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Bristol Bay rainbow trout's last meal of 19 vermin mystifies scientists

Joseph Miller
Trout are known to eat small vermin including mice and voles. But molish shrews? Biologists studying Bristol Bay fish diversity get a lesson in local cuisine from a rainbow trout that died with 19 shrews undigested in its gut.
Togiak National Wildlife Refuge image

Control over one’s appetite, while long believed to be an exclusive human concern, apparently also affects trout living in the Kanektok River, which runs through the Togiak National Wildlife Refuge northwest of Bristol Bay, a multibillion-dollar commercial fishery in Southwest Alaska.

Last month, a team of state biologists were hip-deep in a tracking project geared at placing radio transmitters on at least 200 of the bigger rainbow trout in the Kanetok. Fisheries are managed in Alaska by state scientists and the tracking research will further understanding of the trout's migration and movements through the Bristol Bay watershed.

Several trout populations exist in the Kanektok, according to the study, meaning that the various groups may need to be managed separately instead of in one large, homogenous group. During the course of catching and releasing them, Department of Fish and Game biologists caught a 19-inch trout that was bleeding and unlikely to survive.

The 480-millimeter-long rainbow was cut open and the scientists found a surprise: inside the swollen fish were 19 well preserved shrews.

What prompted the trout to gorge itself on so many vermin? Turns out trout enjoy dining on small mammals like shrews, voles, mice and other little critters. But Terry Fuller, an educator at the refuge, said the trout's gut-full of shrews startled everyone and left biologists perplexed.

How did the score of shrews meet their demise?

“From my understanding, trout do make use of shrews for food often enough, but we’ve never seen this many mammals inside of a trout before,” said Terry Fuller from the refuge. “This might not be a world record, but it sure seems like it.”

Shrews are not a type of rodent but are instead more closely related to moles. Some fishermen use this knowledge of the fish’s mammalian appetite to their advantage. Small lures or patterns that mimic the appearance of a rodent are sometimes used to tempt an ambitious trout. The lures are most commonly a wad of deer hair tied on a hook and then trimmed to look like a rodent before casting it out on the water.

Depending on the location and the affinity of the fish, these lures can look pretty irresistible to the common trout.

“They do make what is called a mouse pattern for fly fishing,” Fuller said. “In the Togiak refuge, I know these patterns are quite common.”

The theories of how the trout was able to gulp down so many shrews are varied, but each one is certainly plausible. Shrews are not very strong swimmers, unlike voles, rodent-like relatives that have been known to swim the entire diameter of a lake.

One theory is that a nest of shrews was most likely swept into the river during a recent instance of flooding and drowned, and the lucky trout in question happened to be downstream at the right place at the right time. To the trout, it must have looked like an all-you-can-eat buffet of shrews coming down the river.

However, the main theory as to how these shrews met their demise might rule out the nest-theory, just from looking at typical shrew behavior.

"Most types of shrews are looking a litter or 6-10, so I have a hard time buying it was a nest. Shrews are solitary creatures for the most part unless they are breeding,” Fuller said, adding:  

 These shrews found in the trout were all roughly the same size and were relatively undigested, meaning that they were eaten in quick succession and the shrews were mostly together when they entered the river.

The main theory, or what makes the most sense, is that there are some islands in the lower end of the Kanektok River. What we were thinking was a surge of water pushed some of them off one of these islands and into the river. It’s also possible that more than one nest on the edge of one of these islands got caved into the river.

Trout have been known to be opportunistic and gluttonous eaters in the summer when trying to load up on food before the long winter, when prey are inactive. Some species of fish, including rainbow trout, enlarge their digestive tracts during summer months in order gorge. The typical diet of the

Alaskan rainbow trout consists of mostly salmon eggs, insects and small fish, but evidently, when shrews are on the menu, they become hard to pass up.

Previously the most shrews found inside of one fish was seven according to biologists at the Togiak National Wildlife Refuge, and these shrews were found inside of a grayling trout. To their knowledge, no shrews have ever been found inside the guts of a salmon.

This article was originally published in The Bristol Bay Times-Dutch Harbor Fisherman and is reprinted here with permission.