Approaches to Improving Groundfish Survey Abundance Estimates by Controlling the Variability of Survey Gear Geometry and Performance

Abstract

Options for using trawl mensuration data to improve survey abundance estimates are discussed, with examples from Scotian Shelf groundfish surveys. Once the survey performance parameters of individual trawls are known, the next step should be to control the variability of parameters which can be controlled. rather than to standardize the catch using catch/parameter relationships which are themselves highly variable or subject to dubious assumptions. It is possible to interactively change warp length usinq spread sensor information to achieve constant door spread, just as distance travelled during a tow is now controlled by interactively monitoring ship speed with Doppler logs. Abundance indices from Scotian Shelf surveys are subject to significant bias from systematic variations in gear spread, confirming the importance of routine trawl mensuration during standard survey sets, and the need for corrective action.