In situ determination of bottom trawl ground gear contact

Abstract

Exact definition of towed distance with proper ground gear contact is an essential parameter in bottom trawl swept area estimates of fish abundance. Errors in this key parameter may lead to a first order bias in abundance estimates of demersal fish. The paper describes the results obtained with a new sensor for measuring the exact timing of trawl ground gear bottom contact and departure, as well as monitoring of the ground gear contact during the haul. Measured towed duration with the new system is compared with the traditional survey measures of this parameter on a number of hauls, and the results are discussed with respect to survey bias.