Concurrent Sessions C: Ecological Consequences of Partial Passage - Headwater Cutthroat Trout Movement Behaviors and Timing

Coastal cutthroat trout Oncorhynchus clarkii clarkii are the most widely distributed native salmonid in the forested watersheds of western Oregon. Populations in headwater streams have evolved on concordance with the physicochemical template that defines these complex aquatic networks. For example, behavioral patterns vary from small tributaries where local movements, within and among stream segments are common, to larger mainstem systems where migratory individuals are the dominant form. Because factors that may influence the relative abundance of these life-history types are poorly understood, we have been studying coastal cutthroat trout in watersheds ranging from 70 to 2,200 ha. Movement and distribution are assessed using continuous electrofishing surveys, passive integrated transponders (PIT tags) with arrays of swim-through antennas, and seasonal mobile antenna surveys. We are evaluating the resulting life-history patterns in relation to physical habitat, fish density, hydrology, connectivity, watershed size, and disturbance at a variety of spatial scales. Because these data span a 10-year period, we have been able to explore the influence of inter-annual variation in the amount, and temporal distribution of water on these critical patterns that are so closely linked to individual fitness and population persistence. The following results are preliminary. Results suggest that a greater proportion of fish exhibit migratory behavior as watershed size or connectivity increases. Depending on channel size, movement was restricted during low-flow periods. The mobile fraction of adult age 1+ cutthroat varied among tributaries within and among watersheds and individuals moving from tributaries to mainstem habitats on average experienced increased growth rates while individuals moving from mainstem to tributary habitats did not.
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