One of my favorite fall activities is to check local streams for salmon runs. Here in the Pacific Northwest, and extending north into Alaska, we have seven species of anadromous Salmonidae: chinook salmon (Oncorhynchus tshawytscha), chum salmon (Oncorhynchus keta), coho salmon (Oncorhynchus kisutch), sockeye salmon (Oncorhynchus nerka), pink salmon (Oncorhynchus gorbuscha), coastal cutthroat trout  (Oncorhynchus clarkii clarkii), and steelhead (Oncorhynchus mykiss). My favorite run is the chum salmon that run up Ellsworth Creek in southwest Washington each fall, but I’m honestly just happy to see any migrating salmon. And as I hike through stands of ancient western red cedar (Thuja plicata), I like to think about the many ways in which these and other forests need salmon for their ongoing health.

Anadromous fish are those that are born in fresh water, spend much of their adult lives in salt water, and then return to fresh water to spawn. Some, like Atlantic salmon (Salmo salar) and some populations of American shad (Alosa sapidissima) are iteroparous, meaning they can make this journey multiple times in a lifetime. Most Pacific salmonids, on the other hand, are semelparous, meaning that they spawn once and then die shortly thereafter. Cutthroat trout and steelhead are generally iteroparous, though many steelhead don’t survive the trip back to the ocean. (From here on out I am going to use “salmon” as a general, casual term referring to both the Oncorhynchus species, and the steelhead and cutthroat trout.)

Why Do Salmon Migrate?

Pacific salmon were originally freshwater fish that inhabited lakes and slow-moving rivers. Somewhere around 25 million years ago, the climate cooled significantly, with average temperatures dropping almost twenty degrees F. We’re not sure at what point after this the salmon began expanding into brackish estuaries and then the Pacific Ocean itself, but when they did they found rich sources of food unlike what they had access to in fresh water. Over time, they evolved a life cycle that let them be born in the relatively safe shelter of freshwater streams, and then go out to the ocean to feast on the banquet found there when they were large enough to have a better chance of survival.

Eventually salmon runs could be found in streams as far inland as eastern Idaho, eastern British Columbia, and the southern two-thirds of Alaska (with some Alaskan runs even crossing over into Canada!) And until the arrival of European colonizers, these streams consistently provided indigenous people all along the Pacific coastline an incredibly important source of food, cultural and economic trade, mythos, and more. Unfortunately, the newcomers overharvested the salmon, dammed and destroyed streams and other habitat, and of course spearheaded the causes of anthropogenic climate change.

All these factors have led to a precipitous decline in the size of both salmon runs, and the salmon themselves. This isn’t just detrimental to indigenous communities, though. It also threatens the health of forests all throughout the salmons’ range.

Forests Need Salmon

A forest isn’t just made of trees. It’s composed of entire plant communities, fungi (including mycorrhizal species), and the animals, bacteria, and other living beings that share space with them. When salmon travel up and down the waterways as fry, and then later to spawn as adults, they have a direct impact on that ecosystem.

Salmon fry are an important source of food for larger fish, amphibians, birds, and other beings that seek food in the water. In fact, part of why salmon lay so many eggs (over 5,000 in the case of chinook!) is because most of the fry that hatch will never make it to adulthood. But adult salmon aren’t safe from predation on their return trip to their birthplaces. In fact, they are caught and eaten by a wide variety of animals from bears to eagles, wolves to osprey, sea lions to bobcats.

Bears are of particular interest here. Brown bears (Ursus arctos) are well-known for gorging on summer and fall salmon runs to build up massive amounts of fat in preparation for winter hibernation. (Katmai National Park even celebrates their bears during Fat Bear Week every October!) You can watch video feeds of several bears hanging out in their favorite fishing spots by waterfalls and in the flow of the river.

Imagine that you are a young bear, perhaps recently forced to independence by your mother who is now focused on your younger siblings. You have to not only start catching fish without her protection from bigger bears, but you also need to make sure those stronger bears don’t steal your catch. What’s the best thing to do? Run far away into the woods to eat your salmon in peace, then leave the remains among the trees and head back for more.

If the fishing is good, bears will often eat only the fattiest parts of the salmon like the brains and skin, and then leave the rest behind for scavengers. The nutrients in the salmon then disseminate throughout the forest, whether carried in the digestive systems of animals, or broken down in place by decomposers. This helps make the nutrients available to the plants, particularly trees which may store massive amounts of nutrients in their trunks; when the trees die, they essentially become a food pantry for younger beings like new seedlings, fungi, and so forth.

Now–what’s so special about the nutrients in salmon? Well, remember that these fish spend years out in the ocean. And the ocean has an entirely different balance of nutrients floating around in it compared to what’s found in fresh water or on land. The salmon are essentially the only way these ocean-borne nutrients can make their way into the forest in any meaningful amount, and they do so on a regular basis each year. The trees near salmon runs fished by bears may be 300% larger than usual, and salmon also provide nearly three quarters of the nitrogen in the forest. That’s a pretty impressive contribution!

And Salmon Need Forests, Too

This isn’t just about how forests need salmon; it’s a reciprocal relationship. While the salmon’s immediate habitats are aquatic, these streams, rivers, and other waterways are directly affected by what happens on the land around them.

Every waterway has a watershed–an area of land from which precipitation drains into that waterway. These watersheds nest within each other; the watersheds of small streams are nested within the watersheds of the rivers the streams feed into. That water carries things with it, from soil to pollutants. So the health of the land has a direct impact on what is found in the water.

But it goes beyond what’s washed downstream, and into how it’s washed down. In a healthy forest, for example, the soil is able to absorb a significant amount of precipitation that falls throughout the year, keeping it from simply cascading down hillsides to create flooding and landslides. Water is also stored in the various living beings in the forest; again trees are often the champions with their great size, but smaller plants help with water retention quite a bit as well, both through internal storage and preventing evaporation from soil. A forest that is badly damaged, such as through a clearcut or wildfire, won’t hold water as well. This can lead to floods, landslides and other erosion, and increase the impact of summer droughts as the land simply can’t store as much water, or for as long.

All of this affects the salmon directly. If the watershed is no longer holding and releasing snowmelt, rain, and other water in a controlled manner, this can lead to flooding in waterways which can wash away salmon eggs and fry. Increased erosion buries the gravel that salmon lay eggs in with silt, smothering the eggs so they never hatch. When a riparian zone–the land along a waterway–is stripped of vegetation, the water loses crucial shaded areas that keep temperatures cool. Salmon easily overheat when temperatures rise even a few degrees. And drought can dry up smaller streams, stranding and even killing young salmon while preventing adults from reaching their spawning grounds.

While not every single salmon run exclusively travels through forests, many of them do. And many spawning grounds are found in forests, or at least areas with significant tree cover in riparian zones. Salmon must have healthy forests in order to continue to survive, and the loss of these forests is just one of many factors contributing to their severe decline.

Thankfully, I am far from the only person concerned about the safety of our wild Pacific salmon. There are numerous organizations working to protect and restore salmon habitat through dam removal, preservation and restoration of aquatic habitat and surrounding land, regulations on salmon fishing, and educating people about sustainable seafood options (or just not eating seafood at all.) And even habitat restoration efforts that aren’t directly in salmon-inhabited waterways still have a positive impact on the forest ecosystem as a whole.

We know that forests need salmon, and salmon need forests. To protect one is to protect the other, and long may they both thrive.

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