The Tiber Reservoir predator-prey puzzle

Winter days can be perfect for working out puzzles. Plenty of time to spend indoors trying to solve a riddle. And so it goes with fisheries workers, who spend cold days inside warm laboratories looking at fish guts.

This is the time of year Fish, Wildlife and Parks fisheries workers look at the contents of Tiber Reservoir walleye and northern pike stomachs, trying to figure out what those predators eat.

"We're trying to balance predators and the forage they eat," says Dave Yerk, a FWP fisheries biologist in Choteau. "The predator-prey issue at Tiber Reservoir is complex. It's not simple."

Stomach contents are gathered in the summer by FWP creel census takers at Tiber fishing access sites and in the fall from fish collected in nets by fisheries crews.

The current state record walleye - 17.75 pounds - came from Tiber just four years ago. Earlier this year, a fisheries crew netted a 16-pound walleye from the man-made reservoir on the Marias River. The fish was measured, weighed and released.

"Like most Western reservoirs, Tiber does not have a diverse forage base," Yerk said. "That means it takes all the pieces of the forage puzzle to come together to produce trophy-sized walleye and northern pike."

In the world of fish science, forage is the small fish that feed the big fish. With the right amount of forage, or prey species, predators like walleye and northern pike can grow big. But it's never that easy, especially at Tiber.

For example, spottail shiners, a minnow species, were put in Tiber in 1984 for walleye to eat. Shiners are now tremendously abundant, but mostly walleye don't use them, Yerk says.

"Spottails occupy an area near the shore where walleye are not normally found," he said.

What Yerk has discovered is the importance of yellow perch to walleye growth in Tiber Reservoir.

"We definitely see improved walleye growth when there are perch present," he said.

Perch reproduce on their own in Tiber, but they can use a little help because of the reservoir's fluctuating water levels. Nature helps out, with wet spring weather and lots of melting snow. Humans help by sinking leftover Christmas trees that perch use for spawning. When it all comes together, the perch reproduce tremendously.

"We saw a huge production spike from them the spring of 2010 and 2011," Yerk said.

Those perch are forage that help walleye grow to about 18 inches. At that length, it's cisco time. Cisco, a member of the trout family, were introduced in Tiber in the late 1990s. They eat plankton and grow rapidly, representing a huge food source for predator fish.

The problem is cisco grow so fast (they can hit seven inches in a year) that walleye have to reach the 18-20 inch length before they can really make use of cisco. Until then, perch are the key for the young, slower growing walleye.

So perch may be the final piece of the puzzle to unlock Tiber's potential as the state's next trophy walleye destination. Something to ponder on a wintry day.

Bruce Auchly is the information officer for Montana Fish, Wildlife and Parks Region 4.