How Many Salmon Does A River Otter Eat?
October 2018, Rocky Brook
Early October, time to watch for spawning activity. The water level has been too low to encourage fish passage up Rocky Brook from the larger Dosewallips River. Still, at night I heard splashing below me (the yurt where I sleep is perched right above Rocky Brook). In the morning I noticed small “clearings” in the tiny gravel and sand. This indicated some kind of spawning activity. They could have been by trout (rainbow or cutthroat) or possibly sculpins. These resident fish tend to spawn under protection of darkness, and at less predictable times. Adult salmon prefer much larger gravel for carving out redds in which to deposit eggs. These might have been made by hatchery fish, as they tend to behave somewhat differently than the wild fish, possibly testing sites. In any case, I could not find the fish responsible for this activity.
Then, at midday during the second week of October, I heard a great splashing. There, right below me in the deep pool under the yurt deck, I saw an otter. Then another, and to my surprise, three more. It must have been a complete family.
I observed the two largest otters surface, each wrestling with its own fish. They began to eat, starting with the head and working down from there. These fish were almost as big as the otters, too big to have been in Rocky Brook at such a low water level. One looked like a five or six pounder, the other slightly smaller. No spawning colors. The Fish and Game biologist who walks Rocky Brook weekly to count fish and redds said that he had seen hatchery coho without spawning colors in the larger Dosewallips, so these might have been coho.
The other otters joined in, the little family feeding, chattering, grooming and sitting on rocks. This went on for an hour. I surmised that the otters had herded the fish up Rocky Brook where it was much easier to trap them, especially by a coordinated group of experienced hunters. I pay attention to the world around me, I’m pretty sure I would have noticed such large fish.
Later that afternoon the otters were still there; they were eating two more smaller fish. Eventually they left, leaving half eaten carcasses. These were gone by morning, I’m not sure if the otters or some other critter got them.
Three days later the otters were back. They had two more fish. These were bright red, clearly spawning coho. I could see the clipped fin, indicating that they were hatchery fish, most likely from the nearby Quilcene National Fish Hatchery, located within an adjacent watershed about ten miles away. After the otters left, I watched a smallish coho female. It had been building a nest, most likely paired with one of the fish that the otters had eaten. Now it wandered around aimlessly, swimming from redd site to nearby pools in a wide circle. The next morning, she was gone too.
By mid October, Rocky Brook was quiet again, save for the resident trout fry. These are being eaten, one by one, by a visiting kingfisher. Luckily there are lots of them and only one bird.
During the third week of the month, I was talking with the Fish and Game biologist during his weekly walk. When I told him about the otters, he said it answered one question he was having. The previous week he had counted eleven redds above me, between the bridge and Rocky Brook Falls, but could not find a single fish (and he is an expert at flushing out and spotting fish). He didn’t doubt that the otters were responsible.
Somehow since these are hatchery and not wild fish, I feel less animosity towards the otters. Its the wild fish that I really want to see survive (at least long enough to spawn), and the wild fish runs for the last two years have been dismal. But, hatchery fish are better than no fish. And while raised in the nearby hatchery, they do carry genes from the wild population in the Quilcene River. Who knows, these fish might eventually learn to survive and return to Rocky Brook year after year, becoming successful resident salmon, eventually mixing with the native coho.
I looked up river otters in various books that I have, wondering how voracious they are. A frequently cited study in Alaska (1985) determined (through scat analysis) that at least 3300 juvenile salmonids were eaten by 2 adult and 2 young over a six week period. If you do the math, it ends up being almost 20 fish per day per otter. Juveniles in freshwater would have been small, less than six inches.
A 1990 study in Scotland of otter predation on Atlantic salmon concluded that during spawning season, otters consumed on average one salmon per night, meeting their daily food requirement with this single fish.
And the family of five I observed in Rocky Brook ate four adult fish in one afternoon. This seems about normal, and they didn’t seem to waste anything, except for maybe the bony tail.
Dennis Lloyd Kuklok
Rocky Brook, October, 2018