Despite extensive investment in estimating when Coastal Oregon coho will return to spawn (Oncorhynchus kisutch, threatened), the mechanism which triggers their upstream migration is not understood. Migration timing of other salmonids has been shown to be an inherited trait, but is also believed to be cued by environmental drivers. While life history diversity allows juvenile coho salmon to exploit a variety of freshwater environments, the time period over which successful spawning can occur is relatively short and must be timed appropriatley. This study aims to ascertain if water discharge and temperature act as cues triggering the start of the spawning migration and whether responses to these cues are consistent across the stream network. Using data from the Umpqua River basin, potential hydrologic cues were investigated by comparing cumulative distribution functions of daily time series data of fish passage, discharge, and temperature data.
Results/Conclusions
Fish count data recorded over the past twelve years on the mainstem of the North Umpqua River at the Winchester Dam (drainage area: 3481 km2) show that the initial coho upstream migration consistently begins after a time lag in peak annual water temperature (which occurs in August) and before the onset of fall storms. Initial coho movement from the mainstem Smith River into tributaries (drainage area range: 3-27 km2) appears responsive to an increase in discharge, though increased discharge and cooler water temperatures are correlated in the small basins. This work suggests that discharge and temperature regimes provide the cues for the initiation of coho upstream migration from the estuary to the mainstem of the river, as well as for the initiation of spawning migration from the mainstem into tributaries. Improved understanding of the potential response of spawning adult coho to changes in stream temperature and hydrology, and the degree of plasticity of this response, will be important in order to guide management actions for coho in the face of potential environmental change.