Life History Variability, Habitat Use, and Migratory Behavior of Coastal Cutthroat Trout in the Salmon River, Oregon

Coastal cutthroat trout exhibit a diverse array of life history strategies to thrive in fluctuating freshwater, estuarine, and marine environments.  The estuary, commonly overlooked as a transitional environment, is used extensively by juvenile and adult cutthroat in Salmon River.  Juvenile cutthroat migrate from streams to the tidal-fresh portion of the estuary to spend weeks to months before returning upstream, and adults use the estuary for days as a migratory corridor or up to a year as a key rearing environment.  This study was designed to describe the variation in life history strategies, quantify what segments of the population use the estuary as a rearing environment, describe patterns of use, and estimate survival of selected migratory strategies.  Individually coded hydroacoustic transmitters were implanted in 46 cutthroat (175 - 320mm length) captured at a rotary screw trap in lower Salmon River during spring 2004, 2009, and 2010. Cutthroat were also implanted with PIT tags in freshwater streams (August – September, 2008-10) and in main channel habitats and marshes in the estuary during all months of the year (2008-10).  Thirty-six to 55% of acoustic tagged fish migrated to the ocean depending on the year. The other acoustic tagged fish remained in freshwater or reared in the estuary. Survival of the ocean migrants was extremely low compared to the estuary resident cutthroat.  Cutthroat sampled in the estuary were composed of all size classes, 58 to 427 mm fork length.  Those caught or detected in marsh channels were typically smaller than those caught in the main estuary river channel, with an average fork length of 155mm compared to 243mm. Cutthroat observed in the estuary included a larger, and presumably older, segment of the population than that observed in the freshwater streams in August and September or at the screw trap in the spring. Of the 110 recapture events in estuary seining, 87 (79%) occurred at the same location that the fish was previously caught (tagged or recaptured). The average growth rate of all individuals recaptured in the estuary was 0.40mm/day, or an average increase of 0.17% of body length per day. The findings in Salmon River highlight the importance of the estuary as a critical rearing habitat for a significant portion of the cutthroat population.  The plasticity of life history strategies in Salmon River has permitted the cutthroat to exploit a variety of diverse environments.