Bivalve cages deep under the surface may be too much trouble for marine mammals
Unlike many humans who will pay top dollar for farm-fresh oysters, sea otters don’t seem to care for them.
“We didn’t see an otter eating any oysters, period,” said Emily Reynolds, a current Alaska Sea Grant state fellow.
In research published in the Journal of Wildlife Management, Reynolds and her colleagues studied how much time sea otters (Enhydra lutris kenyoni) spend foraging in oyster farms compared to areas without oyster farms. “We found that there was no discernable difference in the overall activity of otters or of the prey they were eating between oyster farms and areas with no farming activity,” Reynolds said.
A rising tide of oyster farming
After near-extirpation, sea otters have reached close to carrying capacity in southcentral Alaska. And they came back with an appetite—they have to consume about a quarter of their body weight a day to fuel their energy-intensive dives in such cold waters.
Sea-based oyster farms have also risen at a rapid pace in the region. The 2024 harvest was estimated at 2.4 million oysters and is forecasted to grow 60% this year. As both the numbers of sea otters and mariculture oyster farms in Alaska are rising, Reynolds saw a need to understand the interactions between the two.
Reynolds and Brenda Konar, her master’s thesis advisor at the University of Alaska Fairbanks, devised a research project on sea otter behavior. Reynolds observed the marine mammals on three different farms, two of which only cultivated oysters and one that grew oysters and mussels. Konar, who has spent over two decades studying marine science in the area, located the farmers who participated in the study. “Many of the farmers were quite interested in the project and happy to collaborate with us,” Reynolds said.
The oysters are typically suspended in cages around 20-40 feet deep, the same range where sea otters forage, Reynolds said. She didn’t observe the creatures exploiting these sources much.
“For sea otters to forage dozens of feet deep is quite energy-consuming,” Reynolds said.
Otters took advantage of an easy opportunity at the one farm that also grew mussels, likely due to a different growing technique.
“With the mussel grow ropes being so close to the surface and not surrounded by cages, it didn’t surprise me,” she said.
Seeing below the surface
Reynolds watched the sea otters from a distance and used what she knew of otter behavior and foraging ecology to decipher what may be happening below the waves. Every 10 minutes, she scanned the study area to take note of what the otters were doing: resting, foraging, grooming or playing. Then, looking through the eye of a high-powered telescope, she watched the behavior of individuals.
“Otters have a very specific dive for when they’re foraging,” Reynolds said. To overcome the buoyancy of their lungs—which are relatively huge for their body size—they lift themselves up out of the water and let gravity help pull them deeper underwater.
When they go deep, they typically eat crabs, clams or sea cucumbers. Sea otters have a pocket by their armpit for storing food. “They can bring up to 20 items, so [these dives] can get really exhausting,” she said.
Through her scope, Reynolds identified the food items to the lowest taxon possible. She used the width of the sea otter’s paw to gauge the size of the food. “This gives you an overall picture of what they’re eating and also how much they’re eating,” she said.
The researchers found no discernible difference in overall otter activity or prey taxa, which surprised them. Oyster cages provide structure, which many other organisms, like mussels or barnacles, will attach to. Reynolds thought the extra biomass would attract sea otters to farms.
Some farmers talk about sea otters eating their oysters, but Reynolds didn’t see it. Besides the otters feeding off the mussel line, they didn’t engage much with the farm.
She did find that in the areas adjacent to the farms, sea otters consumed 2.6 prey items per dive compared to 4.8 prey items in areas without oyster farming. This means that otters are eating fewer, higher-quality prey items closer to farms, which could be caused by a difference in environmental conditions, she said.
“The farm might be sloshing off organic matter and carrying these nutrients into adjacent areas,” Reynolds said, which could benefit animals there. There aren’t enough studies on water conditions and prey availability in and around oyster farms, though. “Hopefully, there will be future studies on prey in these areas as well as their environmental conditions.”
This article features research that was published in a TWS peer-reviewed journal. Individual online access to all TWS journal articles is a benefit of membership. Join TWS now to read the latest in wildlife research.
