The effect of electrical stimulation on the footrope and cod-end selection of a flatfish bottom trawl

The beam trawl fishery targeting sole is known for their substantial bycatch of flatfish below the minimum landing size. Pulse trawls were developed to improve the selectivity by replacing mechanical stimulation with electrical stimulation which immobilises fish in front of the footrope. Results are presented of an experiment on board of a commercial pulse trawler studying the effect of electrical stimulation on footrope and cod-end selectivity for three flatfish species -sole (Solea solea), plaice (Pleuronectes platessa) and dab (Limanda limanda) -in 29 paired hauls with electrical stimulation alternating between the starboard and portside gear. It was shown that electrical stimulation increased the footrope selection by a factor 2 in plaice and dab and a factor 7 in sole. The effect on sole is related to the specific response of sole which cramps into a U-shape. Footrope selection showed a small diurnal pattern with the highest selectivity of the pulse trawl during the day. Electrical stimulation and catch weight were shown to have a small effect on the cod-end selectivity retaining slightly more marketable sole. Cod-end mesh selection factors (SF) were estimated at 3.00 (se = 0.02), 2.11 (se = 0.02), and 2.3 (se = 0.1) for sole, plaice and dab, respectively. Selection ratio (SFA = selection range/mesh size) was estimated at 0.45 (se = 0.03), 0.23 (se = 0.02), and 0.41 (se = 0.04) for sole, plaice and dab, respectively. The SF and SFA of the pulse trawl is comparable to values reported for conventional beam or otter trawls.

Automated summary

The electrical stimulation of pulse trawls significantly improves the footrope selection in flatfish, particularly in plaice and dab, by a factor of 2, and in sole by a factor of 7. There is a slight diurnal pattern in footrope selection, with higher selectivity during the day. The cod-end selectivity is also slightly enhanced by electrical stimulation, retaining more marketable sole, with selection factors comparable to traditional trawls.