A Multi-Spatial Scale Evaluation of Migratory Fish Behavior in a Heavily Urbanized and Managed Waterway Using Fine-Scale Acoustic Tracking

The Lake Washington Ship Canal (LWSC) is a 13.8-km-long heavily urbanized and industrialized waterway linking Lake Washington with Puget Sound.  Each year, juvenile Chinook salmon (Oncorhynchus tshawytscha) - listed as Threatened under the U.S. Endangered Species Act - must navigate this waterway during their smolting migration from fresh to saltwater.  Policy makers and resource managers trying to protect these fish have been hindered by lack of information regarding Chinook movement and habitat use within the LWSC and behavior near anthropogenic structures.  To help fill these knowledge gaps, we used a Hydroacoustic Technology, Inc. (HTI) fine-scale acoustic tracking system to perform a multi-year, multi-spatial scale evaluation of Chinook salmon smolts in the LWSC.

In 2007 and 2008, we deployed four tracking systems at key locations in the LWSC: two bridges, a commercial marina, a shoreline with both altered and semi-natural features, and a lock-and-dam facility at the downstream end of the LWSC.  Each system contained 10-16 hydrophones, which allowed for high-resolution tracking (± 1 m) of tagged fish at each of the four study sites.  In addition, each hydrophone array spanned the full width of the channel, which allowed for broader scale analysis of movement (i.e., from site to site).  In both years combined, 494 Chinook smolts were tagged and released in groups throughout the migration season.  Between 9-89% of tagged fish were tracked at the study sites.

The HTI system yielded high-quality data despite some challenges with operating the equipment in this type of setting.  Subsequent analyses provided a comprehensive picture of Chinook movement through the LWSC and behavior at specific locations, as well as likely factors that contributed to substantial inter- and intra-annual variability that we observed.  Some of the more striking observations included: 1) influence of water temperature on behavior and choice of exit route at the lock-and-dam; 2) influence of water quality parameters on horizontal and vertical distribution of fish; 3) affinity for structure edges in deep water; 4) influence of bridges on behavior and migration; 5) spatial and temporal overlap with predators; and, 6) influence of artificial lighting on nighttime behavior.  The wealth of information provided by the tracking data has been extremely valuable in guiding policy, management, and mitigation efforts.  In addition, the tracking data is being incorporated into hydraulic-water quality-fish behavior models that may be useful in predicting fish response to management actions that affect conditions in the LWSC.