Climate change, hatchery competition spur developmental changes in Bristol Bay sockeye
Climate change is shifting the life cycle of sockeye salmon in Bristol Bay to spend less time in freshwater and more time at sea, and creating more competition between the wild stock and hatchery releases, according to new research by the University of Washington.
Since the University of Washington began collecting data on the watershed in the 1940s, winter ice on the Wood River system lakes in Western Bristol Bay are breaking up an average of two weeks earlier and the summer water temperature is several degrees warmer.
Those changes are good news for juvenile salmon because they lead to longer and more productive growing seasons both smolt and the microscopic organisms they feed on, according to the study.
"Of course the big concern is how much more warming these systems can tolerate," said Daniel Schindler, professor of Aquatic and Fisheries Sciences at the University of Washington. "Given these last few years of really extreme hot temperatures in Alaska, we have yet to see the system tip over to the other side where they've become too warn for salmon. That's somewhere on the horizon presumably, but we haven't seen it yet."
Schindler and other researchers found that the salmon throughout the seven major river systems in Bristol Bay have undergone significant changes in their life-history.
Sockeye typically spend either one or two years in freshwater before going out to the ocean. The researchers found that with increased lake temperatures, the juvenile sockeye spend less time in freshwater before moving to the ocean.
However, researchers found much of the strong start for sockeye in freshwater was counteracted by extra time in the ocean.
Bristol Bay sockeye spend either two or three years in the ocean. Schindler says they are seeing more fish spending only one year in freshwater and three years at sea.
One reason behind that extra year in the ocean may be slowed growth due to more competition from hatchery fish. Although there are no hatcheries in Bristol Bay, the diets and spaces of sockeye, pink and chum salmon overlap in the ocean.
The study found that as the total biomass of salmon in the North Pacfic increased, the size of the sockeye returning to Bristol Bay after two years at sea decreased.
“This finding provides evidence of negative impacts of hatchery augmentation on wild stocks at the scale of the North Pacific, particularly through interaction with the effects of ongoing climate change in freshwater. A hatchery-climate interaction can be expected in the ocean, as the quantity of thermal habitat for salmon in the North Pacific is expected to decrease in the future and warming temperatures may intensify competition for food,” study authors wrote.
In addition to the long term changes in the fish population, the warming climate also created immediate impacts in the 2019 season.
Several rivers across the state saw salmon stalling at the mouth waiting for cooler temperatures.
In Western Bristol Bay, a thermal barrier prompted the Alaska Department of Fish and Game to restrict fishing on the Igushik River.
“We’ve definitely seen fish in the river just dying,” Tim Sands, Area Management Biologist for Bristol Bay - Westside. “The warmer the water, the less dissolved oxygen it can carry, and so the fish just couldn’t breathe.”
Sands says that once cooler weather moved in, the fish continued their run and the river met its escapement goal.
“The Igushik goes up to the village of Manokotak and there’s a really dedicated set net effort down at the mouth of that river, so they were impacted a bit because they had reduced fishing time, but they still had the sixth best harvest there in the last 20 years there, so it wasn’t a disaster,” Sands said. “Things are good, but the outlook if we continue to have warm summers like this, it could be an issue for returning adults.”
In Bristol Bay, sockeye have relatively short distances to travel upriver to reach spawning grounds compared to other watershed. Fish returning to six of the nine major rivers in the Bristol Bay management area only have to travel less than 50 miles before reaching a lake.
“In other rivers long migrations are particularly difficult for salmon when the water is warm, and in Bristol Bay that doesn’t seem to be the case in most of the rivers,” Schindler said. “They’re short and they stay cool, at least in the current period of observation.”
Despite the evidence of dead fish likely linked to warm water temperatures, Schindler says the real impact may take years to become evident.
“If the adults die you can usually see them so it’s pretty obvious that a certain heatwave would impact the salmon, but the reality is that you need to wait for the offspring of those fish to return to really determine the biological responses,” Schindler said. “So if there were negative impacts of all this warm weather this summer, we probably won’t see it until 5 years from now when these fish return to spawn.”