Road Ecology Center

Large anthropogenic infrastructure such as major bridges in and near waterways can influence the ecological dynamics of the nearby aquatic environment. These influences may affect behavior, habitat use, fitness, and survival of fishes. Chinook salmon (Oncoryhnchus tshawytscha) spawning in tributaries to Lake Washington typically spend three to five months rearing in Lake Washington before travelling through the Lake Washington Ship Canal to Puget Sound. Most salmon smolts in Lake Washington must pass beneath the SR520 Bridge en route to Puget Sound. Plans to replace the existing bridge have sparked interest in how smolts and potential predators behave around and use the bridge.

To address this interest, we tracked Chinook smolts, smallmouth bass and northern pikeminnow in a 17.2 ha area along a 560 m stretch of the SR520 bridge during June-July 2007 and 2008 using fine-scale acoustic tracking. During the 2007 tracking season a total of 171 smolts were released in three June release groups and 162 were successfully tracked in the study area. Repeating the study design in 2008, 181 smolts were released and 133 were successfully tracked in the study area during a total of four release groups occurring in June and July. Although this study focused on the SR 520 bridge, many fish were also observed at a downstream tracking station approximately 2-miles downstream allowing us to evaluate movements within and between sites. Different release groups appear to exhibit different behaviors, some release groups rapidly migrated through the SR 520 tracking area in < 3 h (“migrating”), while other release groups were often detected ≥ 2 days (“holding”). The bridge appeared to delay some migrating smolts. These delays were typically short in duration as salmon would move along the bridge – typically towards the shoreline – prior to migrating past the bridge. Many holding smolts used areas near the bridge extensively. Timing of migrational cues, physiological smolt status, water temperature and clarity, and macrophytes may have influenced movement timing and habitat use. During the same study periods small numbers of northern pikeminnow and smallmouth bass were also tracked. Bass preferred habitats under overwater structures, including the bridge – particularly near bridge columns. Pikeminnow preferred macrophytes and overwater structures other than the bridge. Predator diets and abundance were also evaluated in and near the study area.

These results suggest that the bridge in its current form may affect the movements of some Chinook smolts and may be preferred habitats for some salmon predators. The SR 520 Bridge Replacement Project is continuing to evaluate these results to help inform design of the proposed bridge replacement.