|Document Type:||Technical Memorandum|
|Title:||Effectiveness of predator removal for protecting juvenile fall Chinook salmon released from Bonneville Hatchery, 1991|
|Author/Editor:||Richard D. Ledgerwood, Earl M. Dawley, Paul J. Bentley, Lyle G. Gilbreath, T. P. Poe, H. L. Hansen|
|Tech Memo Number:||NMFS-NWFSC-9|
Despite a belief that removal of northern squawfish (Ptychocheilus oregonensis) would increase survival of juvenile salmonids (Oncorhynchus spp.) in the Columbia River Basin, there has been no direct demonstration of the benefit of predator removal. In 1991, we assessed the survival increases for juvenile salmon before and after the removal of northern squawfish in the vicinity of the hatchery release site, while the U.S. Fish and Wildlife Service tested the effectiveness of electrofishing to remove northern squawfish. Short-term survival differences among release groups of juvenile salmon were assessed from comparisons of coded-wire tagged (CWT) fish recovered near the upper boundary of the Columbia River estuary at Jones Beach (River Kilometer 75). Captured northern squawfish were examined to determine the effects of predator size and density on the rate at which juvenile salmonids are consumed. A total of 2,012 northern squawfish were removed from nine transect areas near the hatchery in about 20 hours of electrofishing between the two release dates. With few exceptions, the daily catch, catch rate, mean fork length, and mean weight of northern squawfish and the number of CWTs recovered in the digestive tracts of northern squawfish (representing ingested juvenile salmon) declined over time. Analysis of CWT-fish recoveries at Jones Beach indicated that the recovery percentages for fish released into the midstream Columbia River were significantly higher than for fish released into Tanner Creek before predator removal (0.37% versus 0.30%; P = 0.01) and after predator removal (0.39% versus 0.33%; P = 0.02). After the removal of northern squawfish, the difference in recovery percentages between the two release sites was reduced from 23.3 to 18.2% (insignificant; P = 0.92). In 1990 and 1989, the difference in recovery percentages between the two release sites (no predator removal) were considerably greater and may have been related to the lower river flows. We speculate that higher flow volumes in 1991 dispersed test fish more rapidly, reduced their exposure time to predation, and resulted in higher survival rates for Tanner Creek releases. In addition, the Columbia River flow increased about 25% between the first and second release dates and the higher flow may have increased survival of the Tanner Creek-released fish regardless of predator removal efforts. The 18.2% difference in recovery between midstream and Tanner Creek release following northern squawfish removal suggests that the resident population of northern squawfish was large and removal of 2,012 predators was insufficient to significantly improve survival of juvenile fish emigrating from Tanner Creek.