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The endangered beluga whales living in Alaska’s Cook Inlet are still declining in number, despite protections put in place 20 years ago and the adoption of a recovery plan in 2016.

The latest population estimate—released in January, based on a survey from June 2018—is only 279 animals, down from 328 just two years before. The federal agency in charge of whale management, the National Marine Fisheries Service, has called the trend “concerning.”

A new study suggests that reduced access to salmon could be part of the problem, but researchers say there are still far too many unknowns about why the belugas are faring so poorly.

“We’re trying to recover a species here and we just don’t know anything about it,” says Bill Bechtol, an independent fisheries researcher based in Homer, Alaska, who drafted part of the beluga recovery plan.

Beluga whales in Cook Inlet were once so numerous they turned the waters into what looked like a sea of whitecaps. But their numbers dwindled rapidly in the 1980s and 1990s, which was attributed to unregulated subsistence hunting. Biologists thought that once hunting ceased in 1999, the whale population—then estimated at 369—would slowly rebuild. Instead, the numbers have fallen.

Cook Inlet lies adjacent to Anchorage, Alaska’s largest population center. It’s home to marine traffic, fishing activity, and an oil and gas industry. Possible threats to the whales include pollution, noise, disease, changes in food availability, and changes to habitat from development or warming waters. The 2016 recovery plan concludes that there is unlikely to be a single “smoking gun,” but that some combination of stressors may be responsible for the whales’ lack of recovery.

In the new study, Stephanie Norman, an independent researcher from Bothell, Washington, took a closer look at one possible factor: the number of salmon that return to spawn in upper Cook Inlet each year. Belugas are seen feeding on salmon near river mouths from May to September.

Norman and colleagues created a model from two data sets—counts of chinook and coho salmon on an upper inlet tributary, and beluga birth rates as determined by aerial surveys. They found a correlation between greater numbers of salmon in the tributary and successful beluga reproduction in the following year or two.

For now the link is only speculative, the authors say. “Here’s one possible contributing factor that hasn’t been looked at much before,” says Norman. “We think it’s at least worth a look.”

The analysis required “several tenuous assumptions,” says Bechtol, but he says it’s a helpful approach for understanding how the whales’ food supply might affect their reproduction.

Other prey species and conditions during the non-summer months are also significant to belugas, says Brian Marston, an Alaska Department of Fish and Game fisheries biologist and a member of a state-federal beluga task force. “It’s not all salmon,” he says. In spring, Marston notes, the belugas rely on herring and eulachon, an oily smelt targeted by a small commercial fishery. Little is known of winter diets, although belugas are thought to feed on benthic species like flatfish and invertebrates.

Beluga research remains a challenge. State and federal agencies responsible for fishery and marine mammal management have suffered significant budget reductions in recent years. This year’s aerial survey of Cook Inlet’s belugas, scheduled for June, has been canceled due to the coronavirus pandemic, and a program that photographs the whales from a small boat has been delayed until at least July.

Meanwhile, threats to the belugas’ recovery only seem to increase. Chinook salmon returns across Alaska have been low in recent years, likely because of warming ocean conditions. In summer 2019, Alaska’s Deshka River hit a record high temperature of 27.6 °C, and salmon in some inlet streams died before spawning. Marston says he expects a lot more dead fish in the future. “There’s nothing you can do about it. It’s hot.”