Why are Kodiak bears letting salmon slip away?

By  | 

ANCHORAGE (KTUU) - As Kodiak summers get warmer, the island's massive brown bears are increasingly walking away from streams that are bright red with sockeye salmon to instead graze on elderberries, a delicacy that is even brighter red which had always ripened in fall.

It's no surprise that many of Kodiak's bears rely on salmon for a large part of their diet -- but questions remain about how bears move across a landscape to "surf" the variable timing of returning salmon, what factors influence their choice of fishing areas, and when bears may utilize other food resources like berries.

A group of biologists discovered the apparent side-effect of climate change -- and related disruptions in the island's food web caused by the overlap -- after they started trying to figure out why bears were abandoning fishing posts en masse in 2014.

Researchers from Oregon State University, the University of Montana, and Kodiak National Wildlife Refuge collaborated on the study, which was published last week in the Proceedings of the National Academy of Sciences.

Not everyone believes the methods used to arrive at the findings are entirely sound, and one biologist for the state Department of Fish and Game believes it is premature to call the occurrence a full-blown trend that is attributable to climate change.

However, Will Deacy, an OSU research ecologist who led the project, said the theory goes back to the moment researchers realized something was amiss three summers ago: "We had been spending lots of time on these streams studying bears," he told Channel 2 News in an interview. "All of a sudden, the bears seemed to disappear, and none of the salmon were getting eaten."

That year and again in 2015, researchers spotted piles of rotting, intact salmon carcasses that died after spawning instead of being harvested by bears, a departure from the shredded salmon carcasses that typically line the shallow, narrow streams around Karluk Lake.

From there, the scientists set up time-lapse cameras at four streams around the lake to keep tabs on whether or not elderberries and salmon runs were in fact occurring simultaneously.

The resulting photographs confirmed the trend over a short period.

So did aerial surveys that provided annual snapshots of the entire bear population's use of salmon fishing spots, as well as data collected from GPS collars placed on 36 female bears.

161 bear scat samples were examined on August 21, 2015, to identify the dominant prey item: seeds and stems are commonly left behind when a bear mainly eats berries, while salmon scat has a distinct color and odor and often salmon bones.

Beyond those steps used to establish a trend over a two-year stretch, which also happened to be among the hottest in the seven decades of record-keeping, it proved difficult to come by data to conclusively prove elderberries had not ripened early before. To that end, the group built a model based on air temperatures.

The National Climatic Data Center has daily temperature records for the Kodiak Airport from 1949 to present, but that is on the opposite side of the island from Karluk Lake and the study streams, roughly 60 miles away. Spring and fall records -- not daily ones -- were maintained at the lake only from 1965 to 1969.

Still, the authors' determined there is a strong correlation between the temperature at the airport and the lake in the years where there are records for both locations, and they used the years with good data at both sites to predict minimum and maximum temperatures at the lake over a span from 1946 to 2016.

Researchers lean on that dataset to theorize that elderberries have been ripening a couple days early every decade: "If (warming) trends continue, by 2070, the average onset of berry availability would occur during the average peak of salmon availability," their report states.

That is where the study's authors lose Nathan Svoboda, who has been an area biologist in Kodiak for the Alaska Department of Fish and Game for more than four years.

"The inferences coming from the work are probably a bit stronger than the data warrants," Svoboda told Channel 2 News. "The analysis is largely based on data collected from the 1960s, a few years of recent data, and then they applied some pretty innovative modeling techniques to bridge that gap.

"Maybe this was just an anomaly, some of these years. 2014 to 2015, for instance, were fairly warm years and in my opinion were sort of outlier years in terms of berry production."

Svoboda also said it is impossible to draw meaningful conclusions that apply to the entire archipelago from a moment-in-time study looking at four streams.

Deacy said no one should take the study to mean that there are similar shifts occurring in eating habits among bears away from Kodiak.

While there could be measurable effects of climate change on bears in nearby places like Brooks Falls in Katmai National Park and McNeil River, those iconic spots -- where bears spend summer swatting salmon in fast-moving waterfalls -- do not have the same abundance of elderberries.

That is part of what makes Kodiak Island unique: elderberries contain almost exactly the amount of protein needed for bears to gain weight, Deacy said. Ongoing research is being conducted by the Bear Center at Washington State University in an effort to figure out what captive brown bears choose to eat when they have a multitude of options, and why.

While blueberries and crowberries are abundant around Katmai and McNeil, they contain far less protein than the berries that line the shores in Kodiak, which Deacy believes is an indication that bears there would be less likely to ditch streams early even if berry season moves earlier into summer.

Another point Deacy emphasizes is that the study is not meant to suggest that there is any gloom and doom in the outlook for Kodiak grizzlies.

He called the island "bear paradise" and suggested that the impacts of salmon runs and elderberries co-occurring are likely to be felt far down the food chain rather than by the apex predators: gulls and weasels that depend on salmon scraps left behind by bears will suffer, while Dolly Varden and Arctic char will flourish because rain tends to push spawned salmon carcasses into lakes.

Ripple effects of the shift are impossible to predict with any accuracy right now, but the disruption is the sort of minute change likely to become increasingly common all over Alaska as the global warming trend continues.

"When people think about climate change and how it affects wildlife, they often think about something overheating or a polar bear standing on an iceberg," Deacy said. "What we find in this study is that we have a much more subtle effect. It's these types of subtle effects that will probably have the larger consequences in the future."

While critical of some of the study methods and the level of certitude attached to the findings, the state biologist does not shoot down the researchers' findings wholesale: "Regardless, I think that their conclusion that bears could be switching from salmon to elderberries when both food sources are available, that could be plausible," Svoboda said.

There is consensus among scientists that the key to get better answers on how climate change is impacting Kodiak animals and plants is to establish long-term monitoring and research efforts.

Kodiak Refuge is a wild place with no established trails. Traveling across county includes bushwacking through thick brush and crossing icy cold rivers and streams. Will Deacy and volunteer Kristina Hsu navigate a particularly cold creek. (Lisa Hupp/USFWS)

Comments are posted from viewers like you and do not always reflect the views of this station. powered by Disqus