Illumination and diel variation modify fish passage through an inclined grid

Fishing trials were carried out with a demersal trawl fitted with an inclined grid in the extension. Fibre optic cables were attached to the grid allowing the illumination of the top and bottom halves of the grid independently. Fish that passed through the top half of the grid entered one codend whereas those that passed through the lower half of the grid entered another. Four lighting configurations were investigated where (i) none of the grid was illuminated; (ii) the bottom-half of the grid was illuminated; (iii) the top-half of the grid was illuminated; and (iv) all of the grid was illuminated. Trials were held at night and during the day.

We demonstrate that the proportion of fish that entered the top codend depends on the grid illumination and has a diel variation. Most species were less likely to enter the upper codend when the grid was illuminated and the results were similar regardless of whether the bottom-half, top-half, or the whole grid was illuminated. There was also a diel effect for all species with a lower proportion of haddock and whiting and a greater proportion of flatfish in the upper codend at night than during the day. The results are more subtle for some species and for cod, illuminating the grid had no effect during the day, whereas for common dab, there was no effect when the top-half of the grid was illuminated but there was when the bottom-half was illuminated. For long rough dab, illuminating the top-half of the grid had a greater effect than illuminating the bottom-half, and illuminating the whole grid had the greatest effect.