This report assesses the impact of human activities and forest fragmentation on fish communities in the western Lake Superior basin. Human-induced activities noted in the report included temperature changes, siltation, erosion, forest cover and forest manipulation, and invasive species. Specific results are reproduced below. “The relative importance of regional, watershed, and in-stream environmental factors on fish assemblage structure and function was investigated in western Lake Superior tributaries. We selected 48 second- and third-order watersheds from two hydrogeomorphic regions to examine fish assemblage response to differences in forest fragmentation, watershed storage, and a number of other watershed, riparian, and in-stream habitat conditions. Although a variety of regional, fragmentation, and storage-related factors had significant influences on the fish assemblages, water temperature appeared to be the single most important environmental factor. We found lower water temperatures and trout–sculpin assemblages at lower fragmentation sites and higher temperatures and minnow–sucker–darter assemblages as storage increased. Factors related to riparian shading and flow separated brook trout streams from brown trout (Salmo trutta) – rainbow trout (Oncorhynchus mykiss) streams. Functionally, fish assemblages at lower fragmentation sites were dominated by cold-water fishes that had low silt tolerance and preferred moderate current speeds, while fishes with higher silt tolerances, warmer temperature preferences, and weaker sustained swimming capabilities were most common at higher storage sites. Our results suggest that site-specific environmental conditions are highly dependent on regional- and watershed-scale characters and that a combination of these factors operates in concert to influence the structure and function of stream fish assemblages. Key points: This study was completed within 160 km of Duluth, Minnesota, in the Northern Lakes and Forests Ecoregion and within two ecological units, the North Shore Highlands (north shore streams) and the Lake Superior Clay Plain (south shore streams)which provided excellent contrast in hydrogeomorphic types. Functionally, south shore fishes tended to be silt-dwelling, trophic generalists with slow current preferences and a tendency towards nest-guarding spawning behavior. North shore fishes tended to be single-bout spawners with fast current preferences. From a management perspective, our results suggest that both timber management and wetland restoration or degradation decisions will need to be considered by resource managers when fish community health is a concern. For example, increasing percentages of mature forest cover should allow for salmonid–sculpin assemblages to become more prevalent in streams with the potential for cool or cold waters. Similarly, increased wetland cover should allow for a greater predominance of healthy warmwater fish assemblages assuming that other landscape features are not too badly degraded. By understanding the species structure and functional character of an assemblage and its relationship to landscape features, managers should be able to make at least a rough assessment of watershed condition. Lacking fish data, it might be easier to simply characterize forest cover and storage as a first step in identifying which watersheds likely contain streams that are degraded. Our results suggest that although in-stream habitat rehabilitation should continue to be used an important tool to improve biological conditions in streams, restoration efforts will have greater success if the potential interactions with landscape conditions are factored into the decision-making process. In some situations, manipulation of forest cover or watershed storage may have a greater impact on fish assemblage integrity than in-stream habitat improvements.”
Brazner, John; Tanner, Danny K; Detenbeck, Naomi E; Batterman, Sharon L; Stark, Stacey L; Jagger, Leslie A; Snarski, Virginia M.
Regional, Watershed, and Site-Specific Environmental Influences on Fish Assemblage Structure and Function in Western Lake Superior Tributaries.
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