The Quiet Love Affair Between Fish and Trees

We all know fish live in water, but many of us don’t realize that their world stretches up onto the banks and beyond. Sure, fish don’t occupy that space. But what happens out of the water can affect them profoundly. This story is about the quiet love affair between fish and trees.

Juvenile Rainbow Trout find shelter under a floating log near the confluence of the Kenai and Russian Rivers in Alaska. Photo: USFWS/Katrina Liebich
Fish love trees…

What happens in the riparian zone — the land next to rivers and streams — doesn’t always stay there. Trees frequently topple over and fall in. Here, they begin a new chapter where fish, not birds, flit about their branches and roots.

Baby salmon seek cover within the branches of a tree (left) while adult Coho Salmon rest behind a submerged tree just downstream (right). Videos: USFWS/Katrina Liebich

A tree that is large enough and falls just right creates a spillway of water that carves out a pool downstream. If you’re a fisherman or woman, you know fish love pools. The slow water offers a place where they can escape the current. For a weary salmon migrating upstream to spawn, this break is welcome. Pools can also provide a watery refuge during drier times of year when channels get shallow.

Big trees along the bank mean big trees in the creek and habitat for fish. Here’s a section of Campbell Creek in Anchorage, Alaska. Photo: USFWS/Katrina Liebich
…but not all trees are created equal

From a fish habitat perspective, it’s not just the presence of trees along a bank that’s important. Size matters and so does the species. Large conifers stabilize banks with their large root systems. When their big trunks fall in streams they tend to have a bigger impact and stay put longer. Red alders might provide important nutrients to the soils, but are thin and rapidly decay once they fall. They’re less effective at creating the complex stream habitats that give fish options. For a fish, having options is the key to survival. Diverse habitats can make all the difference for a salmon that needs a moment of rest before finding suitable gravel to lay her eggs. Or a juvenile Cutthroat Trout that would have become dinner for a predator if it weren’t for a rootwad or log jam.

Red alders (left) grow relatively thin and decay rapidly in the rainforest habitat of Prince of Wales Island, Alaska. Large conifers (right) — the stuff of old growth forests—have lasting power in terms of fish habitat when they fall. Photos: USFWS/Will Rice
Trees provide shade.

A wooded stream may be a few degrees cooler than a stream that’s lost its riparian vegetation. And a few degrees can mean life or death for fish: each species thrives within a specific range of water temperatures. Salmon get stressed when water temperatures exceed 17°C (~62°F), and may die after prolonged exposure to water just a few degrees too hot.

Is native vegetation lining the banks of the lakes or rivers you care about? Photos: USFWS/Katrina Liebich
Trees are a key link in the food chain…

If you happen to know any fly fishing fanatics, they will happily show you prized tackle boxes full of flies. Some flies imitate the aquatic larvae of winged insects. Beneath the surface, these larvae consume, break apart, and collect bits of leaves and wood. As they drift downstream, they’re snatched up by baby salmon or adult trout. Others flies in the tackle box are tied to “match the hatch.” They resemble the nymphs undergoing metamorphosis into adult winged insects that fish key in on as they hatch en masse and dance across the water surface.

Clockwise from top: Leaves litter the water’s edge in Alaska’s Mat-Su Valley (USFWS/Katrina Liebich); Caddisfly larvae shred leaf litter from the safety of cases they built from leaves and stones (J.J Casas); A Dolly Varden prepares to make a meal of a recently hatched insect (Mark Stadsklev/artwithinnature.com)
…and fish feed the trees.

In death, sometimes fish return the favor. The reciprocal exchange of nutrients from fish to trees is perhaps best illustrated with Pacific salmon. Juveniles that initially draw life from aquatic insects eventually head to sea where a smorgasbord of larger, nutritious prey awaits. With the nutrients they assimilate at sea, salmon grow big bodies that can carry thousands of plump eggs back to their home river. The return journey ends in death, and the decaying fish carcasses release the nutrients gained at sea. Wildlife and occasional flooding bring carcasses beyond the banks of the river, into the woods and meadows. Scientists have found that trees along salmon streams have the advantage over their salmon-less counterparts: the marine-derived nitrogen from the natural fish fertilizer helps the trees grow faster. In perhaps a stroke of genius, the salmon help the big trees that will someday provide big pools and habitat complexity for future generations of fish. Hopefully the love affair continues.

Salmon carcasses feed the trees. Photo: Mark Stadsklev/artwithinnature.com
You can help
  1. If you’re a waterfront property owner, DO maintain or replant native trees and shrubs along the bank and explore nature-based solutions to erosion (like rootwads from locally-harvested trees — see video below). DON’T clear vegetation down to the waters edge or use traditional riprap that has minimal value as fish habitat (e.g., concrete).
  2. Find out if your local/state governments have ordinances that call for riparian setbacks. An intact riparian zone along rivers is good for fish, wildlife, and people.
  3. Help build awareness by hitting ♥ and sharing this story. For more on Alaska’s fish and their habitats and how to help with conservation follow us on facebook and subscribe to our newsletter.
View from a waterfront property along Alaska’s Kenai River. The landowner worked with the U.S. Fish & Wildlife Service and Alaska Department of Fish and Game to replant native vegetation. USFWS/Katrina Liebich
Spruce trees with rootwads attached are the stars of this nature-based solution to erosion along Alaska’s Chena River. Watch 300ft get a week+ long fish-friendly makeover in 30 seconds flat. USFWS/Mitch Osborne & Katrina Liebich

Katrina Liebich is based in Anchorage, Alaska where she has served in her current position as Fisheries Outreach Coordinator for the U.S. Fish & Wildlife Service since 2010. Excerpts included in this story courtesy Will Rice, 2016 U.S. Fish & Wildlife Service Directorate Resource Assistant Fellow and current graduate student researching the human dimensions of public lands with Penn State University’s Parks Studies Unit. See more of his work here: For the Fish: The Stories of Alaska’s Fish Habitat Partnerships