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The mathematical modelling of fish behaviour in relation to towed fishing gear based on quantitative analysis of stimuli of the gear and fish response, was investigated in order to increase the selectivity and efficiency of the gear. The brightness contrast of the netting and other components of the fishing gear such as floats and bobbins was studied by comparing their luminance to that of their visual background of water, sand-cloud or sea bed. Luminance of each component was measured with a luminance meter in the tank and by the video image processing of nets and gears in the sea with light intensity and grey levels. The brightness contrasts of the gear are increased with diameter of twine, light level and viewing zenith angle. A number of contrast modules are developed, based on the measurements, for predicting the visual stimulus for reactions of fish to towed fishing gear. The underwater luminance against which a fish sees the net components is calculated for the prediction process. The visual stimuli of trawl were represented through the predicted light field and methods are developed for calculating the visibility of the trawl components and sand clouds as viewed from any fish position within the net. Fish behaviour in relation to towed fishing gear was observed and analysed using a net panel towed in the gantry tank and bottom trawls at sea observed by a SIT video camera system. When fish displayed stable swimming as in an optomotor response, the variations of relative swimming speed and angular velocity were relatively low and regular in their amplitude and period. However, when fish moved suddenly in panic reaction to escape or rise up, the variations of these measured components were high, irregular and nonperiodic.