The Ecology of Glacial Rivers–Su River runs of humpback, sockeye, and coho


Adult Coho Salmon–photo credit Susitna Salmon Center


The seventh in a series from the Susitna Salmon Center. This segment by Jeff Davis deviates from the ecology theme to tell about the runs of the other four species of  salmon in the Susitna River drainage. From tagging studies, Department of Fish and Game biologists have information about when the runs are, where most of the salmon spawn, how long they spend in freshwater habitats, and other details of the spawning season. Chinook salmon were covered in the previous episode.

Until recently, it was commonly believed that glacial rivers, like the Susitna River, served primarily as migration corridors for Pacific Salmon and that they provided little spawning, rearing, or overwintering habitat. Recent studies tracking radio-tagged salmon in the Susitna River showed that a portion of adult salmon spawn within the Susitna River mainstem. In the previous episode we summarized information on the run timing, length, and spawning destination of Chinook, or King salmon. This episode summarizes the run timing and spawning locations of the other four Pacific salmon found within Susitna River drainage, Pink salmon or Humpies, Sockeye salmon or Reds, Coho or Silver salmon, and Chum or Dog salmon.

There is a large amount of overlap in the run timing of these four salmon species. From 2012 through 2014, Pink salmon catches in the fish wheel at Curry began around July 21 and extended through the middle of August. Approximately 50 percent of the run passed the capture site by July 31st. Average fish length from mid eye to the fork of the caudal fin was 45 cm or 17 inches. Pink salmon were one of the fastest migrating fish traveling up to 30 km or 18 miles per day. Over 6 percent of the Pink salmon that were tagged at the Curry station spawning within side channels of the mainstem Susitna River, more than any other tributary except for the Indian River and Portage Creek. Salmon eggs incubate through the winter and fry or alevins emerge in the spring. Incubation time varies with water temperature. Pink salmon fry migrate directly to the ocean where they will stay for one year before returning to spawn. Therefore, Pink salmon return in 2019 is from fish that spawned in 2017. The Susitna River has a larger return of Pink salmon during even years. For example, the Pink salmon return at the Deshka River Weir, was 65,000 in 2016 and 25,000 in 2017.

Sockeye salmon began to be captured in the Curry river fish wheel in the middle of July and the run extended through the end of August. Sockeye salmon life history is generally associated with lakes. Sockeye spawn in lake tributary streams or in upwelling areas along the lake shore. Emergent fry migrate to the lake where they rear for one or two years. However, glacial rivers, like the Susitna River support “river type” sockeye salmon. These salmon spawn within side channels and side sloughs of the Susitna River and rear in backwater sloughs or beaver ponds. Over 44% of the Sockeye salmon tagged at the Curry Fishwheel spawned within the mainstem side channels and sloughs of the Susitna River. Most of the Sockeye salmon tagged by the Alaska Department of Fish and Game in 2008 spend 1 year rearing in fresh water and 2 or 3 years in the ocean.

Coho salmon arrive in the Middle Susitna River near the end of July and the run extends into September. The average length of Coho salmon captured from 2012 to 2014 was 52 cm or 20 inches long. A surprising portion of the Coho salmon that entered the Middle River, approximately 10%, spawned within side channels of the Susitna River. More coho salmon spawned within the main channel of the Susitna River than in Portage Creek, a large-clear water tributary. Most Coho salmon in the Susitna River drainage spend 2 years rearing in fresh water and one year in the ocean before they return to spawn.

Chum salmon began to show up in Curry fish wheel catches in late July and the run extended through August. Chum spawning occurs from late August through September. Chum salmon were 55 cm or 22 inches long (mid-eye to fork). Approximately 13 percent of the Chum salmon that entered the Middle Susitna River spawned within sloughs and side channels. Chum salmon fry, like Pink salmon, migrate directly to the ocean after they emerge from spawning gravels, where they spend 3 to 5 years before the return to spawn.

All Pacific salmon die after they spawn. The return of adult Pacific salmon to spawning areas is a large transfer of energy and nutrients from the ocean to glacial rivers.


The Ecology of Alaska’s Glacial Rivers

Episode 6. Mainstem Susitna River spawning by Coho, Chum, Pink, and Sockeye salmon

By Jeff Davis

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