Symbiotic Fish and Forests

With the arrival of fall, salmon spawning is in full swing in the Pacific Northwest. Being a fish, they obviously interact with a lot of other sea creatures and aquatic life. They also have a symbiotic relationship with the forest, believe it or not.

Fish and forests, at first glance, are as different as, well, fish and forests. But despite being a water-based critter and chunk of land, they interact with each other splendidly. They feed each other. I’ll explain how, starting with the how the forest helps the salmon.

McClane Creek, an example of healthy salmon habitat

Forest trees are crucial to salmon habitat. Despite living in a river for part of their lives, salmon lean heavily on trees for survival. Salmon need cool, clean water. The forest keeps the river clean by slowing the flow of surface water and the sediments it carries. Sediments can clog their gills and cover the rocks where they lay their eggs during spawning. Shade from the trees keeps the water temperature at acceptable levels during the summer. Warmer water loses dissolved oxygen, and salmon are very picky about temperatures.

Pink salmon returning to the cool, shaded water of Nisqually River

Fallen trees in the river create habitat diversity. The large woody debris forces the current to change, creating deep pools on the downstream side of the obstruction and changing the complexity of the stream bed. The deep pools provide a place to hide from predators and respite from high flows during the rainy season. The large woody debris adds nutrients to the ecosystem as it decomposes. It also contains or attracts tasty insects for the salmon to munch on.

Artificial logjams like this are used as habitat enhancement
in salmon streams.

The salmon help replenish the forest by provided nutrients (nitrogen in particular) from the sea when they die. Predators are the immediate beneficiaries of the ocean’s bounty, but the forest as a whole benefits. Plenty of leftovers decompose both in the river and on land. As the fish decomposes, or after a bear poops in the woods, the trees and other plants absorb the nutrients so they can continue to give back to the salmon and the cycle continues for another season.

Salmon bones replenishing the Hoh Rainforest

It was once thought that removing obstacles like large woody debris was an improvement to the riparian habitat. Now we know better. In some areas, workers are dropping downed trees into rivers. In others, engineered logjams do just as well. Watch this short video about riparian restoration for the benefit of salmon and flood control in Oregon.