Evaluation of the Northern Squawfish Management Program : Final Report of Research, 1990-1996 [report]

David L Ward
1998 unpublished
Development of the hydropower system in the lower Columbia and Snake rivers has resulted in increased losses of juvenile salmonids to resident fish predators. The native northern squawfish Ptychocheilus oregonensis is the dominant predator of juvenile salmonids, but introduced smallmouth bass Micropterus dolomieu and walleye Stizostedion vitreum are also abundant. A large-scale management program for northern squawfish was begun in 1990 to increase survival of juvenile salmonids in the Columbia
more » ... and Snake rivers. The Northern Squawfish Management Program (NSMP) consists of a public sport-reward fishery, and agency-operated dam-angling and gillnet fisheries that target northern squawfish ≥250 mm fork length, approximately the size at which northern squawfish become important predators on juvenile salmonids. The goal of the program is to sustain annual exploitation of 'predator-size' northern squawfish at 10-20%, which may reduce losses of juvenile salmonids by as much as 50%. We evaluated the management program to determine if annual exploitation of northern squawfish was maintained in the target range. We also monitored predator populations to describe the response of northern squawfish, smallmouth bass, and walleye to the management program. From 1990-96, over 1.1 million northern squawfish ≥250 mm fork length were removed from the lower Columbia and Snake rivers. Annual exploitation averaged 12.0%, and ranged from 8.1% to 15.5%. The sport-reward fishery accounted for 86.5% of the harvest. All fisheries targeted large, piscivorous, northern squawfish (96.1-99.5% of reported catch). We found no evidence that surviving northern squawfish compensated for sustained removals. Indices of northern squawfish abundance and consumption of juvenile salmonids were consistently lower from 1994-96 than 1990-93. Size structure of northern squawfish populations appeared to decrease in response to removals of large fish; however, we found no trend of increased growth, fecundity, or year-class strengths. We found no evidence of smallmouth bass or walleye response to sustained removals of northern squawfish. No trends in smallmouth bass density, consumption of juvenile salmonids, population structure, growth, mortality, or year-class strength have been realized concurrent with the NSMP. Variations in walleye density and population structure appear to be driven by variations in year-class strength, not by response to removals of northern squawfish. We found no trends in growth or mortality of walleye. We found no evidence that diets of northern squawfish, smallmouth bass, or walleye changed in response to sustained removals of northern squawfish. Piscivory and salmonid predation varied annually for smallmouth bass and walleye, but did not increase coincident with removals of northern squawfish. Piscivory by northern squawfish declined over time from 1990-96 Losses of juvenile salmonids to predation have probably decreased since implementation of the NSMP. Results indicate that if all variables other than exploitation of northern squawfish were held constant, predation by northern squawfish on juvenile salmonids has decreased to 62% (range 45-75%) of pre-program levels. Lack of response by surviving northern squawfish and other predators, and lack of changes in diet of these fish increases confidence in the hypothesis that sustained removals of northern squawfish increases survival of juvenile salmonids. iii PREFACE This document is the final report of research conducted from 1990-96 by the Oregon Department of Fish and Wildlife (ODFW) to evaluate Bonneville Power Administration (BPA) project 90-077, the Northern Squawfish Management Program (NSMP). The Summary of Project summarizes and integrates the results, conclusions, and recommendations of the evaluation. The report contains research papers that describe how we addressed project objectives, how we reached our conclusions, and why we made our recommendations. The papers are listed and numbered consecutively in the Table of Contents and the numbers are used to reference each paper in the Summary of Project. It is the integration of these individual papers that provides the best picture of the current status of the NSMP. ACKNOWLEDGMENTS Many thanks to all the people who helped us accomplish our objectives over the years. ODFW personnel worked long hours to collect, summarize, and analyze the data presented in this report. Those that worked multiple seasons from boats under extreme conditions include
doi:10.2172/1282 fatcat:wvabxmyvajf2rlrjvz4ivnwzsi