Holding areas must have adequate cover and low temperatures to reduce metabolic losses and pre-spawning mortality. Proper incubation temperature requires accumulation of necessary thermal units so that development rate results in best emergence timing. Prolonged autumn high temperatures or early spring warming can result in early emergence if they prematurely fulfill thermal unit cumulation. It is vital for smolts to begin their downstream migration early enough to avoid higher temperatures that discourage migration. Temperature regulation and monitoring must be done at a watershed scale in order to fully protect the species found along the river continuum.
Survey data are very useful in establishing the downstream limits of distribution.
Bull trout and redband trout represent the lower and upper extremes in thermal tolerance. The general uniformity in thermal response among stocks of a species tends to obviate the need for geographically different biological standards to protect endangered species. Densities of various salmonid species have been shown to decline from a maximum under near optimum conditions to zero at the downstream distribution limit. Distribution is a function of temperature, but species are also distributed along multiple environmental gradients. Optimum habitat for a species involves temperature regime, but also is strongly influenced by channel gradient, substrate composition, pool frequency, large woody debris and other cover elements, oxygen concentration, and competitors or predators.