THE LAST CAST

• JOHN NICKUM

Trout Stream Carrying Capacity

Carrying CapacQ ity seems to be an ecological concept about the total productivity of an ecosystem. Trout streams vary considerably in how many fish they produce. What determines the carrying capacity of a trout stream? Can an angler predict the carrying capacity of a favorite stream by looking at it, or are a lot of details based on systematic studies required?

Yes… a lot of deA tailed information is needed. Many factors, including watershed geology, water quality analyses, stream morphology and physical structure characteristics, as well as climate conditions are involved in determining stream productivity. As with all things in nature, there are also elements of chance. Complex combinations of inter-related factors plus chance, creates conditions that cannot always be predicted accurately. (This tends to frustrate engineers who want high levels of predictability). Fisheries management biologists need professional training and background in limnology, soil science, aquatic plant biology, stream flow dynamics, and other sciences, in addition to their basic education in vertebrate zoology with an emphasis on fish.

Water is “the universal solvent,” so the minerals in the rocks and soils of the watershed determine the chemical makeup of lakes and streams. When combined with the physical structure of each stream, these factors determine its productivity, provided there are no “limiting factors” that prevent full utilization of the fundamental resources. Just as the factors supporting full development of carrying capacity vary widely, limiting factors vary from ecosystem to ecosystem.

My first job after completing graduate school involved teaching courses in aquatic sciences and fisheries at South Dakota State University – training future fisheries biologists for positions managing and/or conducting research on the fisheries and waters of South Dakota and other Midwest waters. South Dakota has some unusual characteristics, which make it an ideal location for young biologists to learn the interconnections and relative importance of the myriad factors influencing the productivity of the waters they would be managing.

South Dakota has three distinct geologic areas: the eastern plains that thousands of years ago were covered by a continental ice sheet, the western prairies that were never glaciated; and the Black Hills, a range of mountains isolated from the Rocky Mountains. Trout streams in the eastern area are small spring-fed water found only in the northeast corner of the state. The waters are rich in calcium and carbonate ions, which makes them ideal for photosynthetic algae growing on the rocks and gravel found in the pool and riffle complexes that are characteristic of these brooks and creeks. In turn, thousands of invertebrate animals—including insect larvae—feast on this productive base. Even though these brooks are small, they can support 200 to 300 pounds of fish per surface acre.

There are no trout streams in the western prairie areas of South Dakota. Man-made ponds, and rivers that originated in the mountains and high country of the area are too

warm for trout. The surface waters found in the man-made ponds and rivers of western South Dakota grasslands are unusual because of their chemistry. Magnesium sulfate is the predominant dissolved mineral. Although this solution is not known to have a laxative effect on aquatic insects and invertebrates, or fish, it is very effective on humans. Do not drink the water when you visit this area! In addition to unproductive water, the streams in western South Dakota tend to have sand bottoms and very little structure. Even if they were colder, they would be unproductive.

The waters of the Black Hills provide an informative overview of the factors influencing productivity in streams. Streams in the highest elevations of the Black Hills originate from granitic “seep basins” fed by melting snow. The mineral content of granite contributes very little dissolved material to the waters; therefore, these small streams are very unproductive. The few insect larvae and micro-crustaceans found in them do not support much of a trout population. As the streams flow down toward the prairies, they flow over limestone rocks and are joined by streams originating in these rocks. These “hard” waters, rich in calcium carbonate, are very productive and maintain abundant trout populations until they are warmed as they flow out onto the prairie.

The descent of Colorado streams do not seem to be marked by such chemical transitions. Changes in productivity over the length of each stream seem more related to flow rates and the substrate structure of the stream. In contrast, the waters in New York’s high country (Adirondack Mountains and Catskill Mountains) are very low in productivity (less than 40 lbs per surface acre), but increase dramatically as they descend into lower elevations where limestone prevails. An unusual change in water chemistry occurs in waters flowing out of the Yellowstone Park area. The Gallatin River is high in arsenic content in the headwaters regions, but this toxic substance does not seem to act as a limiting factor on trout production.

Ultimately, carrying capacity is a streamby-stream, and even a reach-by-reach matter. Anglers and management biologists just have to analyze each situation and develop evidencebased conclusions.

About The Author

John Nickum, is a retired PhD. fishery biologist whose career has included positions as professor at research universities including Iowa State and Cornell University, director of the Fish and Wildlife Service’s fisheries research facility in Bozeman, MT, and science officer for the Fish and Wildlife Service’s Mountain-Prairie Region. He was inducted into the National Fish Culture Hall of Fame in 2008.

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