An Echo Counting and Logging System (ECOLOG) for Demersal Fish Size Distributions and Densities

Abstract

Target strength of fish is dependent on their size, behavior, and density, and uncertainties about these parameters under field conditions affect the interpretation of acoustic survey data. The echo counting and logging system, ECOLOG, appears to offer information on these important parameters. The dual-beam system is described and results of a test cruise are analyzed. Published data on the relation of target strength to fish size are used to generate an "acoustic size-composition" from otter-trawl catches. Correspondence between the converted otter-trawl samples and acoustic data suggest that the system is generating direct information on demersal fish size and density and may thus be useful in verifying the results of echo sounder surveys. Main components of variation among samples and between sampling methods are ascribable to small-scale spatial heterogeneity of size distributions, although day–night behavior differences contribute significantly to the total heterogeneity.