Critical Thermal Maxima of Captive-Bred Devils River Minnows (Dionda Diaboli) (Report‪)‬

The Devils River minnow (Dionda diaboli)is federally listed as threatened (United States Fish and Wildlife Service, 1999) and is endemic to spring-fed waters of the Rio Grande drainage in western Texas and Mexico (Hubbs and Brown, 1956; Garrett et al., 1992). This species is at risk because of reduction and alteration of habitat, depletion of water, limited geographic range, introduction of exotic species, and global warming (Matthews and Zimmerman, 1990; Contreras-Balderas et al., 2003). Garrett et al. (1992) suspected that reduced flow of water was the primary cause of lower abundance and smaller geographic range of this minnow. A population of Devils River minnows recently was discovered isolated in the headwaters of Pinto Creek, Kinney County, Texas, and reduced flows of springs could threaten their existence (Garrett et al., 2004). Throughout its range, the Devils River minnow may be most at risk during summer when temperatures of water away from springs increase. Some fish may seek thermal refuge closer to springs (e.g., Blue Springs, ca. 1 km upstream from Dolan Falls, Devils River; temperature averages ca. 23[degrees]C). If flows of springs become substantially reduced, temperatures could rise and possibly harm the isolated population. We do not know thermal tolerances of this species. Critical-thermal methods quantify thermal tolerances by exposing test animals to a constant change in temperature until a near-lethal endpoint is reached, either the critical thermal maximum (heating) or critical thermal minimum (cooling). These measures are valuable for evaluating thermal requirements of organisms (Lutterschmidt and Hutchison, 1997b) and are especially useful for comparing thermal tolerances (Beitinger et al., 2000). The critical-thermal method has gone through several modifications since it was coined as critical maximum by Cowles and Bogert (1944). More recently, the endpoint of the method was defined as a pre-death thermal point at which locomotory movements become disorganized and a fish loses the ability to escape from conditions that may ultimately lead to its death (Beitinger et al., 2000). To begin understanding thermal requirements of captive Devils River minnows, we determined critical thermal maxima at different temperatures of acclimation. These data estimate upper limits of temperature that would lead to demise of the species (something to avoid in culture) and can be useful for determining suitable sites for restocking, and for management of wild populations and habitat. In addition, the Devils River Minnow Recovery Plan (United States Fish and Wildlife Service, 2005a) calls for determination of habitat requirements, including temperature.