Upper Thermal Tolerances of Rio Grande Cutthroat Trout under Constant and Fluctuating Temperatures

The Rio Grande Cutthroat Trout Oncorhynchus clarkii virginalis is the southernmost subspecies of Cutthroat Trout, and as with the other subspecies, stream temperature regulates growth, reproductive success, distribution, and survival. An understanding of the upper thermal tolerance of Rio Grande Cutthroat Trout is important for developing water temperature standards and for assessing suitable habitat for reintroduction and management. Hatch success of Rio Grande Cutthroat Trout eggs was determined under static temperatures. The thermal requirements of fry and juveniles were also assessed under static and fluctuating temperature regimes using the acclimated chronic exposure method. Egg hatch success was 46-70% from 6 degrees C to 16 degrees C but declined significantly at 18 degrees C and 20 degrees C. Maximum growth of fry that were fed to satiation occurred at 15.3 degrees C. The 30-d ultimate upper incipient lethal temperature (UUILT) was 22.6 degrees C for fry and 21.7 degrees C for juveniles. Survival during fluctuating temperature experiments was dependent upon the daily maximum temperature and the daily fluctuation. The upper thermal limits for Rio Grande Cutthroat Trout were lower than those of Rainbow Trout O. mykiss but similar to those of other Cutthroat Trout subspecies. The low UUILT of Rio Grande Cutthroat Trout relative to some salmonids may increase the risk of deleterious effects brought about by a changing climate, habitat alteration, and sympatric nonnative salmonids, which are known to outcompete Cutthroat Trout at temperatures above the species' optimal range. Daily mean water temperatures near the Rio Grande Cutthroat Trout's optimal growth temperature of 15 degrees C would be suitable for reintroduction of this subspecies. Depending on the daily temperature fluctuation, daily maximum temperatures within reintroduction streams and current habitat should remain at or below 25 degrees C to ensure long-term persistence of a Rio Grande Cutthroat Trout population. This information will aid in establishing water quality standards to protect habitat where the subspecies currently occurs. Received March 12, 2013; accepted May 28, 2013