Development of an Index to Bird Predation of Juvenile Salmonids within the Yakima River, 2000 Annual Report.

Abstract

Avian predation of fish is suspected to contribute to the loss of juvenile spring chinook salmon in the Yakima Basin, potentially constraining natural production. In 1997 and 1998, the Yakama/Klickitat Fisheries Project (YKFP) and the Washington Department of Fish and Wildlife (WDFW)--whose goal is to increase natural production historically present within the Yakima River--initiated investigations to assess the feasibility of developing an index to avian predation of juvenile salmon within the river. This research--conducted by Dr. Steve Mathews and David Phinney of the University of Washington--confirmed that Ring-billed Gulls and Common Mergansers were the primary avian predators of juvenile salmon, and that under certain conditions could impact migrating smolt populations. Beginning in 1999, the Washington Cooperative Fish and Wildlife Research Unit (WACFWRU) was asked by the YKFP and the WDFW to continue development of avian consumption indices. Monitoring methods developed by Phinney et al. (1998) were adopted (with modifications) and monitoring of impacts to juvenile salmon along river reaches and at areas of high predator/prey concentrations (colloquially referred to as ''hotspots'') continued through 2000. In 2000, piscivorous birds were counted from river banks at hotspots and from a raft or drift boat along river reaches. Consumption by gulls at Hotspotsmore » was based on direct observations of foraging success and modeled abundance; consumption by all other piscivorous birds was estimated using published dietary requirements and modeled abundance. Further development of the avian consumption index model provided an estimation of smolt consumption for the 2000 survey season. Seasonal patterns of avian piscivore abundance were identified, diurnal patterns of gull abundance at hotspots were identified, predation indices were calculated for hotspots and spring and summer river reaches, and the efficacy of aerial surveys for estimating bird abundance within river reaches was evaluated. The only change in survey methods in 2000 was the shortening (in river miles) of surveys on the North Fork of the Teanaway River and the shifting of start and stop dates for river drifts and hotspot surveys. Primary avian predators in 2000 were ''gulls'' (California and Ring-billed) at hotspots and Common Mergansers within upper river reaches. Estimated consumption by gulls at both hotspots combined (10 April-30 June) was 163,475 fish. Assuming a worst case scenario (all fish taken were smolts) this represented 6.0% of all smolts estimated passing or being released from the Chandler Dam area during the 2000 smolt migration season. Total estimated take by Common Mergansers across all strata surveyed was 7,654 kg between 10 Apr and 30 Aug, 2000. Seventy-three percent of that consumption was within the upper river reaches (Stratum 1) where there is a known breeding population of mergansers.« less