Weird Nature: Shrew-Eating Trout!

The story of rodent-eating trout at The Nature Conservancy's Silver Creek Preserve has been one of our blog's biggest hits. But those Silver Creek trout look like dainty eaters compared to this one. Meet the shrew-eating trout documented by researchers at Alaska's Togiak National Wildlife Refuge. And how did this many small mammals end up in a trout's stomach?

If you’re a small mammal, it’s definitely not safe to go back in the water…

One of the most popular posts on Cool Green Science has been the story of rodent-eating trout at The Nature Conservancy’s Silver Creek Preserve. A trout research effort there confirmed that large brown trout were keying in on voles (small rodents that happened to be in the midst of a population explostion).

One trout had three freshly-eaten voles in its stomach, which seemed particularly gluttonous. Until it’s compared to this one.

Researchers conducting a trout study at the U.S. Fish and Wildlife Service’s Togiak National Wildlife Refuge in Alaska released this photo on its Facebook page last week. They wrote:

“This rainbow was caught on the Kanektok River during a rainbow trout project. It wound up being a mortality capture so it was cut open to see what it had been feeding on. Surprise! The answer was shrews, and a whole lot of them.”

By my count, this trout ate twenty shrews. Twenty.

There are a couple of really interesting points with this incidence.

For one thing, the rainbow trout was 19 inches (480 mm). That’s certainly a decent-sized fish, but as the researchers point, it’s “fairly pedestrian by Togiak standards.” It is hard to imagine a fish of this size consuming so many small mammals.

And the trout preyed on shrews, small insectivorous mammals that are interesting in their own right. Shrews are not rodents. Many rodent species – like the montane voles at Silver Creek – have periodic population explosions.

As every fly fisher knows, trout really key in on abundance: a mayfly hatch induces a feeding frenzy, as does a vole irruption.

But I can find no literature indicating shrew population explosions. As small predators, it seems hard to imagine them becoming as numerous as rodents. So how could a trout so effectively target shrews?

The staff at Togiak speculate: “How did so many shrews make it into one trout? It’s anyone’s guess but perhaps a nest by the river eroded, dumping all of the shrews into the water where this rainbow likely came away feeling like a lottery winner.”

That sounds as good an answer as any. But it would be interesting to look more into shrew ecology and see if there might be something else going on—a high density of shrews, or some shrew habit that trout have learned to exploit.

The Togiak National Wildlife Refuge looks like a fabulous place, a refuge with not only very big trout but also walrus and caribou and salmon lots of other cool critters. You can enjoy more photos of this place, and support the refuge’s conservation, by following Togiak NWR on Facebook.

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  1. Shrew families max out at 6-8 individuals. It is highly unlikely that 20 shews shared a nest. It is more likely that the trout learned to exploit a shrew habitat – perhaps the shrews found a good feeding ground near the river side. Were the shrews young or mature? Shrews, like trout, feed on insects. Perhaps they both found a rich insect source at the water’s edge and a happy accident one day introduced the trout to a new food (shrew).

    At the University of Washington’s Burke Museum’s Mammology Department we have two shrews that were found in the stomach contents of a Greyling. It is conceivable that some individuals (Trout or any animal) will go to some length to obtain food or a favorite rich food they find in their home range.

    My wife had a garter snake years ago she caught near a pond that would only eat frogs – wouldn’t touch a mouse or any other prey offered to it. It preferred frogs from it’s pond side home. Perhaps when food is abundant, individuals can afford to have preferred foods.

  2. Fly Fisherman in the Northern Michigan key on this trait…

    Beginning in July; those in search of the largest trout in our river systems fish flies that replicate small mammals, fished after dark.

  3. Marko says:

    In Europe we have a species of Shrew that is gladly exploiting water habitats, being a very good diver and swimmer. Its Latin name is Neomys anomalus, Southern Water Shrew. Both Trouts and Danube Salmon can easily catch one.

    If the shrew species from the article is Sorex palustris and if this species has a similar behavior as its European sibling, than the question how the trout caught one is no mystery, but to catch 20 is remarkable and shows that this trout has learnt something about the shrew.

  4. All the so-called experts above are totally overlooking the obvious ‘fishbus’ explanation.

  5. Blythe says:

    This is my all-time favorite post. That picture is freaking crazy. Love it!

  6. Excellent point, Marko. North America is also a home of the water shrew, Sorex palustris. Here it has a white underbelly, but is hard to tell in this picture with the wet bodies, if these are the same. The trout and shrews may have both been feasting on the same aquatic insect hatching or breeding insect swarm, and the trout discovered the shrew was a better meal. The count of 20 shrews suggests a trout that was an experienced hunter with a preference.

  7. A trout eating TWENTY shrews…that’s unbelievable. Looking forward to the follow up on this, and if we get an update on what is really going on.

  8. Ivan says:

    It is obvious to me that there is some sort of retaliation going on here. I am sure that at some point the shrew population placed a hit on this trout’s family due to his father’s refusal to cower to the shrew mob’s extortion attempts. After the shrews whacked the trout’s father, there was nothing else he could do but answer with swift and directed vengeance. I just hope that this bloodshed can end with this Hamlet-like death for this trout and that the future generations can live in shrew/trout harmony.