Preservation of juvenile hake (&lt;i&gt;Merluccius merluccius&lt;/i&gt; L) in the western Mediterranean demersal trawl fishery by using sorting grids

Francesc Sardà,Balbina Molí,Isabel Palomera

doi:10.3989/scimar.2004.68n3435

Abstract

The Mediterranean fishery has experienced a decline in catches over the past 20 years due to an excessive increase in effort caused by both increased trawler engine power and rapid technological advances in fishing technology and fish location. This has led to overexploitation, in which immature individuals support an increasing portion of the catches. The present study was undertaken to test a sorting grid and square-mesh-panel as juvenile exclusion systems. Our experience was a pilot study of such a system in the western Mediterranean. The purpose of these exclusion systems was to help juvenile hake escape from the net. The results demonstrate that the use of sorting grids for small fish in trawl gears in the Mediterranean is an efficient and practical means of avoiding the capture and discarding of unwanted individuals, with escape rates of over 50% (ranging between 50 and 90%). The grids were efficient and useful for excluding hake ( Merluccius merluccius ) on bottoms located at depths between 50 and 300 m, where hake are found all year round. A bar spacing of 20 mm yielded escape rates of L 50 at 18.8 cm TL (L 25 = 16.8 cm and L 75 = 20.9 cm). Panels made of square meshes achieved poorer results than the grids. The grid system was effective for most of the species caught in the study area.

Highlights

Selection methods based on increasing the size of traditional diamond-shaped meshes have often failed to improve the escapement of juveniles because diamond meshes stay tightly closed during trawling and individuals that escape through these meshes may suffer high mortality rates (Suuronen, 1995; Suuronen et al, 1996; Figuerola et al, 2001).there is evidence that post-selectionHAKE PRESERVATION USING SORTING GRIDS 435survival can be improved by using fish sorting grids in trawl gears (Broadhurst et al, 1996; Suuronen et al, 1996; Broadhurst et al, 1999)
Mediterranean is an efficient and practical means of avoiding the capture and discarding of unwanted individuals, with escape rates of over 50%
The grids were efficient and useful for excluding hake (Merluccius merluccius) on bottoms located at depths between 50 and 300 m, where hake are found all year round

Summary

Introduction

Selection methods based on increasing the size of traditional diamond-shaped meshes have often failed to improve the escapement of juveniles because diamond meshes stay tightly closed during trawling and individuals that escape through these meshes may suffer high mortality rates (Suuronen, 1995; Suuronen et al, 1996; Figuerola et al, 2001).there is evidence that post-selectionHAKE PRESERVATION USING SORTING GRIDS 435survival can be improved by using fish sorting grids in trawl gears (Broadhurst et al, 1996; Suuronen et al, 1996; Broadhurst et al, 1999). Countries in northern Europe and America are developing sorting grids to improve size selection of fishes, allowing juveniles to escape through the net with minimal injury and stress before they enter the codend (Larsen and Isaksen, 1993; Chopin and Arimoto, 1995; Lehtonen et al, 1998; Valdemarsen and Suuronen, 2003 ). Other methods based on gear designs that include square meshes or escape windows in different parts of the net have been tried for various species (Ulmestrand and Larsson, 1991; Briggs, 1992; Campos et al, 1996; Broadhurst, 2000; Halliday and Cooper, 2000; Madsen et al, 2001; Massutí et al, 2002; Campos et al, 2003). The effect of towing speed and gear size on the selectivity of bottom trawl nets has been tested (Dahm et al., 2002)

Methods

Discussion

Conclusion