Alewives threaten Champlain salmon restoration
Invasive species causing a thiamine deficiency in salmon, hindering their ability to reproduce naturally
U.S. Fish and Wildlife Service biologist Zach Eisenhauer holds 11-pound salmon he trapped on the Boquet River during a fish survey.For years, biologists have worked to improve conditions for the native fish in Lake Champlain. They’ve removed old dams to help spawning salmon migrate up rivers and reduced the population of sea lampreys that prey on salmon and lake trout.
Now scientists are trying to fully understand how salmon are impacted by alewives, an invasive species that has become one of the main sources of food for salmon.
Alewives were first discovered in Lake Champlain’s Missisquoi Bay in 2003. Since then, their numbers have skyrocketed. They’ve replaced native rainbow smelt as the main forage fish for Lake Champlain’s predators, and they’re likely here to stay.
“There’s not much we can do to manage alewives,” said Lance Durfey, a fisheries manager with the New York State Department of Environmental Conservation. “They are going to do what they are going to do, and the fish are going to use them as a prey base.”
Alewives, which are found from Newfoundland to North Carolina, are a type of herring that grows up to fifteen inches in length. They dwell in the Atlantic Ocean and spawn in coastal rivers. They are categorized as a “species of concern” (slightly at risk) by the National Marine Fisheries Service in their native habitat, but their numbers are high in many lakes where they aren’t native.
Lake Champlain alewife die-off
Alewives have been spreading to inland lakes, particularly the Great Lakes, for decades. They occasionally die in large numbers when spawning in non-native waters in the spring. At times, the shores on Lake Champlain have been lined with hundreds of the dead fish. Reasons for these die-offs may include weakness from a lack of winter food and the temperature shock of moving from deep, cold water to warmer spawning waters.
Lake Champlain alewife die-off
Lake Champlain is stocked with salmon, but has not had a self-sustaining population for two hundred years. Scientists are trying to nurture the population back to health, but alewives undermine those efforts by interfering with the reproductive cycle of salmon.
Alewives posess an enzyme that kills thiamine, or vitamin B1, in fish that prey on them. Since salmon consume a lot of alewives, scientists say, they end up with a deficiency of thiamine, or vitamin B1. As a result, salmon have trouble reproducing and maintaining a population naturally.
Salmon that eat alewives may grow large and appear healthy, but a shortage of vitamin B1 in their eggs leads to problems for hatchlings.
“They have development abnormalities associated with the low vitamin B1, and they can’t orient very well in the water column and they get very lethargic,” said Bill Ardren, a senior biologist with the U.S. Fish and Wildlife Service. “It can cause really high mortality.”
Ardren said the deficiency can be overcome in hatcheries by bathing salmon eggs in a thiamine solution for thirty minutes.
Ardren also said there is evidence that low thiamine levels impact spawning adult salmon. A recent study on the Boquet River in New York showed that spawning salmon injected with thiamine were more persistent in attempts to get up cascades to their spawning ground than those salmon injected with water, which acted as a placebo.
Some salmon may be able to overcome the thiamine deficiency and the species may evolve to cope with it. Last summer college students found evidence that salmon were reproducing in the watershed.
He also said that scientists have noticed a lot of “redds,” places where salmon lay eggs in gravel, in the Winooski River in Vermont and the Boquet River. “But we are not seeing as many fry [young fish] come out of those redds as we would expect,” Ardren said.
That could be a consequence of the vitamin B1 deficiency. Or it could be the result of another problem yet undiscovered. That seems to be the case with lake trout, another large predator that eats alewives, according to Ellen Marsden, a professor at the University of Vermont at Burlington, who has studied the lake trout in Lake Champlain for twenty years.
She recently challenged the longstanding theory that alewives are interfering with the reproduction of lake trout, noting that it had been tested in hatcheries but not in the wild. “Our new hypothesis is that lake trout could get plenty of thiamine in their diet to make up stuff they are missing in the wild before they need it,” she said.
She said lake trout may get enough thiamine from zooplankton, a type of alga, soon after emerging from eggs, offsetting the deficiency at birth. She noted that the number of juvenile lake trout in the lake seems to have increased in the last few years.
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