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A comparison of bycatch and discard mortality in three types of dredge used in the Portuguese Spisula solida (solid surf clam) fishery

Published online by Cambridge University Press:  08 April 2009

Francisco Leitão ,
Miguel B. Gaspar ,
Miguel N. Santos  and
Carlos C. Monteiro
Francisco Leitão
Affiliation:
Instituto Nacional de Recursos Biológicos (INRB, IP) /IPIMAR, Avenida 5 de Outubro s/n, 8700-305 Olhão, Portugal
Miguel B. Gaspar
Affiliation:
Instituto Nacional de Recursos Biológicos (INRB, IP) /IPIMAR, Avenida 5 de Outubro s/n, 8700-305 Olhão, Portugal
Miguel N. Santos
Affiliation:
Instituto Nacional de Recursos Biológicos (INRB, IP) /IPIMAR, Avenida 5 de Outubro s/n, 8700-305 Olhão, Portugal
Carlos C. Monteiro
Affiliation:
Instituto Nacional de Recursos Biológicos (INRB, IP) /IPIMAR, Avenida 5 de Outubro s/n, 8700-305 Olhão, Portugal
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Abstract

Despite the importance of the dredge fishery in Portugal, few studies have focused on bycatch and discard mortality. Catches from three types of dredge: north dredge (ND); grid dredge (GD); and traditional dredge (TD), currently used in the Spisula solida (Bivalvia: Mactridae) fishery, were compared in order to characterize bycatch and to estimate discard mortality. In TD and ND, the catch was retained in a net bag, whereas in the GD a metallic grid cage was used. Tows were carried out on sandy bottoms at depths ranging from 5 to 10 m. Bycatch differed significantly between dredges. The ratio of bycatch abundance to S. solida was 0.22:1 for ND, 0.10:1 for GD and 0.33:1 for TD. The weight ratio was 0.15:1, 0.14:1 and 0.32:1 for ND, GD and TD, respectively. Although a lower bycatch was observed for GD, higher mortality (ND: 10%; TD: 20%; GD: 36%) was estimated for this dredge. However, taking into consideration the fishing yields obtained for each dredge combined with the fact that this fishery is managed by daily quota per vessel, our results showed that, at daily quota level, discard mortality was lower when GD was used. Our study suggests that in output-controlled fisheries, there are obvious advantages in developing dredges with rigid retention structures, such as metallic grids, because these are more selective and efficient than dredges that use net bags. At species level, the bycatch mortality to attain quota varied among dredges. A “selective” and persisting significant removal of bycatch species from biota may change benthic community structure. Therefore, clam fisheries should apply exploitation strategies that are goal-oriented and adaptive to the evolution of the macrofauna communities, which might include the use of different dredge types, thereby diversifying the bycatch and consequently avoiding the cumulative removal of specific macrofauna taxa.

Keywords

BycatchClam dredgingDiscard mortalityMolluscBivalveSpisula solidaAtlantic Ocean
Type
Research Article
Information
Aquatic Living Resources , Volume 22 , Issue 1 , January 2009 , pp. 1 - 10
Copyright
© EDP Sciences, IFREMER, IRD, 2009

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