Seasonal Occurrence of American Shad in the Lower Columbia River Estuary

            The Columbia River basin has become the permanent home to numerous non-native fishes and aquatic invertebrates.  These exotic species often have negative consequences to the native species whose niches are being invaded.  In the lower Columbia River estuary, however, American shad is the only exotic fish species we have regularly encountered in over a decade of sampling the estuarine fish community.  American shad are both consistently caught and abundant in our catches, typically representing 10-15% of all fish caught (by number) and present in over 70% of our hauls.  They are also present in the estuary throughout our sampling season (spring, summer, and fall).  Based on the size distribution, we see at least three size classes of shad in the estuary.  These length-based size groups are approximately 60-170, 180-250, and 260-350 mm fork length.  The timing of these groups varies seasonally, with the largest fish (presumed reproductive adults) having the most limited temporal presence in the estuary.  Given concern for native fishes in the Columbia River basin, especially Pacific salmon (many of which receive protection under the U.S. Endangered Species Act), we will discuss the dual role that American shad may play in the estuary with respect to juvenile salmon.  While they may compete with juvenile salmon for prey, they may also serve a more important function as alternate prey for avian predators that otherwise might prey upon juvenile salmon.  Finally, we will explore the historical presence of shad in the estuarine fish community based on studies conducted over the last 40 years.