Size Selectivity and Bias in Estimates of Population Statistics of Smallmouth Bass, Walleye, and Northern Squawfish in a Columbia River Reservoir

Abstract

We describe size selectivity of five types of gear, size selectivity in combined gear samples, and potential bias in estimates of population statistics of smallmouth bass Micropterus dolomieui, walleye Stizostedion vitreum, and northern squawfish Ptychocheilus oregonensis in John Day Reservoir on the Columbia River. We sampled from 1983 to 1986 with two types of gill nets, boat electrofishers, trap nets, and rod and reel. Different gears selectively sampled different sizes of each species. Recapture rates indicated that different sizes of fish remained differentially vulnerable to capture even when samples from all gears were pooled. Vulnerability of smallmouth bass declined with increasing size. Vulnerability of walleye was not related to size. Vulnerability of northern squawfish increased with size. Size selectivity of gear resulted in estimates of abundance potentially biased by 2–16%, estimates of proportional stock density (size structure) biased by 11–46%, and estimates of annual rate of mortality biased by 17–69%. The bias was negative in estimates of abundance and varied in estimates of size structure and mortality in ways that depended on the pattern of vulnerability. In any long-term monitoring of a population, investigation of the nature of the bias resulting from size selectivity will be prudent.