Why are trees important for salmon? Let us count the ways

Ballinger Creek
Photo by Londa Jacques

By Jim Halliday – LFP Stewardship Foundation Advisory Board Member, and 
Doug Hennick, Aquatic Ecologist, Wild Fish Conservancy and LFP Stewardship Foundation Board Member.

Aquatic life depends on an arboreal setting because all parts of trees contribute to enriching the habitat and improving water quality.

Trees, wherever they are in the watershed, help stream water quality by improving the soil so it removes pollutants (like heavy metals) from storm water.

And if they are near streams, trees help by stabilizing banks and hillsides because tree roots stabilize the soil in ways that reduce sediment from entering streams, which otherwise would smother fish eggs and keep fingerlings from finding food by making the water too muddy to see through.

Boeing Creek
Photo by Kyle McQueen

Exposed roots also make pockets of slow water, where fish can rest while being ready to dart out into the current to snatch a bug as it goes by in the fast water.

When trees and their large root balls fall into streams, they become “large woody debris” that provides more pocket water for fish to use, and surfaces for algae to grow on, which in turn feeds more insects for fish to eat.

The surfaces of logs in streams grow three times more algae than the same surface area of rocks, so fallen trees in streams greatly increase aquatic insect food, and the extra insects become extra salmon food.

Also, the terrestrial insects that accidentally drop into streams from overhanging trees are a valuable and bountiful fish food, and the leafy debris that falls into streams is food for macroinvertebrates that also feed young salmon.

The soil of forested watersheds absorb rainfall, and release it slowly ensuring year-long continued underground flow to streams to support spawning and rearing fish.

Twin Ponds feed Thornton Creek
Photo by Melissa Banker

Trees and vegetation cool streams in summer. This is important because warm water kills salmon.

And a lot of the rain and snow that falls on trees throughout a watershed evaporates before it even reaches the ground, thus minimizing the violence of storm flow in streams. That violence can kill fish in numerous ways, from flushing them downstream into places where bigger predators live, to moving the gravel so much that fish eggs wash away.

Chinook salmon in Thornton Creek

Also, just like trees help salmon live and grow, salmon help trees by bringing fertilizer to them.

Marine-derived nutrients from the carcasses of salmon stimulates the growth of conifer trees along northwest salmon streams and rivers.

The carcasses get into the forests by raptors such as eagles and ospreys carrying them to their nests, and by bears and other mammals dragging salmon into the woods for eating in peace.

This activity contributes to the increased growth of the trees and shows a positive correlation with the number of salmon returning the previous year.