Research scientists are tipping the scales on wild, sea-run Atlantic salmon. That is, they are tipping them onto microscope slides to better understand why the population of this endangered species keeps declining even as harvesting them – either commercially or recreationally – has been banned for decades.

Let me clarify a few points up front to avoid upsetting fishermen or confusing eaters. First, landlocked salmon, a freshwater relative in the Salmo salar family, native to a handful of Maine lakes and stocked in many other lakes, rivers and streams statewide, are still fair (and legal) game for recreational fishermen. Secondly, the Maine-sourced Atlantic salmon on your plate – and the one-pound piece of fillet on my cutting board – almost certainly comes from an operation run by Canadian-based Cooke Aquaculture Inc., where it was raised to market size (about 11 pounds) in netted pens in Machias Bay.

And, finally, Dr. Kathy Mills, who heads up the Gulf of Maine’s Research Institute’s Integrated Systems Ecology Lab, and her team are neither weighing in on the environmental issues surrounding salmon farming in Maine nor commenting on the legislative efforts to regulate that industry at this point in time.

They are simply examining scads of information about physical fish scales extracted from sea-running salmon as they have made their way back to Maine rivers to reproduce over time. Since salmon are anadromous fishes – they spend the early part of their lives in rivers, move into the ocean for one to three years to mature, and then return to their natal rivers to spawn. The consistent decline in populations in multiple rivers in Maine indicates something is happening during the salmon’s time at sea that makes it harder for them to survive.

Fishermen and researchers have been collecting wild salmon scales for a long time, taking note of when and where they were collected. Using a high-powered microscope, researchers can now see the mineralized ridges, or circuli, that salmon deposit on their scales as they grow. The pictures I’ve viewed of these scaled-up scales resemble a cross section of tree trunk crossed with a thumbprint. Unlike trees that produce one growth ring per year, fish scales seem to record multiple circuli per year of life. An international body of researchers comprising folks from the Maine Department of Marine Resources, NOAA-Fisheries in the United States, and Department of Fisheries and Oceans in Canada, have archived the images of scales collected over time and organized information about them into databases.

Mills’ team is analyzing this data. She says the distance between a scale’s circuli (measured with precision by computers) can tell a lot about a particular fish. When the rings are further apart, that means the salmon experienced a fast period of growth. These data points can help reveal how long a particular salmon spent at sea, how long it took to mature and how much it grew compared to other members of its cohort when it returned to its natal river to spawn.

Mills’ team works to link growth patterns, information about factors that may influence growth (such as changing environmental conditions, access to nutritious zooplankton and capelin, and migratory patterns) and Atlantic salmon marine survival rates.

For example, Atlantic salmon prefer ocean temperatures between 4-8 degrees Celsius and grow more slowly at the lower, colder end of that range. More narrowly spaced circuli were probably formed, then, during the colder winter months or during periods of poor prey availability. Circuli spaced further apart likely align with warm summer months and favorable feeding conditions.

The more we can understand how changes in the marine ecosystem are affecting Atlantic salmon growth, the higher the likelihood scientist will be able to pinpoint the barriers to a wild Atlantic salmon comeback and identify conservation and management efforts needed to further protect the species.

Mills says her team works closely with NOAA’s Northeast Fisheries Science Center and researchers there participate in several Atlantic salmon-focused international science and management bodies that develop the scientific advice for managing wild Atlantic salmon and then adopt regulations based on that advice.

“Our research is pointing towards marine ecosystem change as a strong driver of Atlantic salmon population declines,” said Mills. There are few measures international regulators can take to control ecosystem change other than continuing to control salmon fishing at sea, but local and state regulators can work to reduce the number of dams salmon must pass and improve in-river habitat conditions to maximize the number of wild healthy juveniles swimming out to sea.

“Our findings can also help management bodies at all scales (domestic and international) by providing expectations for salmon productivity into the future,” says Mills. Just to level set expectations, while she’d love to see a viable wild Atlantic salmon fishery come back to Maine, she’s not optimistic that will happen. “Ocean conditions have changed substantially in ways that are unfavorable to Atlantic salmon, and climate projections indicate these changes will continue into the future.”

Considering those projections, salmon lovers in Maine can choose farmed salmon, fly in wild caught Pacific salmon from Alaska, or employ adaptable seafood recipes that can make use of the species local fishermen are reeling in.

Christine Burns Rudalevige adds peppers to Curried Seafood and Sweet Potato Stew. Ben McCanna/Staff Photographer

CURRIED SEAFOOD AND SWEET POTATO STEW

This recipe is adaptable to what seafood is available, which vegetables are in season, and how spicy you want to make it. For the rendition photographed, I had salmon and scallops in the freezer, chose sweet potatoes for substance and red peppers, green beans and purple carrots for color, and ran with Thai yellow curry paste, the milder alternative to Thai red and green curry pastes.

Serves 4-6

1 small bunch cilantro (15-20 stems)
2 tablespoons vegetable oil
1 onion, peeled and finely chopped
1 tablespoon chopped fresh ginger
1 teaspoon chopped garlic
2 tablespoons yellow curry paste
1 large, sweet potato, scrubbed and cut into ½-inch cubes
1 (14-ounce) can coconut milk
3 cups fish or vegetable stock
1 tablespoon brown sugar
1 sweet red pepper, cut into 1-inch pieces
½ pound fresh green beans, trimmed and cut into 1-inch pieces
2 carrots, peeled and sliced
1 ½ pounds fish or shellfish of your choice, cut into large chunks
2 limes
1 serrano chili, sliced
White rice or soba noodle, to serve

Remove cilantro leaves from their stems. Tie the stems together with a piece of kitchen twine. Finely chop the leaves. Set both aside.

Heat oil in a large heavy-bottomed pot over low heat. Add onion and cook until softened, about 5 minutes. Add ginger and garlic and cook for 1 minute. Stir in curry paste and cook for 1 minute. Add sweet potatoes and toss together well. Add coconut milk, fish or vegetable stock, brown sugar and tied-up cilantro stems. Bring curry to a simmer and cook for 8 minutes until the sauce is thickened and the sweet potato pieces are almost tender. Fish out the cilantro stems and compost those.

To the pot, add red pepper, green beans and carrots. Bring curry back to a simmer. Gently place fish pieces into curry, making sure they are covered with sauce. Cover the pot, turn off the heat, and allow the residual heat in the curry sauce to gently cook the seafood until it is opaque, 4-5 minutes.

Stir in zest and juice of 1 lime and the chopped cilantro. Add sliced serrano chili to taste. Serve with rice or noodles and wedges of lime.

Christine Burns Rudalevige is a food writer, recipe developer, tester and cooking teacher in Brunswick, and the author of “Green Plate Special,” a cookbook from Islandport Press based on these columns. She can be contacted at: cburns1227@gmail.com

Curried Seafood and Sweet Potato Stew. Ben McCanna/Staff Photographer