The Daily Bucket is a regular feature of the Backyard Science group. It is a place to note any observations you have made of the world around you. Rain, sun, wind...insects, birds, flowers...meteorites, rocks...seasonal changes...all are worthy additions to the bucket. Please let us know what is going on around you in a comment. Include, as close as is comfortable for you, where you are located. Each note is a record that we can refer to in the future as we try to understand the patterns that are quietly unwinding around us.
December 2014
Salish Sea, Pacific Northwest
Since the 1970s the populations of Rockfish, colorful good-sized tasty bottom dwellers, have declined in the Salish Sea primarily due to overfishing by commercial and sport fishing (fishermen turned to Rockfish after Salmon populations were decimated). Several species of rockfish are now federally protected and no-catch zones have been established in these waters to help bring these slow-growing long-lived fish back.
A question raised by fishermen, now restricted in their pursuit of Rockfish, is whether it isn't natural predators driving the population crashes, specifically the increasing numbers of marine mammals like Harbor Seals, Sea Lions and River Otters. I've become interested in this question after watching the local Otters near my home. They do a lot of fishing in plain view.
I drift around in the local bay frequently in my kayak. Otters come through intermittently as they wander their range and if I'm lucky my time overlaps with theirs. Sometimes it's the big old loner male with the missing tooth, other times it's groups of youngsters. Recently a troupe of 5-6 has been visiting the bay. Sometimes they hang around rocking the boat where Mr O is working. Sometimes they nap or play on the dock.
Most of the time they are fishing though, and occasionally I drift close enough to see what they are eating. Is it Rockfish?
(All photos by me. In Lightbox...click to enlarge)
It's difficult to be sure, what with the otters' constant motion in brief moments at the surface and partial views of their prey, but I've been able to identify some basic categories, at least, using guide books such as Fish Identification Guide for the San Juan islands. Here is what I have seen so far.
Flounder:
Gunnel. In the second photo it is partly chewed. Otters bring small fish up to the surface, chew it up and swallow it there. I have quite a few photos of otters and birds eating gunnels.
Sculpin:
If a fish is too big to manage on the surface an otter swims to shore and eats it there. This one is carrying a flounder to the new dock. I could hear it crunching for a while. The yellow and black bars on the fins identify this as a Starry Flounder.
All these are common slow-moving fish. A general principle for carnivores is they maximize prey calories with the least expenditure of energy. Their survival depends on it. Otters are opportunistic and will catch whatever is easiest. At their resting spots on shore I see piles of crab shells along with their scat...crabs are even slower than fish.
I found a study of otter prey as measured by their scat that describes what otters eat on three of the other islands in the county. It is consistent with my informal observations on this island. They eat mostly fish, and in order of abundance in their scat, various species of: gunnels, sculpins, pricklebacks, flounder, clingfish, codfish, rockfish, and others (26 species of fish listed, plus crabs, chitons, shrimp, barnacles and other invertebrates). Clearly otters are feeding on what's there, and that's rarely Rockfish.
Harbor Seals and Sea Lions are also scapegoated for depleting salmon and rockfish. Studies show they are also opportunistic feeders, for the same reason. The primary prey of my local sea lions, in order of its occurrence in scat is: dogfish, herring, skates, sand lance, salmon, stickleback, codfish and walleye. In other words, they eat salmon only if they are abundant. This fall there was a big run of Coho (aka Silver) salmon in the Salish Sea, which is what this Steller's Sea Lion has caught. Correction thanks to readers: this is a Chum salmon! Thank you :)
I have much more observation to do of my local marine mammals. Sometimes I get the feeling they are thinking the same thing.
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Time for you to share your observations of nature from your part of the world. What's new on this winter day?
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