Effect of strobe lights on catches and length distributions of euphausiids collected by Bongo nets

Euphausiids are difficult to catch quantitatively with most net sampling systems due to avoidance of the nets. Previous studies have shown that bright lights may reduce euphausiid net avoidance from relatively large net sampling systems. This study describes experiments where strobe lights were mounted on the smaller Bongo net frame, and net avoidance evaluated by repeated towing through layers of euphausiids, as evidenced from acoustic backscattering data, with the strobe lights either on or off. The study was conducted both in fjord and offshore environments. The results show that the use of strobe lights significantly increases the catches of Thysanoessa raschii, T. inermis and Meganyctiphanes norvegica, and that the increase is mainly due to increased catches of the older components of the populations, whereas juveniles of the spring generations are relatively unaffected. The results for T. longicaudata are less conclusive although they indicate that the catches of this species are also positively affected by the strobe lights. The results lend support to the hypothesis that the use of the strobe lights increases catches by reducing net avoidance behavior of the euphausiids rather than by attracting them, and the catches therefore better representing the composition of the stock than if the lights were not used.