Hand dredging for the wedge clam (Donax trunculus) in the Algarve coast (southern Portugal): fishing yield, bycatch, discards and damage rates

ABSTRACT

This study aimed to characterize the catches, quantify fishing yield, bycatch and discards, and also estimate damage and mortality rates using hand dredges with two different mesh sizes (20 and 25 mm) for harvesting the wedge clam (Donax trunculus). A total of 160 tows were performed in Lota Beach and Armona Island in the Algarve coast (southern Portugal). Overall, were caught 12,319 individuals (3985 in Lota Beach and 8334 in Armona Island) belonging to 48 taxa distributed among seven phyla (14 taxa in Lota Beach and 44 taxa in Armona Island). Bycatch proportions ranged from 13.1% to 32.0% in abundance and from 10.7% to 32.9% in biomass, with more bycatch using hand dredges with 20 mm mesh size. Commercially undersized D. trunculus (MCRS = 25 mm SL) prevailed within the bycatch, representing over 40% of discards using both mesh sizes. Due to different size selectivity, the target catch (CPUE) and bycatch per unit effort (BCPUE) in abundance and biomass, as well as fishing yield as a function of tow duration and towed area, were lower for hand dredges with 25 mm mesh than with 20 mm mesh. In general, damage and mortality rates were fairly low (1.3–2.6%) independently of mesh size, reinforcing the importance of releasing live bycatch to the sea immediately after sorting the catches. Overall, this study prompted the need to develop a good practice guide for hand dredgers, aiming to mitigate avoidable indirect discards mortality and improve the sustainability of this traditional harvesting activity.

KEYWORDS:

• Manual harvesting
• fishing yield
• bycatch
• discards
• fishing impacts
• damage score

Acknowledgements

The authors are grateful to João Soares (IPMA - Centro de Olhão) for performing the fishing surveys with hand dredges, to Miguel Carneiro (IPMA - Lisboa) for providing the technical design of the wedge clam hand dredge, and to the Portuguese Hydrographic Institute (IH) for the data on surface seawater temperature during the study period. The authors also acknowledge Dr. Carolin Löscher (Editor), Dr. Francesc Maynou (Subject Editor) and two anonymous reviewers for their useful comments and suggestions to improve the present article.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was performed within the framework of the project ‘Estudo Integrado dos Bancos Naturais de Moluscos Bivalves no Golfo de Cádis para a sua Gestão Sustentável e Conservação dos Habitats Associados - VENUS’ (POCTEP - Spain-Portugal Cooperation Programme - Interreg V-A), co-financed by the European Regional Development Fund (ERDF 2014-2020). The author Laura Sordo was supported by a research grant (Ref: IPMA-2018-008-BI) awarded by the project ‘VENUS’ and the author Lídia Nicolau was supported by a research grant (Ref: IPMA-2019-054A-BI) awarded by the project ‘Contributo para a Gestão Sustentada da Pequena Pesca e da Apanha (PESCAPANHA)’, funded by the Fisheries Operational Programme (MAR 2020) and co-financed by the European Maritime and Fisheries Fund (EMFF 2014–2020).