Beluga food sources shifted from marine to freshwater over past 50 years

Over the past fifty years Cook Inlet’s endangered beluga whales have changed their feeding habits, eating less prey that originates in marine waters and more prey influenced by fresh water, according to a new study of chemical traces in beluga bones and teeth.

The study, lead by Alaska Department of Fish and Game biologist Mark Nelson, measured isotope ratios in beluga skulls and teeth dating to the 1950s to “provide evidence of a long-term shift to more freshwater-influenced feeding habitat and ecology,” the study states.

“How this behavior relates to the continued decline in (Cook Inlet beluga whales) or their possible recovery, however, remains unknown,” the study states.

Cook Inlet belugas declined from an estimated 1979 population of around 1,300 to a present 340. In September 2017, the Alaska Department of Fish and Game received $1.26 million from the National Oceanic and Atmospheric Administration to study why the beluga population has failed to recover.

The recently released study — which measures isotope ratios of carbon, nitrogen, and strontium — agrees with observed changes in beluga habitat, but casts questions on the timing of those changes.

As the population dropped, observers saw the space belugas inhabit also shrinking. Summer surveys in 1978 and 1979 found them clustering around river mouths as far south as Tuxedni Bay, but by 2008 surveys found them concentrated in the Knik and Turnagain Arms, and around the mouths of the Susitna and Beluga Rivers.

“We’ve seen the belugas moving higher in the inlet, not spending as much time in the southern inlet where the waters would be marine, and the strontium (isotope research) shows that,” said Alaska Department of Fish and Game wildlife biologist Lori Quakenbush, a co-author of the study. “We still don’t know why that is, but now we have a sense that things are starting to fit together.”

The beluga population suffered its most serious drop in the 1990s, being cut in half between 1994 and 1998 at the height of the unregulated hunting commonly thought to be the cause of the decline. However, since hunting was limited in 1999 and stopped in 2005, the population has continued to drop by about 1.3 percent annually. The isotope research suggests that factors other than hunting are also responsible for the change in feeding habits, which began before the 1990s — at least as far back as the 1950s, the study concludes.

“If this change in habitat and prey is related to the decline in (Cook Inlet beluga whale) abundance, then we must look farther back in time (i.e. before the documented decline in the 1990s) to determine what was responsible for the greater freshwater influence in CIBW feeding ecology,” the report states.

Nelson said he’s pursuing the question by seeking older beluga bones to sample. Quackenbush said examining fisheries records for changes in known beluga prey is another possible approach to the question.

“Maybe there was something going before what we think of as too much harvest pressure — maybe something was already stressing the population,” Quackenbush said. “That fits pretty well with why we haven’t seen a recovery with the removal of the harvest stress. We still can’t say what’s going on, but it seems like if the population was stressed before the harvest pressure, maybe we’re still dealing with some part of the system that’s still a problem.”

The study tracked beluga diet by sampling the carbon, nitrogen, and strontium that belugas incorporate from digested prey into their bones and teeth. Researchers traced the origins of these elements to either freshwater or marine water by measuring ratios of stable isotopes, or forms of the elements that vary by the number of neutrons in their atomic nuclei. The variance in isotope ratios between different bodies of water is also present in the bodies of fish living in that water, and in predators such as belugas that feed on those fish.

Bone powder from a single skull can provide a long diet history, since belugas may live up to 50 years. Moreover, their teeth grow in layers that build up like tree rings through the whale’s life, storing a chronology of the animal’s feeding habits. Nelson’s research used the skulls of 20 whales that died between 1964 and 2007, and teeth from 26 whales.

One element — strontium, a metal that animals build into bone similarly to calcium — may be traceable to specific waterbodies. Traditionally used more by geologists than biologists, Nelson said, strontium isotopes originate in geological formations and enter rivers from the underlying rock, printing creatures that live off the river with the same strontium isotope ratio that can be found in water samples from that area.

“The interesting thing with strontium is that in the world’s oceans — because essentially all rivers flow into the oceans — it’s a global marine value: one ocean looks like pretty much like another ocean,” Nelson said. “And one part of the ocean — or one part of Cook Inlet — looks like the other parts. But as soon as you go from ocean water to freshwater, those ratios change quite dramatically… A fish that lived in a river, picking up those levels of strontium, and the beluga that eats that fish, is going to look a whole lot different than the beluga that eats a fish that spends all its time in Cook Inlet or out in the ocean somewhere.”

Though ecologists have often used carbon and nitrogen isotopes to study food webs, strontium isotopes are a newer method. In Alaska, UAF researchers have used the technique since at least 2014, when a Ph.D student mapped strontium ratios in Bristol Bay’s Nushagak watershed. The technique potentially allows researchers to identify marine-caught salmon with their rearing streams via samples of the fish’s otolith, a disk-shaped ear bone. Professor Matthew Wooller, director of UAF’s Alaska Stable Isotope Facility, was an advisor to that project and is a co-author of the recent beluga paper.

Though he’s shown that beluga food sources are now more freshwater than marine, Nelson said “we don’t know exactly what that freshwater source is — it’s probably a combination of sources.” He and his fellow researchers plan to continue the project by “collecting water samples from as many rivers as we can in Cook Inlet, and as many places as we can in Cook Inlet itself.”

“By sampling more rivers and comparing it to past ratios in belugas, we can hopefully tell which areas of the inlet have been important in the past and which are important now,” Nelson said. “And from a conservation standpoint, which areas we need to be protecting and thinking about for the future.”

Nelson also hopes to track the diet changes through briefer periods of time.

“Teeth are continuously grown — potentially even daily, the tooth grows a tiny bit,” Nelson said. “That continues throughout the year. Once those little tiny layers are formed, they remain biochemically unchanged. What we were limited by in this study is whether we could drill out enough powder to take a sample. To do that we had to take a whole year’s worth of powder. What we’re hoping to do is come up with a method to sample those teeth in a more fine-scale way, so we can get multiple samples in a single year.”

Finer tooth-ring samples could show variations in the seasonal diets of belugas — one of the population’s enduring mysteries. Though the whales are opportunistic feeders known to eat out-migrating smolt and in coming salmon runs through out the summer, as well as shrimp, worms and crabs, what their winter food is and where it comes from is less understood and may be important for explaining their failure to recover.

Reach Ben Boettger at bboettger@peninsulaclarion.com

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