Using field data to estimate the realized thermal niche of aquatic vertebrates

The field-derived thermal niche of aquatic vertebrates is potentially useful in determining whether resource management plans are adequate to protect sensitive vertebrates. Our objective was to use field data to estimate the thermal niches of 16 species of aquatic vertebrates and to compare these values among five geographic regions in Oregon. Thermal niche values varied among regions; for example, the upper thermal limit for rainbow trout Oncorhynchus mykiss was 22.4 degrees C in the Blue Mountains ecoregion and 16.9 degrees C in the Cascades ecoregion. Nonmetric multidimensional scaling (NMS) analysis of aquatic vertebrate assemblages revealed that level-three ecoregions grouped vertebrate assemblages more cohesively than the third-order hydrologic unit code (basins). Analysis of similarities of Bray-Curtis distance measures supported NMS findings that the structures of aquatic vertebrate assemblages coincide more with ecoregions than with basins. The realized thermal niches calculated in this study are generally comparable to the maximum growth temperatures and upper thermal limits established by other field and laboratory techniques. This information is valuable for managers who devise water temperature criteria as well as fisheries ecologists interested in quantifying or delineating thermal habitat.