Trout and salmon movements in two Idaho streams as related to temperature, food, stream flow, cover, and population density

Many juvenile salmon and trout migrated from the Lemhi River drainage each fallwinter-spring period. Seaward migration of anadromous trout and salmon normally occurred inthe spring but pre-smolt anadromous and non-anadromous fishes also left the stream usuallybeginning in the fall. I compared data on temperature, food abundance, stream flow, cover andpopulation density with movements and conducted field and laboratory tests to determinereasons for the two types of movements.Smolts of the anadromous species migrated for an obvious reason but none of the factors Iexamined appeared to \u27stimulate or release\u27 their seaward migration. Movement frequentlycoincided with changes in water temperature and stream flow, but I could not establish aconsistent causal relationship and concluded that photoperiod and perhaps growth must initiatethe physiological and behavioural changes associated with seaward migration.Non-anadromous and pre-smolt anadromous species emigrated from the streams for differentreasons than the smolts. I postulated that fish found the stream environment unsuitable duringthe winter. Stream temperature declined in the fall as fish began moving from the streams but Icould not induce more fish to stay in test troughs with 12 C water versus troughs with 0-10 Cwater. Fish emigrated before abundance of drift insects declined in winter. Emigration occurred inspite of the relatively stable flows in both streams. Population density modified the basic migrationpattern by regulating the number and percentage of fish that emigrated and to a limited extenttime of emigration.Movements of non-smolt trout and salmon correlated best with the amount of cover provided bylarge rubble substrate. Subyearling trout emigrated from Big Springs Creek which contained norubble substrate but remained in the Lemhi River which did. In both field and laboratory testsmore fish remained in troughs or stream sections with large rubble substrate than in troughs orsections with gravel substrate. Trout and salmon in many Idaho streams enter the substrate whenstream temperatures declined to 4-6 C. A suitable substrate providing adequate intersticesappears necessary or the fish leave