ESTIMATION OF THE CAPTURE EFFICIENCY OF TRAWL GEAR .2. TESTING A THEORETICAL-MODEL

This paper uses published data to test a theoretical model of trawl gear efficiency. The objective is to use the model described in Part I and find if the results from four separate experiments can be fitted to it coherently and without contradiction. The four sets of experiments were: the measurement of escapes at the groundrope of the bottom trawl (bag experiments); comparative fishing of rockhopper vs. standard groundrope both with standard sweeps; long sweeps vs. standard sweeps; and short sweeps vs. standard sweeps. The species concerned are cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.). Estimates are made of absolute efficiency, meaning catch/encounters, of both the bobbin and the rockhopper nets. Using the measured parameters of otterboard spread and net spread the theory is applied to derive estimates of sweep efficiencies and otterboard effect, which fit all cases. This makes possible improved estimates of local abundance and length frequency distribution. The differences in effective spread by size group and between gears is marked.