival in the Gulf of Maine (figure 2, below). The probability of catching a fish with a lean body type (grade C or worse) in June 1991 was 30% compared with 70% in 2004. Good quality fish, those with rounded bodies that are graded as B or better, now comprise less than one percent of the commercial catch at the Yankee Fisherman’s Co-op.
Unfortunately, the team’s findings confirm numerous observations by fishermen and fish brokers throughout the years that there are fewer tuna being caught in the western Atlantic and the captured fish have poorer body condition, which makes them fetch lower prices on the open market. However, this decrease in overall body fat is more important than simply worrying about the taste of the fish: it also has important implications for reproduction and for the long-term survival of the species.
“One of the big consequences of not fattening as much is the potential impact it could have on reproduction,” Golet explained. “Reduced energy stores can often force a fish to skip spawning in a particular year.”
Bluefin tuna are a highly migratory pelagic species whose spawning grounds are located in the Gulf of Mexico and in the Mediterranean Sea. Due to their migratory ways, they have high energy demands so this reduction in fat stores could affect their migration patterns. Even though the bluefin tuna’s seasonal movements are poorly known, there is mounting evidence suggesting that their migration is more complex than once thought. For example, many bluefin tuna may be traveling long distances to reach the Gulf of Maine. In fact, some tagged fish were discovered to have traveled from the central and eastern Atlantic Ocean, which results in far greater than expected energy demands because they are swimming greater distances against stronger currents.
Further, UNH researchers documented a dramatic decline in overall numbers of the fishes: in the mid-1990s, they recorded between 500 to 900 schools of bluefin tuna in the Gulf of Maine, each averaging 100 to 150 fish each, said Molly Lutcavage, a co-author of the study and director of UNH’s Large Pelagics Research Center. She said only a “few” schools would be seen in today’s waters.
What is causing this dramatic decline in bluefin tuna numbers and quality? Many on the Western side of the Atlantic claim that overfishing by Eastern Atlantic and Mediterranean fisherman is responsible for the decline. The Western Atlantic’s bluefin quota is only about 2,500 metric tons per year, while the Eastern’s is more than 10 times that, with many fisherman catching even more fish than that, Lutcavage said, decreasing populations stateside. But the evidence suggests that there are likely other factors contributing to the tuna’s decline.
For example, even though stock assessments of herring, which comprise the bulk of the tuna’s diet, show that the populations of this fish are at historically high levels, the herring might have experienced a decline in quality as well, or maybe they dispersed into smaller schools, thus requiring a greater energy output to locate. There also is the so-called “junk food hypothesis” that proposes that changes in tuna diet from high-energy food like herring and mackerel to less energetic species like haddock or sand lance results in lower caloric intake, and thus, poorer body quality as well as lower survival rates in tuna. Irregardless, the source of the tuna’s problems is not immediately obvious.
“I’m very convinced that it’s multiple factors working with each other,” observes Golet.
Walter J. Golet, Andrew B. Cooper, Robert Campbell, Molly Lutcavage (2007). Decline in condition of northern bluefin tuna (Thunnus thynnus) in the Gulf of Maine Fisheries Bulletin, 105 (3), 390-395 [free PDF].
University of New Hampshire press release (quotes).
You should also read this story that describes how 90% of the oceans’ large fishes have disappeared.