The invasive species causes a thiamine deficiency in salmon, hindering their ability to reproduce naturally
By Mike Lynch
For years, biologists have been working to improve conditions for the native fish in Lake Champlain. Among other things, they have removed old dams to help spawning salmon migrate up rivers and have 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 a main source of food for salmon, a keystone predator that eats smaller fish.
Alewives were first discovered in Lake Champlain’s Missisquoi Bay, in Vermont, in 2003. Since then, they have grown in number and replaced native rainbow smelt as the main forage fish for predators in the lake, and they are 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 coastal rivers, where they spawn. 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.
Alewives have been spreading to inland lakes for decades, particularly the Great Lakes. They occasionally die in large numbers when spawning in non-native waters in the spring. At times, shores on Lake Champlain have been lined with hundreds of the dead fish. Reasons for the die-offs may include weakness from a lack of food in winter and temperature shock caused by moving from deep, cold water to warmer spawning waters.
Lake Champlain is stocked with salmon, but it has not had a self-sustaining population for two hundred years. Scientists are trying to nurture the population back to health, but alewives are undermining their efforts by interfering with the reproduction cycle of salmon. Alewives have an enzyme that kills thiamine, or vitamin B1, in fish that prey on them. Since salmon consume alot 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 heathy, 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 said there is also 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 attempting to get up cascades to spawning ground than those salmon that were injected with water, which acted as a placebo.
Ardren said some salmon may be able to overcome the thiamine deficiency and that the species may evolve to cope with it. He noted that 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 been studying 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 sufficient thiamine from zooplankton, a type of alga, soon after emerging from eggs, thereby 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. ■