Estuarine Migration and Expression of Anadromy by Coastal Cutthroat Trout in Two Lower Columbia River Tributaries

During the past decade, review of a proposal to list lower Columbia River coastal cutthroat trout under the Endangered Species Act highlighted poor understanding of coastal cutthroat trout life history and relationships between resident and migratory cutthroat in the lower Columbia River.  We used PIT-tag and acoustic telemetry to quantify migration tendency and evaluate estuarine migration behavior of cutthroat trout originating in Big Creek and Bear Creek, two adjacent tributaries that join the Columbia River estuary approximately 30 kilometers from the ocean.  Both tributaries contain artificial barriers to upstream migration of trout, but a substantial portion of Bear Creek is barrier-free, allowing comparison of trout populations above and below barriers to anadromous adult return.  Cutthroat trout (n = 1303) were PIT-tagged during summer electrofishing surveys throughout both basins, and stationary PIT antennas were installed near the head of tide on each stream.  The proportion of PIT-tagged trout that migrated to the estuary varied among tagging locations but was greater on average at sites below migration barriers in Bear Creek (28% migrants) than at sites above migration barriers in both streams (3% migrants).  Putatively migrant cutthroat trout (n = 109) were captured and tagged with acoustic transmitters at smolt traps in both tributaries.  Sixty percent of acoustically tagged trout entered the Columbia River estuary, and 25% were detected entering the ocean.  Ocean migrants moved rapidly through the estuary and entered the ocean one to six days after leaving tributaries.  Marine and estuarine survival, indicated by the proportion of tagged trout that returned to the two monitored streams after entering the estuary, was low (≤8%).  Up to 15% of PIT-tags known to have entered the estuary in any year were later recovered on a Caspian tern breeding colony, suggesting avian predation may be a substantial source of cutthroat trout mortality in the estuary.  Our results demonstrate rates of coastal cutthroat trout anadromy in lower Columbia River tributary populations both above and below migration barriers and suggest limited estuarine rearing and low survival (or high straying) of Columbia River cutthroat that emigrate from natal streams.