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Winter hazards affect summer trout populations

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Kelly Nicholson fishes the Gallatin River on a winter day. PHOTO BY RICH ADDICKS

By Stephanie Lynn Gallatin River Task Force

Montana winters are not for the faint of heart. And for trout living in local rivers and streams, cold weather can be particularly perilous.

According to Pat Byorth, director for the Trout Unlimited Montana Water Project, “Trout suffer three primary risks during the winter: physiology, ice and oxygen.”

Cold-blooded westslope cutthroat trout thrive when the water temperature falls between 50- and 63-degrees F. However, average winter conditions on the Gallatin River drop below this ideal range, forcing fish to enter torpor, a sluggish state, to survive. In this condition, trout require less food due to reduced metabolism, but lack the energy to escape predators.

To complicate matters, ice constricts habitat at a time when rivers are already low. Anchor ice growing from the streambed covers hiding spots and can entomb young trout while frazil, or slush ice forming on the surface of rapidly cooling rivers, causes damage to gills. Finally, collapsing ice dams can trap or crush unwitting fish.

Although oxygen levels are typically high in cold rivers and streams, including the Gallatin, ice and snow can limit aeration in shallow ponds with decaying vegetation. On occasion, low oxygen levels cause winter fish kills.  

To endure these threats, trout move from summer feeding lanes to winter habitat when the snow flies. Juveniles hide between streambed boulders and large woody debris while adults hunker down in large, deep pools to await the return of spring. Pools and pockets, in particular, are sensitive to human impacts, such as water use that lowers stream levels, barriers that alter connectivity, and sedimentation that fills hiding holes.

Dave Moser, Montana Fish, Wildlife and Parks fisheries biologist, said, “The limiting factor for many Rocky Mountain trout populations is overwintering habitat, particularly adequate pool depth in high elevation streams. If fish can’t make it through the winter, or, in the case of fall spawners, if their eggs don’t make it to hatch, that can limit numbers of fish and extent of occupied habitat.”

Despite these challenges, trout are adapted to survive winter in the Big Sky area. And for those willing to weather the cold, the fishing can be extremely rewarding. Josh Berry, a fly-fishing guide with Gallatin River Guides, said, “From my perspective, I like guiding in the winter more, there’s more open water, fewer people, and the fish are still eating.”

Berry explained that catch-and-release anglers should follow summer fishing best practices to avoid harming fish. “It’s even more important in the winter to keep fish in the water because their gills could freeze,” Berry said.

During the next high pressure system, when it’s 35 degrees and sunny, take a break from the ski hill and try winter fishing. Just remember to handle fish with care.

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