Environmental

Study finds salmon deplete fat stores while stopped by dams

Study finds salmon deplete fat stores while stopped by dams

When Atlantic salmon are stopped by dams during their upstream spawning migration, they can deplete needed fat stores, according to research from the University of Maine.

Restoration of critically endangered Atlantic salmon is an important issue in Maine. The population has been declining since the 1800s, according to the researchers. Atlantic salmon return to the rivers from the ocean every spring to swim upstream and spawn in freshwater.

A new study has been performed to quantify the time and energy lost by Atlantic salmon stopped by dams. When the fish are stopped by dams, they are stuck in warmer waters for longer than expected, which can deplete the fat they have stored up to migrate, produce gametes, develop secondary sexual traits and spawn.

“Salmon limit the food they eat in freshwater, so excess energy lost during their migration doesn’t get replaced,” says Sarah Rubenstein, who completed her masters of science in wildlife ecology at the University of Maine in 2021. “Spawning takes a lot of energy, so the more energy reserves salmon have left after migrating and during spawning, the more likely they will be at successfully reproducing.”

For her graduate thesis, Rubenstein led a group of researchers from UMaine and the Maine Department of Marine Resources to radio-tag adult Atlantic salmon at the lowermost dams of the Penobscot and Kennebec Rivers —Milford and Lockwood dams, respectively — and track their movements upstream. Milford Dam impounds water for an 8 MW hydro plant and Lockwood Dam has an associated 7.2 MW hydro powerhouse. The researchers measured fat stores when the fish were tagged and again once they had ascended the dams’ fishways, which are meant to mitigate impacts by providing a detour route for migrating fish. 

The scientists found that the tagged fish were delayed on average 16 to 23 days at the dams and lost 11% to 22% of their fat reserves, in large part due to water temperature but also due to the delay in time. One of the results of such dramatic energy loss could be the decline in repeat spawners — adults that survive the spawning process, return to sea and live to migrate another season to spawn — which have nearly been eliminated from Maine’s populations. Moreover, warming waters as a result of climate change will likely exacerbate the energy depletion.

The study suggests that restoration efforts should focus on providing migrating Atlantic salmon with more rapid access to the upstream habitats to get to cooler waters faster.

“We studied the impact of delay at only a single barrier. However, Atlantic salmon often encounter multiple dams or other barriers along their migrations and this can increase the likelihood of a cumulative loss of energy that becomes detrimental,” Rubenstein says. “Improving access to quality habitat benefits migrating fish species and is likely to promote increased spawning, post-spawning survival, and reproductive success in Atlantic salmon.”