Can vertical separation of species in trawls be utilized to reduce bycatch in shrimp fisheries?

Roger B Larsen,Jure Brčić,Kåre Nolde Nielsen,Mark J M Lomeli,Manu Sistiaga,Adnan Tokaç,Bent Herrmann,Nadine Jacques,Elsa Cuende,Kristine Cerbule

doi:10.1371/journal.pone.0249172

Abstract

Several shrimp trawl fisheries use a Nordmöre sorting grid to avoid bycatch of fish. However, small fish can pass through the grid. Therefore, the retention of juvenile fish often remains an issue during shrimp trawling. We investigated the vertical distribution of deepwater shrimp (Pandalus borealis) and dominant bycatch species at the point where the Nordmöre grid section is installed. This was achieved using a separator frame which split the net vertically into three compartments of equal entry size. Our results showed that shrimp predominately follow the lower part of the trawl belly, whereas species such as redfish (Sebastes spp.), cod (Gadus morhua), polar cod (Boreogadus saida) and American plaice (Hippoglossoides platessoides) preferred the mid-section in the aft of the trawl. Haddock (Melanogrammus aeglefinus) primarily entered through the upper section of the trawl belly. Using these results, we predict that a vertical separation device installed forward of a 19 mm Nordmöre grid combined with a 35 mm codend would result in a significant reduction in bycatch with only minor loss of shrimp.

Highlights

The deep-water shrimp (Pandalus borealis) has a widespread distribution in the North Atlantic and the North Pacific Oceans and is a commercially important species in many countries [1]
We considered k of up to an order of 4 with parameters vX = as our experiences from prior studies have demonstrated that this provides a model that is sufficiently flexible for modelling the vertical separation for different species [26,27,30]
Besides deepwater shrimp the catches contained sufficient number of American plaice, cod, polar cod, redfish and haddock to be included in the investigation (Table 1; Fig 4)

Summary

Introduction

The deep-water shrimp (Pandalus borealis) has a widespread distribution in the North Atlantic and the North Pacific Oceans and is a commercially important species in many countries [1]. Deepwater shrimp and other species in the family Pandalidae live most of their life relatively close to the seabed [3,4] and are almost exclusively harvested with bottom trawls. The use of this gear results in excessive bycatch of juvenile fish and other marine organisms [5,6].

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Conclusion