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

P Molenaar,J Batsleer,A.D Rijnsdorp

doi:10.1016/j.fishres.2021.106104

Abstract

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.

Highlights

In many countries, fisheries land the marketable part of the catch and discard undersized or unwanted species
This paper studies the effect of electrical stimulation on the catch
The proportion of sole caught in the pulse net was on average 0.87 of the total number caught, as compared to 0.69 and 0.72 in plaice and dab, respectively

Summary

Introduction

Fisheries land the marketable part of the catch and discard undersized or unwanted species. Discarding is pro nounced in bottom trawl fisheries. Discarding reduces the sustainable yield and may cause unwanted ecological consequences. FAO estimated global discards at 27 million tonnes in 1994 and 7.3 million tonnes in 2005 (Alverson et al, 1994; Kelleher, 2005). In order to reduce dis carding the EU has imposed in the 2013 reform of the Common Fisheries Policy an obligation to land all fish caught (Borges, 2015; Uhlman et al, 2019). A ban on discarding is expected to create an incentive for fishers to develop discard avoiding technologies improving the selectivity of the gear or avoid fishing on fishing grounds with large quantities of discards (Condie et al, 2014; Gullestad et al, 2015; Guillen et al, 2018; O’Neill et al, 2019)

Methods

Results

Conclusion