Recently I stopped at FWP Region One headquarters in Kalispell and asked Fisheries Biologist Amber Steed what effects wildfires have on fish.
“One of the biggest negative effects of wildfire is the increase of sediment flowing into the stream in erosion after rains and snow melt.”
According to Steed, higher levels of sedimentation may affect fish in two ways.
“Sediments can suffocate eggs following the spawning process, thus reducing the numbers of eggs that mature into small fish.
“Also, stream-spawning fish often select gravelly areas of the stream where oxygen-laden water percolates up from the streambed. This is especially true of bull trout and westslope cutthroat trout.
“Higher levels of sedimentation may fill in the spaces between the pieces of gravel and reduce the oxygen available to the eggs.”
“When streamside trees burn and lose their crowns, there is a corresponding loss of shade which may contribute to higher water temperatures, but, more importantly, loss of shade means fewer hiding places for fish.”
“On the positive side,” added Sneed, “burned trees are more likely than live tress to fall into a stream and, once in the water, can serve as cover for fish, helping them to avoid predators, and to provide some shade otherwise lost (temporarily) when nearby vegetation burned.”
“Another possible positive effect of fire is a pulse of nutrients released into the soil in burned areas, then transported to streams during rain. These nutrients can help increase the aquatic food base for fish.”
Steed emphasized that fish living in streams are probably not affected by a sudden increase in water temperatures because they have the ability to move to more optimal conditions.
“What actually happens depends on the nearness of the fire to streams,” said Steed. “In Northwest Montana, the fact that cooler water may be entering the stream tempers the rise in stream water temperatures.”
While the immediate impacts of wildfire on fish might be minimal, long-term impacts can be more significant.
“What we’re having now with wildfires is much more likely to impact us than the fish!”