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Bluefin Grading Records Show Quality Decline

August 01, 2005

Commercial Fisheries News
August 2005
By Lorelai Stevens
Found at: Commercial Fisheries News

DURHAM, NH While New England fishermen have been frustrated by the shortage of bluefin close to home in recent years, something even more insidious has been happening in the fishery – the quality of the bluefin they have managed to catch in the Gulf of Maine has declined significantly.

Scientists at the University of New Hampshire’s Large Pelagics Research Lab have for the first time documented the fat/oil content and shape of a large number of fish over a 14-year period.

“A lot of people in the commercial bluefin industry have said they’ve seen what has appeared to be a decline in the quality of the fish,” said Walt Golet, principal scientist in the investigation. “We wanted to take any available data and statistically determine if what they were seeing appeared to be happening.”

The results of that investigation have raised serious questions about the quality, abundance, and availability of bluefin prey and even the possibility of a major shift in the Gulf of Maine ecosystem.

“The quality decline implies either that the quality or quantity of prey has declined,” Golet said. “The fish are still feeding here but they don’t seem to be putting on the weight they used to.”

Detailed records

What made the investigation possible was the fact that Bob Campbell, manager of the Yankee Fishermen’s Co-op in Seabrook, NH and an experienced tuna grader, has kept detailed records of every single fish he has graded since 1991.

Those records, which are laid out in several spiral-bound and student notebooks, allowed Golet and his scientific collaborators to analyze 3,834 observations of bluefin fat/oil content and 3,082 observations of shape for fish landed from 1991 through 2004. Each of the observations was accompanied by the month and year in which the bluefin was caught.

The results of that analysis have been detailed in a soon-to-be submitted paper titled “Shape and Fat Content of Atlantic Bluefin Tuna in the Gulf of Maine.” Listed as authors are Golet, Campbell, and Andy Cooper and Molly Lutcavage of the large pelagics lab.

Grading system

Bluefin grading practices, which vary somewhat depending on who’s doing the grading, take into account many characteristics of each fish.

However, the researchers determined that two – fat/oil content and shape – were the best indicators of a fish’s overall condition. That’s because, unlike color and freshness, for example, those two characteristics cannot be altered by the type of gear used, the length of the fight, or at-sea handling practices.

The researchers assumed that fish with good shape – a “filled-out” appearance – and a high fat/oil content had been “feeding in excess of their daily metabolic demands,” which meant they were eating enough to put on weight. Those “without good reserves may be, at best, maintaining everyday metabolic requirements,” the paper said.

The fish in the investigation ranged in size from 73” to 120” curved fork length. In dressed weight, they ranged from a 119-pounder in extremely poor health to a 772.2-pounder. Campbell did all of the grading himself using the same grading scale for the entire 14-year period.

To avoid problems with small sample sizes in some of the categories, particularly marginal categories, the researchers “collapsed” Campbell’s 15-category scale for fat/oil into five categories. Proceeding from best to worst quality, the categories used in the investigation were “B+ or better,” B, B-, C+, and “C or worse.”

Likewise, while Campbell used 10 categories to evaluate shape, the researchers collapsed those into four – “B or better,” B-, C+, and “C or worse.”


The investigation showed that, among Campbell’s samples, early season fish captured in the Gulf of Maine during the month of June have been showing up in poorer condition than they did in the early 1990s.

For example, the researchers observed that only 30 percent of the fish landed in June 1991 were “C or worse” quality. By June 2004, the “C or worse” figure had climbed to 70 percent.

Even more striking were the results for the proportion of fish that fell into the C+ category later in the season – even after having several months to fatten up.

The researchers found that for August and September of 1991, 16 percent and nine percent of the fish were in the C+ category respectively. For August and September 2004, those figures had risen to 68 percent and 76 percent respectively.

“As would be expected with a proportions model, more fish entering the lower categories means the better quality fish are making up less of the total catch,” the researchers said. “What would be considered good-quality fish, such as B and ‘B+ or better’ individuals, now comprise less than one percent of the commercial catch at this particular co-op.”

The analysis also revealed significant decreases in the shape grades over time.

Possible causes

So what’s the cause of this “significant decline” in the quality of Gulf of Maine bluefin tuna? Clearly something has changed. But what that may be is not yet known.

The researchers put forward a number of possible causes.

A change in bluefin migration patterns. Different migration routes could mean the bluefin that wind up in New England are coming from farther away, which could account for the increase in poor-quality fish landed in the month of June.

However, the researchers noted that there is not enough tagging data at this time to support such a hypothesis.

A change in bluefin spawning areas and/or times. June fish could be in poorer condition if they had spawned closer to the feeding grounds than expected.

In their paper, the researchers cited a study being prepared by researcher Jen Goldstein that showed a third of bluefin sampled in the Gulf of Maine in June and July were capable of recent spawning. Furthermore, the study says, there may not have been enough time for the tuna to have reached New England from traditional Gulf of Mexico and/or Mediterranean spawning grounds.

Forage fish depletion in southern waters. If bluefin migration routes have not changed then maybe the availability of feed along traditional bluefin migration corridors has.

The researchers pointed to a study suggesting that small-mesh trawl fisheries targeting prey such as squid, silver hake, and butterfish may prevent bluefin from feeding on their preferred prey during their travels to northern waters.

Forage fish depletion in the Gulf of Maine. Bluefin can stay in the Gulf of Maine for up to six months, which points to changes in the forage base in this area since the bluefin here aren’t putting on fat like they used to.

Bluefin are known to eat both herring and sand lance. Herring is considered the best forage for bluefin because it has the highest lipid content and energy density of any prey tested in the region and is considered highly digestible. Studies suggest bluefin turn to sand lance when herring is not available. The bluefin may also be “skinny” because they are expending energy hunting for prey that is present in less dense quantities than in the past.

A problem with the quality of the available herring. Finally, the researchers suggested that if stock assessments are correct and herring abundance is high then maybe the quality of the herring itself has declined. They pointed to a study that documented significant changes in the “energy densities” of herring in adjacent Canadian waters.

Smoking gun?

Golet stressed that all of these hypotheses were just that and that more studies were needed, such as an investigation into the historical fat content of Gulf of Maine herring and other environmental factors.

“It’s all speculation,” he said of the possible causes. “We’ve documented that the problem exists. Now, we have to find out why the problem exists.”

In the minds of industry people who have argued that midwater trawling is causing localized depletion of herring stocks, the study simply confirms their point.

“You can look for exotic reasons for the dramatic drop in oil content, shape, and fat content over the last few years, but to me the obvious reason is that it coincides so closely with the development of the midwater trawl gear fishery,” said Rich Ruais, executive director of the East Coast Tuna Association (ECTA).

“This is the first piece of hard evidence we have to move our case away from the anecdotal,” he said. “The bluefin have to expend more energy looking for prey. That also explains why they don’t stay here longer. They have to move to richer feeding grounds.”

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