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As sea otter populations rebound in the northeast Pacific, shellfish farmers, crab fishermen and others are concerned about how their voracious eating habits – which can see them consume around a quarter of their body weight each day – could affect the richness of the ocean. Scientists have studied the effect of sea otters on major shellfish populations, hoping to better predict the results of ongoing reintroduction efforts, which began in the 1960s.
A recent study, however, adds a wrinkle to this effort. As Nicole LaRoche, a graduate student at the University of Alaska Fairbanks, and her colleagues have shown, there are significant differences in eating habits from one sea otter to another.
Alaska’s dark, freezing winters make it difficult to study sea otter feeding habits year-round. But LaRoche and his team overcame that by watching the otters feed during the warmer months and then measuring changes in carbon and nitrogen isotopes of otter whiskers collected by the US Fish and Wildlife Service. Agency officials routinely collect whiskers for scientific purposes when capturing sea otters or recovering carcasses. Since sea otter whiskers tend to grow at a steady rate, measuring changes in the length of a whisker can give scientists a way to see if what otters eat changes throughout life. winter.
The researchers weren’t able to match warm weather observations with winter whisker measurements for specific sea otters, but they did find individual dietary variance.
Through warm-weather observation, LaRoche and his colleagues found that sea otters depend on crabs, sea cucumbers, mussels, snails, sea urchins and various types of clams. Butter clams, in particular, seem to be a favorite of the foraging otters they have observed. Sea otters “get what they pay for,” says LaRoche. Butter clams “are easy to dig, they won’t swim, and they pack a lot of calories for each.”
It’s tempting to interpret these individual differences between what sea otters eat as food preferences, but LaRoche says competition with other otters and prey availability also play a role in determining which otter eats what. .
Previous research has shown that sea otter feeding habits can change as the regional population grows, says Tim Tinker, a wildlife biologist with the United States Geological Survey who was not involved in the study. When sea otters spread into a new area, juvenile males arrive first and seek out large, high-calorie prey like Dungeness crabs. As females and then cubs are added to a growing colony, otters begin to eat a wider variety of prey. This means that the early impact of sea otters on some prey species may be high, but should stabilize over time.
“What [this] the paper,” says Tinker, “is to dig a little bit into how diets are starting to diversify.”
LaRoche hopes that a new understanding of clam consumption by sea otters in Southeast Alaska, in particular, will draw attention to the need to balance the demands of hungry otter populations with those of hunters. subsistence farmers who also collect clams.
“People tend to think of sea otters affecting commercial fishing like crab and sea urchin, which they do,” LaRoche says via email. “But I think the focus should be more on personal use, especially subsistence use. Commercial rigs, especially crab, have the ability to fish much deeper than otters typically dive, but subsistence use is usually in the same area that sea otters also forage.
Despite the difficulties that may arise from the rebound in the sea otter population, LaRoche hopes that researchers will continue to pin down the nuances of their interactions with various species and hopefully find new ways to help sea otters, their prey and humans to coexist.