Modelling the visual stimulus of towed fishing gear

摘要

A number of modules are developed for a model with the aim of predicting the strength of the visual stimulus that might initiate fish reactions from any viewing point within a towed fishing gear. The first part of the model defines the properties of the undersea light field through which a trawler might tow a fishing gear including allowances for geographical position, weather, water quality and diurnal changes. A variety of light measurements are made to check day and night light levels used in the model. The underwater luminance of water, sand cloud and sea bed which form the background against which a fish sees the net components is then calculated. An example constructing the space in which a fish may react within a towed trawl gear is developed for the BT130C, North Sea trawl. This part develops an existing net frame model that creates the three-dimensional shape of the towed net from the designers net drawings. Analysis of previous observations of this trawl form the bases for simulation of sand clouds produced by the ground gear and otter board as part of the fish stimulus. Finally the simulated trawl is towed through the predicted light field and methods are developed for calculating the visibility of the net components including netting, ropes, floats, bobbins, otter boards and their sand clouds as viewed from any fish position within the net. An example of the result of these predictions, maps the stimulus strength for one fish position. (