Salmon in the Imnaha

By Rick Ahrens, Science & Education Committee

Many hikers at this year’s Summer Camp saw salmon in the Imnaha River. I don’t know much about fish, but I did some research and here’s what I found. The big guys we were seeing are the official State Fish of Oregon, the Chinook salmon. Early arrivals begin showing up on their spawning grounds in May and so this group is called spring Chinook. Sometimes you also hear them referred to as “king” salmon, because they are the largest of the five Pacific salmon species.

Most people are familiar with the incredible journey salmon make to the sea and back. Anadromous is the technical word for this life history, traveling from fresh to salt and back again to fresh water. Their migratory instinct is guided by an extremely keen olfactory sense. Salmon literally smell their way upriver, to locate the exact stream where they were born. It’s amazing to think that the fish we saw had to travel more than 550 miles from the ocean and negotiate eight dams to get to their spawning grounds near our camp. After they reach the mouth of the Columbia, the salmon stop eating and live off their fat reserves. The meat is still good to eat early on, but by the time they are spawning it has deteriorated significantly.

A “run” is a group of salmon that return to spawn at the same place at about the same time every year. An estimated 104 wild runs of salmon in the West are already extinct. The spring Chinook run in the Imnaha is currently federally listed as threatened.

The Chinook run in 2001 was exceptionally good on the Imnaha. The runs in 2002 and 2003 were slightly lower, but still very good given recent trends. Early estimates were that about 4100 Chinook would return to the Imnaha this year. Sixty percent of these returning salmon were hatchery raised, the rest were wild salmon. The reasons for this recent increase are complex. Good water flows in recent years may have helped to flush young salmon, as well as nutrients downstream. Water temperature and nutrient cycling are the two key components of survival of young salmon once they reach the ocean. The latter has to do with the upwelling of nutrients offshore. Collectively referred to as “ocean conditions” they are cyclical, not fully understood, and connected to larger processes.

More enlightened stream practices instituted in recent years may also have paid off. Among these are leaving large woody material in streams, regulating water flows for young smolts heading downstream, screening irrigation diversion channels, minimizing the impact of cattle on riparian areas, keeping and planting trees along streams to shade and cool the water and reduce sediment loads, monitoring pollution, and enforcing fishing regulations.

Chinook returning from the ocean average 18–34 pounds, with the record fish taken in Alaska weighing in at 128 pounds. The age of return varies from one to six years. Very small males are called “jacks” (about 5% of Chinook) and may only spend one year in the ocean before returning to spawn. They can’t compete with the dominant males, but attempt to sneak in and fertilize eggs. The vast majority of Chinook spend two to three years in the ocean. Early arrivals begin passing the Bonneville Dam in mid-April. After arriving in their home streams, they then wait for several months in deep pools before they begin to spawn.

The males were the fish we were seeing chasing each other around. The dominant male guards the female until she is ready to lay her eggs. The females were the fish turning on their sides and digging a circular depression in the gravel. These nests are called “redds”. A female salmon lays a total of four to six thousand eggs, often utilizing more than one redd. Clean, cool water and sediment-free gravel are vital for survival of the eggs and developing young.

Incubation takes an average of 50 days (at 50°) and is dependent on water temperature. Eggs are timed to hatch in late winter. The young “alevins” remain in the gravel for several more weeks after hatching, absorbing a yolk sac still attached to their bellies. The “fry” then emerges from the gravel and begin to feed on insects and other tiny animals. They stay in the area or drift downstream until the following spring, when the young “smolts” begin their perilous journey to the ocean. (Only one in a thousand eggs laid makes it back upstream to spawn.) The young salmon then linger in the estuaries near the ocean, to allow their bodies to adjust to salt water. They then head out into the open ocean where they may travel as much as 3,000 miles feeding on baitfish (sandlances, herring, pilchard) before returning to spawn.

After they have finished spawning the Chinook, like all the Pacific salmon, die. The bodies of the dead fish provide nutrients for the stream, which helps to ensure the survival of the next generation. There is also evidence that the young salmon obtain nourishment from feeding directly on the carcasses of adult salmon. Salmon are called a “keystone species” because they are vital to stream ecosystems. They transport the riches of the ocean upstream to hundreds of tributaries. They make the ultimate sacrifice not only for their offspring, but for the ecosystems on which they depend.

The other good sized fish we were seeing in the Imnaha were most likely bull trout (formerly known as Dolly Varden). This species, like the Chinook, is also federally listed as threatened.

[Information for this article was gathered from ODF&W literature and talks with fish biologists in Enterprise and Eugene - R.A.]

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