Efficiency of point abundance sampling by electro-fishing modified for short fishes

Abstract

Summary The assessment of fish densities using point abundance sampling by electro-fishing requires information about the size of the sample area. For electro-fishing the effective fishing range depends on biological effects such as species and length of fish as well as physical effects like conductivity of water or substrate type. The present study investigates systematically the impact of conductivity and substrate type on the extension of the electrical field of a battery-powered electro-fishing gear (DEKA 3000, Marsberg, Germany), modified for larval and juvenile fishes. Threshold values for galvanotaxis were examined for juvenile fishes of five species in terms of current densities. Based on 71 experiments a general function relating body length to current density threshold values was developed. Optimal electrical current flow periods of 10 s were determined. For three different substrate types (gravel, sand, mud) a formula has been developed to quantify biological and physical effects on the effective fishing range. Each equation included information on the length of fish and the ambient conductivity. An increase in the effective fishing range of about 10% every 0.1 mS cm−1 was established. Reduction of the fishing range over muddy substrate was about 20–30% compared with coarse gravel or sand. This study provides a sufficient tool to calculate area-related densities of larval and juvenile fishes in different habitat types of a large river system using point abundance sampling by electro-fishing. Finally, calculated fish densities were evaluated by different types of fishing gear.