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The Back Porch


Solving Tiber’s Predator-Prey Puzzle

This story is featured in Montana Outdoors July-August 2012 issue

When Montana Fish, Wildlife & Parks fisheries workers survey a lake or stream this time of year, they usually look at the outside of the fish they catch. The length, for instance. Or whether a fin has been clipped (indicating a stocked fish).

But sometime they are equally interested in the fish’s insides.

That’s the case at Tiber Reservoir, an impoundment of the Marias River on the Hi-Line east of Shelby.
Stomach contents of walleye and northern pike are gathered by FWP creel census takers in summer at Tiber fishing access sites and in fall from fish collected in nets by fisheries crews. The partially digested material is preserved in formula. In winter, fisheries workers in laboratories poke through the stomach contents trying to figure out which fish species those predators consumed.

Glamorous work, no. Important, yes.

“The predator-prey issue at Tiber Reservoir is complex,” says Dave Yerk, FWP fisheries biologist in Choteau. “We’re trying to balance predators and the forage they eat.”

Tiber produces some massive predator fish. Northern pike over 30 pounds have been reported. The state record walleye—17.75 pounds—came from Tiber just five years ago.

To ensure that more pike and walleye like those swim in the reservoir, FWP looks closely at what they eat. “As is the case in most western reservoirs, Tiber doesn’t have a diverse forage base,” Yerk says. “All the pieces of the forage puzzle have to come together to produce trophy-sized fish.”

In the world of fisheries science, forage species are the small fish that feed the big fish. With the right amount of forage, or prey species, predators species can grow big. Making that happen is never easy.

For example, in 1984 FWP put spottail shiners, a minnow species, in Tiber for walleye to eat. The good news is shiners are now tremendously abundant. The bad news is that Tiber’s walleye rarely eat them.

“Spottails swim near the shore where walleye are not normally found,” Yerk says.

Perch, on the other hand, live in the same areas walleye do. And walleye very much enjoy eating their smaller, striped cousins. “We definitely see improved walleye growth when perch are present,” Yerk says.

Perch reproduce on their own in Tiber, especially when high water floods the vegetated shoreline. Perch spawn best on flooded trees and shrubs. Unfortunately, Tiber’s water level fluctuates so much that vegetation has a hard time taking root. Plus, the area is arid and the soil grows few plants.

Nature helps out with wet spring weather and lots of melting snow. Humans help by sinking leftover Christmas trees that perch use for spawning. FWP workers and the Great Falls Chapter of Walleyes Unlimited coordinate this work each winter.

When it all comes together, perch reproduce very successfully. “We saw a huge production spike in spring of 2010 and ’11,” Yerk says. Those perch are forage that help walleye grow to about 18 inches.
At that length, it’s cisco time.

FWP introduced ciscoes into Tiber in the late 1990s. These salmonids eat plankton and grow rapidly, representing a huge food source for predator fish. The problem is that ciscoes can grow too quickly, reaching up to 7 inches long in a year. Not until walleye are 18 to 20 inches long, about at age four, are the predator fish large enough to eat ciscoes.

Northern pike, on the other hand, are such gluttons they can start eating ciscoes at age two. A lifetime diet of these fatty fish is what produces 20- and 30-pounders.

Walleye need perch until they get big enough to get their jaws around a cisco for supper.

That means perch may be the final piece of the puzzle to unlock Tiber’s potential as the state’s next trophy walleye destination.

It’s something to ponder the next time you’re there, waiting for a fish to bite.Bear bullet

Bruce Auchly manages the regional Information and Education Program in Great Falls.