- Salmonids in arid high-elevation streams find themselves at the fringe of their tolerance range. Under the conditions they endure in such environments, long-distance migratory movements among widely dispersed habitats may be an important mechanism for some fish to persist, and even thrive. The Donner und Blitzen (Blitzen) River redband trout (Oncorhynchus mykiss gairdneri) population is known to have a migratory life-history component, but little is known about the timing, spatial patterns, or ecological context of the migration. I tracked trout movements and passage delays at diversion dams with radio telemetry and passive integrated transponder (PIT) tags from March 2007 to November 2008. PIT tag readers established at four locations (river km 1, 35, 48, and 76) recorded large-scale movements of trout. Three of the readers also recorded ladder entry time, ladder ascending time, and total passage time of each trout.
My results indicated the existence of a life-history in which redband trout made long-distance migrations both as sub-adults and as adults. While adult trout migrated for reproductive purposes, movement of immature individuals appeared to be a response to
seasonal stream temperature conditions. Trout migration rate was positively correlated with fork length and mean river discharge but was not associated with stream temperature, tagging method, or the number of dams passed. The upstream-most detection location for radio-tagged adults that reached potential spawning habitat was not different for trout from the lower river versus the middle river (t(42) = 1.07, p = .29); however, lower river trout reached spawning habitats an average of 20 days later (t(42) = 3.78, p < .001). Passage times at the dams were not affected by discharge, fork length, or tagging method. At a jump-pool ladder trout were 7.23 times more likely to pass during day than at night (2(1, N = 40) = 9.96, p = .002). At a Denil ladder, trout were 25.6% more likely to pass with each degree Celsius increase in stream temperature (2(1, N = 22) = 4.39, p = .036). At all three dams, a greater proportion of the total passage time involved trout finding and entering the ladder than ascending the ladder after initial entry. Trout passage times varied significantly between each of the dams.
This study highlights the importance of landscape-scale distribution of habitats and refugia in determining life-history and movement patterns of trout. Migratory behavior was related to both life-stage and environmental conditions. Maintaining
connectivity with efficient passage throughout the river is a critical element in managing migratory fish populations like the one in this study. The well-functioning fish ladders were those that provided adequate fish attraction, while those ladders with poor attraction delayed migrating trout for long periods.