Differing Catchability among Lakes: Influences of Lake Basin Morphology and other Factors on Gill-Net Catchability of Northern Pike

Abstract

Understanding factors that affect catchability is important for interpreting fish catches from research index netting and comparing fish populations from different habitats. Population density estimates and gill-net catch rates were compared for age-2 and older northern pike Esox lucius in 16 north-central Minnesota lakes that varied in physical characteristics and northern pike populations. When northern pike density was calculated using total surface area of the lakes, numerous factors appeared to influence gill-net catchability. Those factors included several lake basin shape metrics, northern pike size structure, and northern pike density, with density dependence in catchability being especially problematic for interpreting gill-net catches. The critical finding in this study was that calculating density using littoral area rather than total surface area caused evidence of density-dependent catchability and all other relationships with catchability to completely disappear. Catchability became independent of northern pike density and lake basin shape metrics when density was calculated using littoral area. Correlations with density (calculated from total surface area) and other factors were simply artifacts of underlying interrelationships between the morphology of lake basins, northern pike population characteristics, and density-dependent processes. Catchability estimates for littoral habitat averaged 0.20 ha/net and ranged from 0.09 to 0.43 ha/net among the lakes. Although the relationship between gill-net catch and population density indicated that gillnetting could be a useful tool for comparing northern pike populations from different lakes, a predictive model fitted to the observed gill-net catch rates and population densities from 16 lakes had low precision. Results from this study illustrate the importance of a comprehensive understanding of fish ecology, including spatial habitat use, when interpreting survey results.