Revisiting a Marine Mammal Mystery, with the Help of New Data and Powerful Statistics
New techniques and a massive dataset have helped USGS scientists and partners determine why sea otter populations in southwest Alaska collapsed in the 1990s.
In the 1990s, sea otter populations in southwest Alaska collapsed unexpectedly, after what had been a strong recovery from losses sustained during the fur trade at the beginning of the 20th century. Researchers first detected the collapse at Adak Island, one of Alaska's remote Aleutian Islands, but soon after found that steep declines had occurred throughout the region. They were puzzled. What had happened to these otters? USGS researchers and collaborators were able to determine that the sea otters were dying at higher rates and not just experiencing fewer births or moving, but there was no obvious and definitive cause.
They first published their findings on the surge in sea otter deaths in 1998, outlining various hypotheses, including lack of nutrients, disease, toxins, and predation, and the evidence for each. USGS researchers had observed several killer whales, also known as orcas, attacking sea otters around this time, something they hadn’t seen in prior years of research. They suggested that predation by orcas might be the cause. Besides the orca attacks that the scientists had witnessed they found that sea otter populations were stable in Clam Lagoon, an area that was inaccessible to orcas, even as populations were in steep decline in an adjacent bay that orcas could access. And the numbers checked out: if orcas were eating the otters, the researchers calculated that they would likely observe about five attacks in six years. Six attacks were actually observed during that time.
Despite these lines of evidence, there was no way to test this hypothesis experimentally, and not everyone was convinced. In the meantime, the otter population in southwest Alaska continued to decline and have yet to show any signs of recovery.
The definitive cause of the sea otter collapse long remained a mystery. Until last year. In early 2021, more than 20 years after that first study was published, USGS scientists and collaborators shed new light on the mystery with the help of additional data and a powerful statistical approach designed to quantitatively determine the most likely driver of the declines.
“Lively debates on what might have caused the decline in sea otters in southwest Alaska went on for years, culminating in these new findings,” said USGS Western Ecological Research Center Director and author Dr. A. Keith Miles.
The researchers assembled a vast dataset covering many aspects of sea otter behavior, physiology and population growth patterns, along with environmental stressors like prey availability, contaminants and predators. Then, they ran simulations to see which prediction was most likely based on all the data. They found that the predation hypothesis, which suggested that the missing otters were being eaten by orcas, was more than two times more likely to happen than the other scenarios.
“Getting to the bottom of this mystery would not have been possible without the kind of long-term data and knowledge that accumulates over decades,” said Miles. While the new study was a big team effort, Miles highlights three USGS Emeritus Scientists who devoted their careers to studying sea otter populations, Dr. James Estes, Dr. James Bodkin, and Dr. Tim Tinker. Their tenacity in investigating the facts meant that the evidence kept building from the 1980s through the 2010s.
Ecological mysteries like the sea otter collapse in southwest Alaska can be difficult to solve. By assembling a vast dataset, with multiple types of data and analyzing the clues in a rigorous, systematic way, scientists can zero in on the most likely answer. And knowing the most likely answer means better information for wildlife managers, who have to decide what to do about declining sea otter populations in southwest Alaska. With the help of research by USGS and partners, wildlife managers can make science-informed decisions to protect wildlife.
Of course, some parts of the story remain unsettled. Exactly why orcas began to eat sea otters in the late 20th century remains controversial—one possibility suggested by Estes and colleagues is that some orcas switched to new prey after heavy commercial whaling in the North Pacific decimated the large whales that the orcas depended on. The jury is still out on that one, but USGS scientists continue to investigate the lives of sea otters and marine ecosystems, one clue at a time.
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