RIGGINS - Idaho is home to 42 different species of fish, from sturgeon to steelhead to smallmouth bass.

Salmon and steelhead season is closed right now - but over the next few months, with the high flows expected to continue on the rivers because of the abundant snowpack, this season's migration may be slowed a bit.

However, over the last few years fish experts say those migration patterns have been altered by other factors.

Sport fishing across Idaho's 26,000 miles of rivers and streams fires a big part of the Gem State's economic engine, pulling in more than $43 million each year, according to a recent survey by Idaho's Department of Fish and Game.

That makes it big business in small river towns like Riggins.

"Without this river we wouldn't have Riggins," says Lenard Hansen with Rubicon Outfitters. "Riggins wouldn't be here."

Hansen has been fishing the Salmon River since the early 90s. In mid-May the water is a little fast and a little cold to do little more than get a line wet.

But it's conditions like water temperatures that Hansen hangs on to know when to fish.

"If we don't get water around mid-50s to high 50s the fish don't bite," he says.

So his success swings right along with the readings on the river.

"We have ups and downs," says Hansen. "And as the water temperatures increase the fish seem to get here a little slower."

It's that slowdown of sockeye and steelhead returning to spawn that sparked Dan Isaak's attention.

"Trout, salmon, anything that lives in a stream or river is a cold water organism," says Isaak, a fisheries scientist with the U.S. Forest Service.

Isaak says cold water fish have very strict thermal boundaries, meaning temperatures have to be within a certain range for them to thrive. With a PhD in zoology and physiology, Isaak has been watching Idaho's water temperatures for 16 years. But he says trends have been ticking upward for a lot longer.

"People have been collecting temperature data in this region for 20 years, before they ever cared about climate change," he says.

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Today's information is tallied from 100 different agencies around the Northwest. Going back four decades,

Isaak says Idaho's river systems have seen an increase of more than a third of a degree every 10 years.

"It's not a big change but you add it up over the course of 40 years and probably your average river in Idaho now is about 1 degree Celsius warmer than it was in the mid-70s," says Isaak.

He says Idaho is fortunately buffered from some of the extreme changes because of its abundance of cold, steep rivers. But those fish that try to return from the ocean have to swim through lower, flatter, warmer water in the middle of the summer migration period.

A problem that peaked just two years ago.

"2015 was kind of the big year that kind of, i think, woke everyone up," recalls Isaak.

That year, a record low snowpack and low water levels combined with a record hot June saw water temperatures in the Columbia River Basin push into the 70s by July.

"And those fish were just swimming into a wall of warm water that they had never experienced before," says Isaak.

That led to a record die off of salmon, with many having to be trucked to Idaho to help them survive.

That summer aside, it hasn't been all doom and gloom for Idaho's rivers.

Says Isaak, "Here we are 20 years later, we still have a lot of salmon, lot of trout in Idaho and other parts of the Rocky Mountains and so these changes, even though they are occurring and they're real, they are happening slowly."

"Fish populations a lot of time can adapt to it," he adds. "And fishermen will have to as well."

Isaak says angler adaptation may mean, years from now, they might find themselves fishing for bass in rivers where they once fished for trout.

He also says another factor in the changing landscape for fish habitat is the variability with the water flow. It just doesn't seem to be as consistent as it once was, with the swings between high water years and low water years happening more frequently.