Interactions Among Aquatic Vegetation, Waterfowl, Flows, and the Fishery Below Island Park Dam

Abstract

Management of Island Park Reservoir has significantly affected abundances of rainbow trout (Oncorhynchus mykiss) and wintering trumpeter swans (Cygnus buccinator) on the Henry's Fork of the Snake River. Trout and swan numbers both increased following dam operation changes that resulted in increased winter flows during the 1970s. Throughout the 1980s and early 1990s, declines occurred in overall macrophyte abundance and in the relative abundance of tall, erect species capable of providing fish habitat and waterfowl forage throughout the winter. This was most likely caused by introduction offi ne sediment into the river resulting from drawdowns of Island Park Reservoir in 1979 and 1992, a series of scouring spring flows in the early 1980s, low winter flows during drought years in the late 1980s, and increased waterfowl herbivory throughout the period. Although fluctuations in the rainbow trout population cannot be tied directly to changes in the macrophytes, our review of the literature suggests that robust and abundant macrophytes benefit the fishery and associated angling opportunities through increased invertebrate abundance, water depth, and trout habitat. We recommend managing Island Park Reservoir to minimize the probability of extensive sediment transport into the river, maximize winter flows, and minimize abrupt flow increases during the spring. Furthermore, we recommend continuing the waterfowl hazing program at Harriman State Park and exploring new techniques for reducing waterfowl abundance on the Henry's Fork between Island Park Dam and Riverside during autumn and winter. The effects of these and other management actions on the Rocky Mountain trumpeter swan populations should be carefully monitored to maintain viability of the Greater Yellowstone population.