Impact of the Potholes Reservoir Caspian Tern Breeding Colony on Out‐Migrating Juvenile Salmonids in the Mid‐Columbia River

Abstract

AbstractWe examined the foraging behavior and diet of Caspian terns Hydroprogne caspia breeding at Potholes Reservoir, Washington, in 2003, 2005, and 2006, and we developed a bioenergetics model to estimate impacts on juvenile salmonids Oncorhynchus spp. from Columbia River stocks. Potholes Reservoir Caspian terns mostly foraged on local fish (68–97% of tern bill loads) rather than traveling to the Columbia River. Our model suggested that only 8,913–94,139 juvenile salmonids were consumed, representing 0.02–0.38% of the salmonids available to Caspian terns. Local foraging is probably explained by higher net profitability of Potholes Reservoir fish prey. Columbia River steelhead O. mykiss were the only juvenile salmonids that were energetically comparable with local Potholes Reservoir fish, mostly due to their high energy density and relatively large size. Passive integrated transponder tag analyses showed that Potholes Reservoir Caspian terns preferred steelhead (predation rates = 0.42–1.06%) to all other salmonid stocks (coho salmon O. kisutch = 0.09–0.36%; subyearling Chinook salmon O. tshawytscha = 0.009–0.040%; yearling Chinook salmon = 0.05–0.17%). Given the proposed reduction in nesting habitat at East Sand Island (ESI; where the current Caspian tern population consumes 3.6–5.3 million juvenile salmonids annually), we estimated the impact on salmonid stocks that would occur if ESI Caspian terns relocated to Potholes Reservoir. Depending on diet scenario, 100% relocation could result in juvenile salmonid consumption between 2.2 and 6.6 million fish, with lower values being much more likely, making Potholes Reservoir the colony location of least likely impact among all sites within the Columbia River drainage. We suggest that selection of sites for ESI relocation should be based on scientific evidence combining a quantitative approach, such as bioenergetics modeling linked to prey population growth rate.