Cook Inlet belugas' diet shifted amid population decline

June 13, 2018

U.S. Department of Defense photo by Chris Garner. A Cook Inlet beluga cow and calf swim in the mouth of Eagle River in Knik Arm in 2009.

A new University of Alaska Fairbanks study has found evidence that the diet of Cook Inlet beluga whales has changed significantly during the past half-century, a shift that may offer clues about why the population has been struggling.

The study, which was published today in the journal Endangered Species Research, used isotope analysis to identify the food sources for Cook Inlet belugas since the 1950s. Mark Nelson, who graduated from UAF in December with a master’s degree in marine biology and also works for the Alaska Department of Fish and Game, wrote the paper as his thesis.

By studying isotope signatures from beluga skulls and growth layers in teeth, the research team found that the marine mammals seem to have shifted to more freshwater-influenced feeding as their range gradually contracted. The study presents the first evidence for a long-term change in the feeding ecology of Cook Inlet belugas.

Those findings are notable because they coincide with a mysterious decline in Cook Inlet beluga numbers. The marine mammals have struggled since the 1990s, when their population dropped roughly in half. Despite greatly reduced subsistence harvests and federal protections, they haven’t recovered. Cook Inlet belugas were listed as endangered under the Endangered Species Act in 2008, and have a current population of about 340 animals.

Evidence of a dietary change isn’t necessarily linked to the decline, but it provides researchers with more data as they look for answers. The findings have spurred a follow-up study funded by the National Oceanic and Atmospheric Administration, Georgia Aquarium and John G. Shedd Aquarium.

“Not only have belugas declined in numbers, and not only have they contracted their range, from this research we’re also able to show there’s also a major change in feeding ecology,” Nelson said. “There’s been a change in where they’re feeding and what they’re feeding on.”

The study analyzed samples of 20 beluga skulls at the University of Alaska Museum of the North, along with analysis of tooth growth layers from 26 belugas provided by the museum and the Alaska Department of Fish and Game. The results indicated a shift toward more freshwater feeding, although specific food sources weren't identified.

“Carbon and nitrogen isotopes indicated a change in feeding ecology,” said Matthew Wooller, director of the Alaska Stable Isotope Facility and a co-author of the study. “The addition of strontium isotope analysis helped refine the study by connecting the belugas’ food sources to locations in Cook Inlet.” Those specific locations included eight freshwater rivers, Turnagain Arm and Kachemak Bay.

Wooller said instrumentation in UAF’s new Engineering Learning and Innovation Facility allows the full isotope analysis to be conducted in Fairbanks, rather than sending samples to a facility in the Lower 48. He hopes strontium isotopes, a less frequently used tool in wildlife studies, will become more commonly applied in Alaska with the establishment of a local facility to conduct these types of analyses.

Other collaborators and co-authors on the study included Lori Quakenbush and Brian Taras of the Alaska Department of Fish and Game, and Barbara Mahoney of the National Marine Fisheries Service.

The Endangered Species Research article is available at http://bit.ly/2t5D3QC.

ADDITIONAL CONTACTS: Mark Nelson, 907-459-7259, mark.nelson@alaska.gov; Matthew Wooller, mjwooller@alaska.edu