Evaluating the Impact of Artificial Light at Night on Juvenile Salmonid Predation Risk in the Lake Washington Ship Canal

The Lake Washington Ship Canal is the only outmigration route to the Puget Sound, making it key to the conservation of native anadromous salmonids. Artificial Light at Night (ALAN) fosters perpetual twilight conditions, increasing predation opportunity for visual foragers effective in low-light conditions. To explore conservation strategies related to reducing ALAN, the Fremont Bridge was chosen for a case study as it is impacted by skyglow, street lights, and an underbridge art installation. Downwelling spectral irradiance was sampled using the Biospherical Instruments C-OPS. Daytime profiles were collected to analyze light attenuation. Nighttime surface light levels were sampled with the art lights on and off. These data were then used to model nighttime spectral profiles. Predator search volumes were modeled using a visual foraging model parameterized for coastal cutthroat trout, a key predator in the system.  The impact of the art lighting is not statistically significant on overall light levels, other than directly under the bridge. Distant enough from the bridge that the spectral fingerprint of the art lighting is not visible, light levels are similar to or brighter than those under the bridge. Spatially modeled predator search volume shows that under-bridge predation risk only slightly exceeds the levels seen beyond the influence of bridge-related ALAN. However, near the bridge where street lines shine, search volume is much greater. Predation risk is significantly elevated near the Fremont Bridge, attributable to unshielded street lights rather than the art lights.

Presentation Preference: Poster

Primary Presenter: Luke Valleli, University of Maine (lvalleli13@gmail.com)

Authors:

Luke Valleli, University of Maine  (luke.valleli@maine.edu)

Tessa Code, University of Washington  (tessa.code@uw.edu)

Dave Beauchamp, USGS Western Fisheries Research Center  (fadave@usgs.gov)