Two avian predators in the Columbia River estuary, Caspian Terns and Double-Crested Cormorants, are known to consume millions of juvenile salmon each spring. However, the factors that influence their consumption of salmon are poorly understood. Diet differences at breeding colonies located in different parts of the estuary in the early 1990s illustrated the importance of alternate prey to salmon consumption. To formally explore this idea, we are examining diet information from these avian predators alongside fish community data collected in open waters of the lower estuary over six years (2006-2012). Initial analyses show that both bird diets and the fish community are highly variable at short temporal scales (2-week periods). But, both bird species are consuming most of the fish species caught during purse-seining, suggesting the fish data at least fairly represents the prey field available to avian piscivores in the estuary. Furthermore, initial analyses suggest that Caspian Terns and Double-Crested Cormorants select salmon in very different ways: tern consumption is highest when salmon abundance is highest, while cormorant consumption is highest when the relative--not absolute--abundance of salmon is highest. This suggests that while terns are actively selecting for salmon (regardless of other fish abundances), cormorants may not be, and the presence of other fish may decrease cormorant consumption of salmon. We provide additional examples of how abundances of particular fish species may be positively or negatively related to their consumption by birds. Our ability to match bird diets with fish community data is providing new insights into potential factors that influence avian predation on juvenile salmon and may help untangle these complex interactions.
Donald Lyons - Oregon Cooperative Fish and Wildlife Research Unit, Oregon State University
Daniel Roby - Oregon Cooperative Fish and Wildlife Research Unit, Oregon State University
Laurie Weitkamp - Conservation Biology Division, NWFSC