They’re digging tiny bones out of tuna heads to help set future quotas

PORTLAND, Maine — Heidi Nydan revved a jagged, bloody reciprocating saw blade to life on Tuesday morning behind a building on Commercial Street, then sent it slashing down the length of a giant bluefin tuna’s decapitated head.

One lifeless, golf ball-sized fish eye stared up at the gray sky as Nydan, an intern at the University of Maine Pelagic Fisheries Lab, finished her whirring cut. Half the tuna’s head then hit the pavement with a wet thud.

“This is so fun,” Nydan said, her face splattered with flecks of fish spray. “I still can’t believe this is what I do when I come to work.”

Nydan, and a small team of other workers from the Portland-based lab, spent the morning cutting open 30 donated fish heads, then extracting tiny, fingernail-sized bones from within them. The small bits can reveal a lot about each fish and will eventually inform federal and international policymakers who set quotas and other regulations in the future.

But it takes a lot of itty bitty fish bones to do that. Lab interns and scientists sometimes process up to 2,000 bluefin tuna heads in a single fishing season.

All of the heads are donated by local fishermen, fish dealers and fishing tournaments. On Monday, the team collected several heads at the Casco Bay Bluefin Bonanza , a South Portland-based tournament going on this week. The batch getting processed on Tuesday came from a dealer in Gloucester, Massachusetts.

“It sure is messy,” said goo-spattered intern Faith Flynn, working alongside Nydan, “but it’s a good way to learn.”

Flynn then poked a long pair of forceps into a fatty fish head cavity and came out with what she was looking for: a pair of bones less than an inch long.

“If you’re a product of the ’70s or ’80s and you remember the game ‘Operation,’ that’s sort of what it’s like. You got to reach in there and get those things very delicately and and pull them out,” said Walt Golet, an assistant professor at the University of Maine’s School of Marine Sciences who heads the lab.

Properly called otoliths, the structures are roughly equivalent to what’s inside a human’s inner ear. They help the fish stay balanced and upright during a lifetime of swimming in the sea. Bluefin tuna cannot pump water over their own gills and must keep moving at all times to breathe.

In the lab, the otoliths can tell scientists two very important things about the fish: its age and origin.

As the fish ages, its otoliths grow with it, leaving a series of distinctive markings not unlike a tree’s growth rings. In summer, when food is plentiful, tuna growth rings are wider. Conversely, in winter, they’re thinner. To tell a fish’s age, Golet simply counts the fatter summer rings.

“Bluefin, in the Gulf of Maine, range from 2 years old to over 30 years old,” Golet said.

By chemically analyzing other material in the otoliths, Golet’s lab can tell whether the fish was spawned in the Mediterranean Sea or on the North American coast. That’s an important distinction because bluefin can easily migrate thousands of miles in just a few months, and the two populations — west and east — are managed separately, though they both mingle in the Gulf of Maine.

“If you’re not accounting for the fact that a lot of eastern fish are coming into the western fishery, you’ll get a false sense of how well or how poor the western population is doing,” Golet said.

In addition to the otoliths, the UMaine Fisheries Lab also takes muscle samples from the fish heads for later genetic testing. Additionally, they take tape measurements along the severed heads and extrapolate the fish’s original length.

The lab team also collects stomachs. Because bluefin don’t chew their food, it’s easy to tell what they’ve been eating. Golet’s team has seen the bluefin’s main diet change over the years, depending on what prey fish are plentiful, going from Atlantic herring, to squid and now to Atlantic menhaden, known locally as pogies.

All the data eventually gets passed up to regulators, including the National Marine Fishery Service and the International Commission for the Conservation of Atlantic Tunas, who use it to devise fishing quotas for them and the smaller fish they eat.

“You can kind of think of it like baking a cake,” Golet said. “In order to get the cake right, you have to have the right ingredients. These are the ingredients that go into the assessment.”

For now, bluefin tuna are doing OK, Golet reckons. Their numbers are doing better compared to five or six years ago.

“Whether or not that stays is anybody’s guess because it’s Mother Nature and populations go up and populations go down,” Golet said. “But right now, bluefin are doing quite well. People are catching them off New York on striper gear from the beach.”