Juvenile salmonid migratory behavior at the mouth of the Columbia River and within the plume

Abstract

Early ocean experience is a critical time period that affects juvenile salmonid survival. Understanding juvenile salmonid behavior in nearshore marine environments and how oceanic conditions (such as dynamic river plume habitats) affect salmonid migration will contribute to salmonid survival studies and conservation and management efforts. Relatively few studies have been conducted on juvenile salmonid behavior as they migrate out the mouth of the Columbia River and some studies suggest that juvenile salmonids typically migrate north immediately upon entry into the ocean from the Columbia River. We present findings from a study that used acoustic telemetry to determine the migratory direction, residence time, and travel rate of juvenile salmonids as they left the Columbia River and entered the marine environment. A total of 8,159 acoustic-tagged salmonid smolts were detected at the mouth of the Columbia River. Of the fish detected at the mouth, an estimated 16% of yearling Chinook salmon, 10% of steelhead, and 26% of subyearling Chinook salmon were detected on a sparse array deployed outside the mouth of the Columbia River in the vicinity of the plume. The travel rate of Chinook salmon smolts decreased as they left the river and entered the marine environment, whereas the travel rate of steelhead increased. Chinook salmon also spent more time in the transitional area between the river mouth and plume compared to steelhead. In early spring, yearling Chinook salmon and steelhead were predominately detected on the plume array towards the edge of the shelf and to the south. Later in the season, yearling Chinook salmon and steelhead smolts were more often detected north of the river mouth. Subyearling Chinook salmon were most often detected on the portion of the plume array to the north of the river mouth. Our study showed that salmonid smolt migration out of the river into the nearshore marine environment appears to vary across species, season, and age class, and may be influenced by local environmental conditions. Direction of movement upon ocean entry cannot be assumed and is likely influenced by oceanic conditions such as wind and currents. We also present, for the first time, the utility of the Juvenile Salmon Acoustic Telemetry System (JSATS) to monitor the behavior of juvenile fish in the marine environment. Our results will help inform future studies using telemetry and hydroacoustics as well as trawl surveys to assess nearshore ocean juvenile salmonid distribution, behavior, and survival.