Effects of avian predators on stream fish assemblages

Conventional models of freshwater food webs assume that fish occupy the top trophic level. However, many terrestrial predators feed and depend upon aquatic prey. Because of the abundance, diversity and trophic position of avian predators, they have the potential to play important roles in aquatic systems. Although we are largely ignorant of what effects they have in regulating aquatic communities, existing information inferred from observations suggests that birds may be top predators in these systems. Our lack of knowledge of the role of these avian predators in natural systems limits our ability to predict how these birds affect both natural populations, as well as stocked game fish populations at hatcheries and reservoirs. The following studies focus on the effects of avian predators on aquatic prey and are among the first to experimentally examine the effects of avian predators in aquatic systems.; The first chapter provides background material. The second chapter focuses on foraging behavior of kingfishers and great blue herons, and shows that netting and perches can be used to successfully manipulate avian predation pressure. The third chapter uses this technique to examine what effects kingfishers and wading birds are having on stream fish assemblages, and shows that there are effects are on two of the most abundant minnow species, striped shiners and central stonerollers. The fourth chapter documents how facilitation occurs between avian and aquatic predators, resulting in more prey being consumed when both predators are together than expected based upon the number of prey each prey consumes in isolation. The last chapter shows that both birds and bass produce non-lethal effects on prey habitat use, movement rates, and foraging activity.{09}Taken together, this work shows that avian predators play an important role in affecting fish populations in aquatic systems.