The Ecology of Glacial Rivers: How fish have adapted to silty water

Longnose Sucker, photo by Annie Helmsworth

Longnose Sucker, photo by Annie Helmsworth

The second of a new series from the Susitna Salmon Center: The Ecology of Glacial Rivers. This segment, by Jeff Davis, describes how suspended sediment changes conditions in main channels of glacial rivers, and how fish have adapted to these changes.

The main channels of glacial rivers carry a large amount of fine sediment suspended in the water column during the summer months when runoff from glaciers is at its highest. This sediment gives glacial rivers their brown or turbid appearance. The amount of sediment can be reduced in glacial rivers that contain lakes, as some of sediment is deposited in the lakes.

Suspended sediment affects the amount of food within a river, the ability of fish to see and capture available prey, and the quality of spawning habitat. Resident fish species that spend their entire lives in glacial rivers and anadromous salmon that spend part of their lives spawning and rearing in fresh water, have developed different ways of adapting to the high levels of sediment. The fine sediment in glacial rivers does not appear to affect the ability of fish to obtain oxygen through their gills as is commonly believed.

The sediment in highly turbid glacial rivers absorbs or reflects a lot of the sunlight that reaches the water surface. The resulting low light levels can reduce the amount of algae growing on the stream bed, which is one of the primary source of food for aquatic insects and ultimately fish in large rivers. In glacial rivers, most of the plant material for aquatic insects during comes from trees and shrubs on the stream bank, from clear water tributaries, or from high algal growth during the spring or fall when the mainstem water is clearer.

The low light also makes it more difficult for fish to see insects and other prey items. Salmon fry or juvenile salmon that feed on insects drifting through the water column must not only be able to see the insects, but be able to tell the difference between an insect, suspended sediment, and pieces of leaves or sticks. However, the sediment also provides protection for these juvenile salmon, by providing visual cover from larger fish, birds like kingfishers or mergansers, or other predators.

Some of the sediment in glacial rivers settles to the river bed filling in the spaces between gravel and larger rocks. This reduces the space for aquatic insects that live between or under rocks and the amount of water flowing through the stream bed. Water flowing through the stream bed is important for supplying oxygen, and carrying away waste materials from incubating fish eggs.

Resident and anadromous fish species have adapted to the large amount of sediment in the main channels of glacial rivers. Resident suckers and burbot have adapted ways to find prey in low light. Suckers have protruding lips which they use to feel along rocks and mud for insects and other food items. Burbot, a freshwater cod, also have a barb below their mouth which they use to feel along the stream bottom and a strong sense of smell which they use to find fish, fish eggs, or carrion. Whitefish, which are part of the salmon family, have a mouth that is directed toward the stream bottom which they use to pick insects from rocks on the stream bed. Chinook and coho salmon parr, or juveniles, are found rearing along the margins of the main channel, but may be more abundant near the mouths of clear-water sloughs, tributaries, or locations where clear groundwater flow is high.

Fish that spawn within the main channel of glacial rivers select times or locations to avoid the high suspended sediment during mid-summer. Eulachon, also known as hooligan, spawn during the spring before sediment levels get too high. Burbot spawn under the ice during mid-winter when glacial rivers are clear. Whitefish and chum salmon spawn during the fall when the amount of sediment declines as colder temperatures reduce glacial runoff. Chum salmon also select areas where ground water is flowing into the river, or upwelling, which keeps their eggs from becoming buried in fine sediment.

Fish that live within the main channel of glacial rivers have found ways to adapt to the high amounts of fine sediment by developing ways to find limited amounts of food in low-light conditions, and by spawning at times or at locations where fine sediment effects on developing eggs are reduced.

The Ecology of Glacial Rivers, from the Susitna Salmon Center, episode 2 on KTNA, by Jeff Davis.

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