MTPR

Linchpin Food For Invasive Trout Seeing Major Changes

Sep 27, 2019

The Confederated Salish and Kootenai Tribes are in the midst of their Mack Days fishing contest on Flathead Lake and their parallel gillnetting efforts aimed at reducing invasive lake trout numbers. Both are showing signs of working, but a this comes as one of the species’ main food supplies is going through some major changes.

On a sunny day this summer, Tribal Fisheries Biologist Barry Hansen was on his way to collect the catch from one of the tribes’ gillnetters fishing along a deep section of Flathead Lake.

As he arrives, the crew is pulling in the nearly three-mile long net filled with about 400 lake trout of all sizes. The operation is part of the tribes’ lake trout suppression program, which aims to reduce lake trout numbers in an effort to restore native trout species.

“In the peak season, we run two boats, and where we’re fishing right now is what we call the trench. It’s 300 to 350 feet deep. It’s heavy with Mysis, so the lake trout really concentrate there now,” Hansen said. 

Mysis shrimp made their way into Flathead Lake in the 1980s from a nearby water system where they were introduced to boost a salmon fishery. The Mysis population exploded and so did the number of lake trout feeding on them.

Over time, Mysis became a linchpin in Flathead’s ecosystem, affecting everything from the top to the bottom of the food chain. However, Mysis populations are changing. Overall abundance of the small crustacean is low compared to historical highs.

“Over the last five years on average it’s been about 60 individuals per square meter. We’ve had as high as 110,” Flathead Lake Biological Research Station Ecologist Shawn Devlin said. 

Devlin says Mysis populations have actually been quite erratic over the last decade, with 50-percent population swings every few years.

Recent research also indicates that Mysis in Flathead Lake are breeding at different times of the year.

“Mysis are breeding more," Davlin says. "They usually had a single cohort in a year and we’re seeing double and, in some years, we’ve seen a third cohort appear.”

How well that second or third cohort survives is likely the direct cause of population changes year to year, but it’s hard to say what’s driving the overall change in reproductive strategy. Devlin says it could signal things are really good for Mysis or that it’s a last-ditch effort to keep the population going.

Mysis from trout stomach.
Credit Aaron Bolton

Mysis have been known to crash out of lake systems with high abundances, but Devlin says it’s too early to predict whether recent population swings are a signpost for a similar event in Flathead Lake.

“I think we’re just going to continue to see this oscillation as this dynamic occurs. I think that the status quo of up, up, down, down, down, up kind of thing is where we’re going to continue to see,” Devlin said. 

Back on the gillnetter, Hansen with the Salish and Kootenai Tribes says that trend hasn’t had a huge impact on lake trout, at least so far. Annual catch assessment numbers are trending downward, indicating an overall population decline, but he says that’s due to the tribes’ suppression efforts. They’re aiming to reduce adult lake trout by 75 percent.

The tribes estimated there were nearly 1.5 million lake trout in Flathead Lake back in 2014 when they began gillnetting. Between that and the Mack Days contest, the tribes are currently harvesting about 110,000 lake trout annually, but the plan is to increase that number.

“The near-term goal is to catch more every year toward the long-term goal, which is in total 143,000 lake trout, and that’s by all methods – by the fishing contest, by gillnetting and by public as well,” Hansen said. 

On shore, Hansen and his crew are unloading the day’s catch at a processing facility. He says if Mysis populations are indeed headed for a decline, that may help.

“If they do go down over time, that would make it harder for lake trout and maybe make our jobs a little easier, but I don’t think it converts that way,” Hansen said. 

It’s unclear exactly what the large changes in Mysis populations will mean in the long term for lake trout and the greater food web. But without immediate ripples in the ecosystem, one thing is clear, Flathead Lake is showing its resilience to change.