Autumn sees several species of native salmon return to their home rivers to spawn. Chinook, coho and chum salmon are among those that can be seen swimming the Columbia River, Willapa Bay and its tributaries.
Although salmon also run in the spring, summer and even winter, the fall runs are the most numerous. All of these anadromous fish were born in the same freshwater they seek as adults, although they spend most of their lives in the Pacific Ocean. Unlike Atlantic salmon, however, local salmon die once they reproduce, making this a unique opportunity to create the next generation.
Salmon have evolved over millions of years, and after the climate cooled about 25 million years ago, it is believed that ancestral salmonids began to leave freshwater lakes and rivers for the bounty of the ocean
But the cold, clear rivers and streams were the perfect safe places for them to lay their eggs in gravel beds, providing fewer risks than the wide expanse of open water. While some salmon travel only a short distance up rivers and streams to spawn, some range for hundreds of miles, with fish seeking spawning grounds as far inland as eastern Idaho.
This life cycle is highly dependent on a careful balance of water availability and temperatures. For millions of years, streams fed by snowmelt and rain runoff have had fairly cold temperatures, and riparian trees and other vegetation that shade the water prevent it from warming further.
Salmon born in these cold waters spend their first few months there, although coho can wait until they are a year or two old before traveling to the ocean.
Migrating salmon fry often spend a large amount of time in estuaries such as Willapa Bay before exiting to the ocean. Once settled in the Pacific, they continue with a cold lifestyle. As anyone who has waded into the surf on a hot summer day knows, even shallow water can be cold year-round.
All this adaptation to cold water makes salmon quite sensitive to warming temperatures. Each living being has a certain temperature range within which it can live; too cold and they can die of hypothermia and too hot means they can die of heat stroke or other heat related illnesses. Because salmon does not have the benefit of air conditioning, it is at the mercy of ambient temperatures.
Brian Alfonse of the Oregon Department of Fish and Wildlife prepares to release an adult salmon into a stream near Clatskanie as part of the department’s ongoing investigation.
Summers on the North Coast have been getting hotter and drier in recent years. Despite the proximity to the ocean, which typically keeps temperatures cooler, the region has had many days that exceed 90 degrees Fahrenheit.
Rainfall is also less predictable. Last year, the rain dried up in early spring, and this year its return in the fall was delayed by weeks. All this is the result of anthropogenic climate change.
Both increased heat and changes in precipitation have dramatic negative effects on freshwater salmon habitats. Adult fish struggling upstream are more likely to overheat and die in warmer waters, assuming they can even return to their actual spawning grounds.
As smaller, shallower streams dry up more often, some salmon are completely cut off from their spawning grounds and die without ever having spawned there. They are naturally programmed to spawn at the exact location of their own birth, so they can’t just choose to spawn somewhere slightly more convenient.
Those salmon eggs that hatch the following spring produce fry that are also at greater risk. They must be able to withstand the summer heat in their native streams and rivers, despite the warming that causes their nurseries to heat up. In addition, the destruction of trees and vegetation along riparian zones, whether due to logging, agriculture, development, or forest fires, means that they no longer have shady places to hide to maintain – get fresh
If a section of the stream downstream ends up drying up, fry can be cut off from their route to the ocean. Some can wait until the rain returns, but others die of heat, starvation or predation in their small, isolated pools.
The problem, however, is not only the heat. Earlier snowmelt and heavier spring rains cause rivers and streams to have stronger and faster flows earlier in the year. Not only can this carry away whole redds (nests) full of unhatched salmon eggs, but young fry can be swept out of safety and killed by the force of the flow itself, or be washed into places less hospitable with less food and shelter, or more predators.
However, they are not out of danger if they reach the ocean. Earth’s oceans have been absorbing massive amounts of additional heat due to climate change; while they are capable of being natural heat sinks, they can and are becoming overloaded. It only takes a few degrees for the average ocean temperature to put cold-dependent fish like salmon in serious danger.