Bycatch and discard survival rate in a small-scale bivalve dredge fishery along the Algarve coast (southern Portugal)

Mariana Anjos,Domitília Matias,Miguel Gaspar,Sandra Joaquim,Karim Erzini,Paulo Vasconcelos,Fábio Pereira

doi:10.3989/scimar.04742.08a

Mariana Anjos, Domitília Matias + Show 5 more

Open Access

https://doi.org/10.3989/scimar.04742.08a

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Abstract

Although the bivalve dredge used on the Algarve coast (southern Portugal) is highly selective for the target species, in some periods of the year the bycatch can exceed the catch of the commercial species. The present study aimed to quantify the bycatch and discards, estimate damage and mortality, and propose management measures to minimize discards and mortality. A total of 15 fishing surveys (60 tows) were performed using two types of dredges (“DDredge” targeting Donax trunculus and “SDredge” targeting Spisula solida and Chamelea gallina). Of the 85257 individuals (392.4 kg) of 52 taxa that were caught, 73.4% belonged to the target species, 22.1% to commercially undersized target species and 4.5% to bycatch species. Bycatch rates were lower for SDredge (13.5% in number and 6.3% in weight) than for DDredge (46.0% in number and 32.9% in weight). Damage and mortality rates were also lower using SDredge (1.3% and 1.0% of the total catches, respectively) than using DDredge (4.0% and 2.8% of the total catches). Survival experiments revealed the diverse vulnerability of the taxa and confirmed the influence of the damage score on the mortality rate. The results gathered in the present study encourage the adoption of a bycatch reduction device to reduce both direct and indirect mortality.

Highlights

Small-scale fisheries (SSFs) are strongly represented in all European Union (EU) member states (Guyader et al 2013), so many coastal communities rely on fisheries and fishing-related activities as a means of subsistence, income and employment (Oliveira 2014) that has created an ancient cultural heritage and socio-economic dependence
The vast majority of the Portuguese fishing fleet is composed of artisanal vessels (Oliveira et al 2007), and the largest bivalve dredging fleet is based on the Algarve coast, with 53 vessels operating an average of 177 days year–1 (DGRM 2015)
In the 60 tows performed during 15 fishing surveys using the two types of dredge, 85257 individuals belonging to 52 taxa distributed through six phyla were caught

Summary

Introduction

Small-scale fisheries (SSFs) are strongly represented in all European Union (EU) member states (Guyader et al 2013), so many coastal communities rely on fisheries and fishing-related activities as a means of subsistence, income and employment (Oliveira 2014) that has created an ancient cultural heritage and socio-economic dependence. On the Algarve coast, the dredging fleet targets mainly three commercially valuable bivalve species: Spisula solida, Chamelea gallina and Donax trunculus (Gaspar et al 2015). The surf clam (S. solida) occurs mainly between 3 and 14 m depth; it is the most abundant bivalve species in this area and has shown increasing densities over the last decade. The donax clam (D. trunculus) occurs within a narrower bathymetric range (0-5 m depth, with higher densities at 3 m depth) (Gaspar et al 2015), but it is among the most important molluscan species commercially exploited in the southern Iberian Peninsula and western Mediterranean Sea (Gaspar et al 1999, Tirado et al 2011)

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