© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
A comparison of direct macrofaunal mortality using three types of clam dredges
a Instituto de Investigação das Pescas e do Mar (IPIMAR), Centro Regional de Investigação Pesqueira do Sul (CRIPSul) Avenida 5 de Outubro s/n, P-8700-305 Olhão, Portugal
b Universidade do Algarve (UAlg), Faculdade de Ciências do Mar e do Ambiente (FCMA) Campus de Gambelas, P-8000-117 Faro, Portugal
c Instituto de Investigação das Pescas e do Mar (IPIMAR), Delegação de Setúbal, Avenida Jaime Rebelo 29-A 1°, P-2900-409 Setúbal, Portugal
*Correspondence to M. B. Gaspar; tel: +351 289 700500; fax: +351 289 700535. e-mail: mbgaspar{at}ipimar.ualg.pt.
The white clam Spisula solida is harvested along the entire coast of Portugal using mechanical dredges. In this study, the total direct mortality of the macrobenthic community caused by three types of clam dredges (north dredge—ND, traditional dredge—TD, and the metallic grid dredge—GD) used in the S. solida fishery was determined and compared. The relationship between mortality and catching efficiency for each type of dredge was also assessed. Our results showed significant differences for total direct mortality between the ND and both the GD and TD dredges. This difference was largely attributed to the mortality of animals that died in the dredge track as a direct result of the physical damage inflicted by the dredge passing. It was also found that the damage to uncaught individuals is directly related to gear efficiency. The lower catching efficiency of the ND (64%) led to a higher proportion of damaged individuals being left in the dredge path, when compared with the more efficient GD (98%) and TD (90%) dredges. Short and long-term implications of the impact of dredging on the composition of benthic communities are discussed. From fisheries management and ecological points of view, there are obvious advantages to introduce into the bivalve dredge fisheries more efficient and selective dredges in order to reduce the number of damaged individuals and by-catch, and consequently decreasing the impact on the macrobenthic communities.
Keywords: direct mortality, dredge, fishing impacts, gear efficiency, macrofauna, Spisula solida
Received 19 September 2002; revision received 28 January 2003; accepted 18 February 2003.
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