Methods for the calculation of the horizontal opening (swept area) of a bottom trawl are discussed. Details of the Scanmar trawl monitoring equipment used for making in situ measurements of horizontal opening and headline height of the FP-1 20 trawl during the 1993/94 UK fish stock assessment survey around South Georgia on the MV Cordella are provided. Empirical equations are derived for the calculation of horizontal opening from other, more easily obtained measurements, such as tow speed and trawI depth. A new multiple regression equation is compared to that used previously for the estimation of biomass parameters of the major fish species around South Georgia. The resulting difference in biomass estimates is of the order of a 1% to 5% increase, depending upon species.