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.
that is cool!!
Are these what are called the masked shrew?
Jeff,
Thanks for your comment. Masked shrews are not found in this location. I am not sure these shrews were identified to species level, but there are 4 species of shrews found at Togiak, where this trout was collected: common, tundra, pygmy and Alaska tiny. Subsequent studies have found that shrews are a fairly common prey item for grayling and rainbow trout in Alaska.
Cheers,
Matt
Matthew:
Not sure what you mean by shrew population explosions as compared to viewing many shrews in one
place which also is seldom reported. I studied small mammals including shrews for three years in the field in New England for my doctorate at the Univ. Conn. under Lowell Getz. In addition, I have spent literally thousands of hrs in the field in n. Vt from about 1959- to date (Now retired). Generally, much of the litt. on shrews suggest they are essentially quite solitary or mostly travel in a family group, at least in the n. latitudes here in N. America. I just wanted to share that I vividly remember one time in the Victory Bog WMA in Victory, Vt at Damon’s Crossing standing in an alder thicket and suddenly becoming aware of tens of shrews moving around me in the alder patch. I was almost shocked to say the least. I should have but never published that observation. It might have been rejected anyway because I had no ‘hard” proof: just my own observations. Basically, it was analagous to seeing a swarm of army ants going through the understory of a leaf litter. The whole observation only last about 5 mins. They moved across the alder thicket surface and on out-of-sight. Had to be more than 20 or so. Some time later a student of mine while on a deer stand reported a similar observation. (sorry, I don’t have the exact dates in my head as I write this. I would have to go through a half century of field journals to retrieve it. I went back later spread out some mouse traps in the area and never caught a single shrew.
This was less than a week later. (I was extremely experienced in trapping small mammals at that time, inc. the smallest shrew in eastern N. America-Microsorex sp.
I have several other amazing obs on shrews that I won’t take space to share here. I did publish one
note on the pygmy shrew years ago in the J of Mammalogy. That is arguable one of the most amazing of
all the soricidae living in Vt, by many measures: size, relative ear size to cranial breadth, etc.
There is a lot of misleading and sensationalized information in mostly the popular and semi-popular
litt. on soricidae in general. Some of it completely erroneous if not sheer nonsense.
I don’t know if you have checked Gordon Kirkland’s many exc. articles on shrew biology. He is unfortunately passed but published several major and very important articles on shrew biology.
I enjoy your Green Blog immensely. I’m sorry I wasn’t aware of it before now. I’m “retired ” now and mostly working on insects but I’m an incurable generalist, basically a hybrid between a naturalist and
a field ecologist. I have published a bit in peer-reviewed journals but where I taught at a small open
admission state college with an extremely heavy teaching load, I had little time for personal research,
nor the lab. facilities required to do much of this. However, I did publish more than most in the same
system and prepared literally hundreds of manuscripts and other teaching materials for my classes, inc.
even technical keys to some taxa (such as the freshwater fish of Vt). Many of my articles were on birds,
but by no means all. Again, I am very impressed by the Cool Green Science site! Thanks.
Don M.
Donald H. Miller, Ph.D. Univ. of Conn.: Zoology/Ecology
TNC member Williston, Vt.
My mom does not believe that the brown trout can eat mice but I do I know it’s true and you know it’s true
They definitely eat mice. In fact, I’ve written about this previously: https://blog.nature.org/2013/02/27/big-fish-rodent-eating-trout/
This should settle the argument. Brown trout eat rodents, and likely lots of them.
Shrews are known for “eating trout”. Fishing shrews have webbed feet and swim into streams to catch and eat trout minnows as a food source. Could this rainbow trout have noticed that the shrews were catching and eating their minnows and followed a protective instinct to destroy them? I have observed salmon species in these rivers chase off predator fish, such as the rainbow trout, that were eating their eggs and hindering their spawning efforts. It is possible that the rainbow trout has a similar protective instinct.
Wow, 20 shrew is impressive. In an area I fish I once caught a 12 inch brook trout. It had swallowed the hook so it was a keeper. When I cleaned it I found 6 salamanders! Not quite as impressive as 20 shrews but impressive none the less!