We used temperature and fish data from streams across Michigan and Wisconsin to estimate upper thermal tolerance limits for brook trout Salvelinus fontinalis and brown trout Salmo trutta. Tolerance limits were estimated for the maximum daily mean temperature (MEANT), maximum daily maximum temperature (MAXT), and maximum daily temperature range (RNGT) at exposure lengths of 1, 3, 7, 14, 21, 28, 35, 42, 49, 56, and 63 d. We found no difference in the upper thermal tolerance limit for brook and brown trout. For time periods of 1-14 d, the upper temperatures tolerated by trout decreased rapidly from 25.38C to 22.58C for MEANT and from 27.68C to 24.68C for MAXT. For time periods from 21 to 63 d, the upper temperatures tolerated by trout declined more gradually from 22.18C to 21.08C for MEANT and from 24.28C to 22.98C for MAXT. The 7-d upper tolerance limit was 23.38C for MEANT and 25.48C for MAXT. The maximum RNGT tolerated by trout varied as a function of mean temperature and length of exposure. Our findings suggest that chronic temperature effects as well as temperature fluctuation play an important role in limiting salmonid distributions and therefore should be considered when developing management objectives and water quality standards.

Although the importance of water temperature to the ecology of stream fishes is well documented, relatively little information is available on the extent of regional variation in thermal regime and its influence on stream fish distribution and abundance patterns. In streams draining the heterogeneous glacial landscape of Michigan's Lower Peninsula, regional variation in summer mean temperature and temperature fluctuation is among the highest reported in the literature. We developed a habitat classification to simplify the description of thermal regimes and to describe the relationships between available thermal regimes and distribution patterns of stream fishes. Changes in community composition, species richness, and standing stocks of key fish species occurred across gradients in mean temperature and temperature fluctuation. These changes were used to identify three mean temperature categories (cold, <19°C; cool, 19–<22°C; and warm, ≥22°C) and three temperature fluctuation categories (stable, <5°C; moderate, 5–<10°C; and extreme, ≥10°C). The combination of these categories resulted in a 3 × 3 matrix with nine discrete thermal regimes. The classification developed in this study provides a framework for descriptions of the realized thermal niche of stream fishes, and can be used as a baseline for measurement of changes in distribution patterns associated with future climate warming. Our results suggest that observed differences in community structure among sites are largely attributable to spatial variation in mean temperature and temperature fluctuation. Thus, accounting for the linkage between regional variation in thermal regime and fish community structure should improve our ability to effectively assess and manage stream resources.

Coolwater streams, which are intermediate in character between coldwater ''trout'' streams and more diverse warmwater streams, occur widely in temperate regions but are poorly understood. We used modeled water temperature data and fish assemblage samples from 371 stream sites in Michigan and Wisconsin to define, describe, and map coolwater streams and their fish assemblages. We defined coolwater streams as ones having summer water temperatures suitable for both coldwater and warmwater species and used the observed distributions of the 99 fish species at our sites to identify coolwater thermal boundaries. Coolwater streams had June-through-August mean water temperatures of 17.0-20.58C, July mean temperatures of 17.5-21.08C, and maximum daily mean temperatures of 20.7-24.68C. We delineated two subclasses of coolwater streams: ''cold transition'' (having July mean water temperatures of 17.5-19.58C) and ''warm transition'' (having July mean temperatures of 19.5-21.08C). Fish assemblages in coolwater streams were variable and lacked diagnostic species but were generally intermediate in species richness and overlapped in composition with coldwater and warmwater streams. In cold-transition streams, coldwater (e.g., salmonids and cottids) and transitional species (e.g., creek chub Semotilus atromaculatus, eastern blacknose dace Rhynichthys atratulus, white sucker Catostomus commersonii, and johnny darter Etheostoma nigrum) were common and warmwater species (e.g., ictalurids and centrarchids) were uncommon; in warm-transition streams warmwater and transitional species were common and coldwater species were uncommon. Coolwater was the most widespread and abundant thermal class in Michigan and Wisconsin, comprising 65% of the combined total stream length in the two states (cold-transition streams being more common than warmtransition ones). Our approach can be used to identify and characterize coolwater streams elsewhere in the temperate region, benefiting many aspects of fisheries management and environmental protection.

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