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Analysis and damage assessment of discards in the hand dredge wedge clam fishery (Donax trunculus Linnaeus, 1758) in the Gulf of Cádiz (SW Spain

Raimundo Blanco1*, Sara Román1, Ana Rodriguez-Rua1, Miguel Coján1, Luis Silva1, Antonio Sánchez2, Esperanza Lisa-García1 and Marina Delgado1
  • 1 Oceanographic Center of Cádiz, Spanish Institute of Oceanography, Spain
  • 2 Facultad de Ciencias, Universidad de Cádiz, Spain

Within the framework of the research project VENUS, the aim of this communication is to provide a first insight of the discards associated with the artisanal hand dredging fishery targeting wedge clam Donax trunculus Linnaeus, 1758 in the sandy beaches at littoral of Huelva, in the south west of Spain. Small scale fisheries represent one of the main types of fisheries occurring in the area with a high socioeconomic importance. D. trunculus constitutes a very important fishing resource due to its high economic value and has been exploited over the last decades using primarily either artisanal hand-operated dredges (rastros manuales) in shallow water and commercial boat-operated dredges from subtidal to deeper waters. The hand dredge wedge clam fishery has been regulated by the Andalusian autonomic government since 2008. Several measures for the control of fishing and protection of this valuable economical resource were established in order to maintain its sustainability: licenses (the protect area Doñana NP has a specific license regulation), daily quota, a minimum size limit and a closed season (Silva et al., 2014). In general fishing trawling is one of the most impactful fishery activities on the seabed. The physical and biological impacts in the communities are variable. They depend upon size and weight of the gear as well as fishing practice and ecosystem. In the area of study, there is a lack of scientific information about of some aspects of the wedge clam fishery with hand dredge: discard structure and composition, and damage caused to specimens during the fishing and the manipulations. A basic tenet of the ecosystem approach to fisheries management is that harvesting should be conducted with minimal impact on juveniles, non-target species, and marine habitats. An analysis of discards and damage will be useful for the adoption and assessment of the discard mitigation strategies for this fishery (Bellido et al., 2011). The study area comprised the low intertidal and the upper (2 - 3 m depth) infralitoral zone of the sandy beaches of the Huelva province coast. The area was divided into six units from the mouth of the Guadalquivir river to Ayamonte (mouth of the Guadiana river): Doñana National Park, Mazagón, Huelva, Punta Umbría-La Bota, Isla Cristina-El Rompido, and Ayamonte. Sampling was carried out using a hand dredge similar to those used by artisanal fishermen. It consists in a wood towing arm, dredge mouth of 44.5 cm width by 15 cm height with an even cutting edge, a mesh sieve size of 7 X 7 mm and a length of 58 cm, with a rear net bag of 200 cm length and mesh size of 5 X 5 mm. The gear digs deeply into the sediment up to 10 cm. A total of 17 hauls were performed at 17 stations, between the lower intertidal and 2 m depth of the infralitoral zone in June 2018. At each station the hand dredge was towed diagonally to the shoreline for 5 min. Every transect was geo-referenced by a GPS. The samples were sorted in the laboratory. Macrofauna and megafauna were identified to the lowest taxon level (mainly species level) with the uttermost precision and their respective abundances and weights recorded. Additionally, a damage assessment was performed on the fauna, taking into account the whole fishing process. A three level damage scale was used to assign the level damage to the individuals of each taxon: D0 (no damage), D1 (low damage), D2 (intermediate damage) and D3 (high damage, not survival after their release) (Anjos et al., 2011, Urra et al, 2017). The preliminary study of the 3897 specimens yielded 25 taxa belonging to 5 Classes: Polychaeta (4), Bivalvia (8), Gastropoda (3), Scaphopoda (1), Crustacea (6), Echinoidea (2) and Actinopterygii (1). However, the number of taxa could be even higher, because Paguroidea and Polychaeta could not be identified to species level. Bivalvia was the dominant group in abundance (94.12%) and biomass (95.41), followed by Crustacea with 4.72% and 2.06%, Echinoidea 0.44% and 2.42%, Polychaeta 0.39% and 0.06%, Gastropoda 0.21% and 0.01, Scaphopoda 0.10% and 0.01% and finally Actinopterygii with 0.02% and 0.03%. The targeted species, D. trunculus, represented 83.34 % of the specimens and 76.58% of the biomass of the capture (3248 specimens and 8447.03 g); The discard fraction was composed by 24 taxa and represented the 16.65% of the collected individual and the 23.42 % of the biomass. Discards were dominated by Bivalvia in taxa richness (7), abundance (64%) and biomass (80.39%). Overall, 82.44% of the discarded specimens were undamaged, 0.92% display low damage, 0.15% display intermediate damage, 16.49% display severe damage. Bivalve was the group with more damaged individuals (66.67%), all belonging to Mactra stultorum (Linnaeus, 1758), followed by brachyuran crustaceans (25.44%), irregular sea-urchins (6.14%) and fishes (1.75%).

Acknowledgements

Study funded within the framework of the project “VENUS” (Estudio integral de los bancos naturales de moluscos bivalvos en el Golfo de Cádiz para su gestión sostenible y la conservación de hábitats asociados) (0139VENUS5E; INTERREG-POCTEP). We would like to thank Rafael Donato, Carlos Farias, Eva García-Isarch, Verónica Duque, Jorge Rey and Teresa Montaño for follaborating in biological sampling. We also thank the ranger team of the Doñana national park and Doñana Biological Station (EBD-CSIC).

References

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Keywords: Discards, damage, Megafauna, macrpfauna, Wedge clam, Small scale fisheries, Ecosystem aproach

Conference: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019.

Presentation Type: Poster Presentation

Topic: Ecology, Biodiversity and Vulnerable Ecosystems

Citation: Blanco R, Román S, Rodriguez-Rua A, Coján M, Silva L, Sánchez A, Lisa-García E and Delgado M (2019). Analysis and damage assessment of discards in the hand dredge wedge clam fishery (Donax trunculus Linnaeus, 1758) in the Gulf of Cádiz (SW Spain. Front. Mar. Sci. Conference Abstract: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) . doi: 10.3389/conf.fmars.2019.08.00091

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Received: 11 May 2019; Published Online: 27 Sep 2019.

* Correspondence: Mr. Raimundo Blanco, Oceanographic Center of Cádiz, Spanish Institute of Oceanography, Cádiz, Spain, raimundo.blanco@ieo.es