An assessment of the lethal thermal maxima for mountain sucker

—Temperature is a critical factor in the distribution of stream fishes. From laboratory studies of thermal tolerance, fish ecologists can assess whether species distributions are constrained by tolerable thermal habitat availability. The objective of this study was to use lethal thermal maxima (LTM) methodology to assess the upper thermal tolerance for mountain sucker Catostomus platyrhynchus, a species of greatest conservation need in the state of South Dakota. Adult fish were captured from wild populations in the Black Hills of South Dakota and acclimated to 20, 22.5, and 25 °C. Four endpoints (3 sublethal, 1 lethal) were recorded, with death being the most precise (lowest SE, easily discernible). The LTM for mountain sucker was 34.0 °C at 25 °C acclimation, 33.2 °C at 22.5 °C acclimation, and 32.9 °C at 20 °C acclimation. Compared to co-occurring species in the Black Hills, the LTM of mountain sucker is higher than that of salmonids but lower than that of 3 cypriniforms. Mountain sucker LTM is intermediate compared to other species in the family Catostomidae. These results suggest that the mountain sucker is not currently limited by water temperatures in the Black Hills but may be affected by stream warming as a result of climate change. RESUMEN.—La temperatura es un factor crítico en la distribución de peces de río. Mediante estudios de laboratorio sobre la tolerancia termal, los ecólogos de peces pueden evaluar si las distribuciones de especies están limitadas por la disponibilidad de un hábitat termalmente tolerable. El propósito de este estudio fue evaluar la tolerancia termal superior del lechón de montaña Catostomus platyrhynchus, una especie con la máxima necesidad de conservación en el estado de Dakota del Sur, con metodología de la temperatura letal superior (TLS). Se capturaron peces adultos de poblaciones silvestres en las Colinas Negras de Dakota del Sur, y se aclimataron a los 20, 22.5 y 25 °C. Se registraron cuatro temperaturas límite (tres subletales y una letal), siendo la letal la más precisa (error estándar más bajo y fácilmente discernible). La TLS del lechón de montaña fue de 34.0 °C a una aclimatación de 25 °C, 33.2 °C a una aclimatación de 22.5 °C y 32.9 °C a una aclimatación de 20 °C. Cuando se comparó con especies que cohabitan las Colinas Negras, la TLS del lechón de montaña es mayor a la de los salmónidos, pero menor a la de tres cypriniformes. La TLS del lechón de montaña es intermedia cuando se compara con otras especies de la familia Catostomidae. Estos resultados sugieren que el lechón de montaña no está limitado por las temperaturas en las Colinas Negras, pero podría ser afectado por el calentamiento de la corriente como resultado del cambio climático. thermal tolerance can be used to draw inferences about the distribution of a species along a temperature gradient. Great Plains stream fishes evolved in habitats often characterized by high temperatures, high salinity, and low dissolved oxygen (Dodds et al. 2004), and the distribution of fishes reflects interspecific differences in physicochemical tolerance (Matthews 1987). In the Brazos and South Canadian rivers, spe cies distributions and abundances in warm and drying pools matched the temperature and salinity tolerances estimated from laboratory studies (Matthews and Maness 1979, Ostrand and Wilde 2001). Thus, thermal tolerance measured in the laboratory may be a predictor of fish presence and abundance in the field and an important variable to consider in the conservation of imperiled fishes (e.g., Smith and Fausch 1997, Torgersen et al. 1999, Selong et al. 2001, Harig and Fausch 2002). Low and high water temperatures limit fish distributions. Low temperatures have sublethal effects, including delayed egg and larval devel opment (Harig and Fausch 2002), whereas high temperatures may increase susceptibility to predation or direct mortality. The effects of climate change are likely to increase the occurrence of high air and stream temperatures (IPCC 2007). Based on an evaluation of stream water temperatures and fish distributions in the United States, a global mean surface air-temperature increase of 4.4 °C would reduce available habitat for coldand coolwater fish by 50% rangewide (Eaton and Schel ler 1996). Laboratory studies can estimate thermal tolerance of fishes, assess the relative vulnerability of different fishes to increasing water temperatures (e.g., Smith and Fausch 1997), and predict habitat overlap between native and nonnative species (Carveth et al. 2006), all of which aid in selecting suitable conservation areas. This study empirically derived the lethal thermal maxima for mountain sucker Catosto mus platyrhynchus. Although the mountain sucker is secure across its range (NatureServe 2011), regional trends suggest declines at finer scales. A series of long-term studies on Sagehen and Martis creeks and Stampede Reservoir in eastern California indicated declines in mountain sucker total abundance, relative abundance, and spatial distribution between the 1950s and 1980s (Erman 1973, 1986, Gard and Flittner 1974, Moyle and Vondracek 1985, Decker 1989). In the Missouri River drainage of Wyoming, Patton et al. (1998) found that mountain sucker distribution had declined on at least 3 spatial scales (i.e., site, stream, and subdrainage) since the 1960s. In South Dakota, mountain sucker is listed as a species of greatest conservation need (SDGFP 2006), and a long-term analysis indicated that the species had significantly declined in density and spatial distribution since routine sampling began in the 1960s (Schultz and Bertrand in press). Since the mountain sucker inhabits mostly coolwater streams (Hauser 1969, Scott and Crossman 1973, Baxter and Stone 1995, Sig ler and Sigler 1996, Belica and Nibbelink 2006) and other catostomids show temperature-regulated distribution (e.g., Li et al. 1987, Eaton et al. 1995, Huff et al. 2005), high water tempera tures likely constrain mountain sucker distibution. As the climate warms (Bates et al. 2008), mountain sucker distribution may be further limited by warming stream temperatures. The objective of this study was to assess the upper thermal tolerance of mountain sucker in the laboratory. These results will improve understanding of mountain sucker biology and factors that threaten peripheral populations. They will also inform management decisions and predictions of the consequences of elevated stream temperatures resulting from climate change in the Black Hills of South Dakota and across the range of mountain sucker.