THE LAST CAST
•
JOHN NICKUM
Trout Stream Carrying Capacity
C Q
arrying Capacity 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?
Y A
es… a lot of detailed 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
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High Country Angler • Summer 2020
“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
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