Do river otters eat endangered rockfish?

In a sea filled with charismatic mammals like killer whales and Steller sea lions, it’s easy to overlook a smaller critter whose name might make you think it’s not even found near saltwater. However, as shoreline residents know, the Salish Sea is home to thousands of river otters. And with their fearless ways and fearsome canines—as well as their webbed toes and ability to dive at least 60 feet deep - these whiskered members of the weasel family are prodigious predators of marine species. A previous study in British Columbia found that otters fueled their high metabolisms in part by consuming a lot of rockfish, with up to a third of all scat samples containing rockfish remains. Since rockfish populations are so depleted that all fishing for them has now been banned on the US side of the Salish Sea, we needed to answer an important question: As we invest in rockfish recovery, are river otters eating up our profits?

To find out, SeaDoc-funded researchers visited otter latrines around the San Juan Islands. Otter scat was examined for fish bones and otoliths (ear bones) to determine species and age of prey. Otters are indeed seafood fanatics: fish were present in 100% of the samples.

Fortunately for our endangered rockfish, though, the otters seem to specialize in the small lower intertidal and shallow subtidal fish such as the gunnels, sculpins, and pricklebacks. Rockfish occurred most frequently in samples from San Juan Island (22%), and most rarely (2.7%) Fidalgo, Island. Also encouraging was that otoliths showed that less than half the rockfish taken by otters were adults - the breeders that are critical to replenishing rockfish stocks.

Tracking scat and identifying otter diet is the kind basic science that, with your generous support, is helping us piece together the incredibly complex ecology of the Salish Sea and understand how we can best restore it.

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Banner photo courtesy of Phil Green/The Nature Conservancy.