Bycatch and discards from two types of bivalve dredges targeting Donax trunculus and Chamelea gallina used in the southern coast of the Marmara Sea, Turkey

Intensive commercial harvesting of the wedge clam Donax trunculus and the striped venus Chamelea gallina was conducted along the southern coast of the Marmara Sea using a mechanical dredge (MD) and a hand dredge (HD). The purpose of this study was to characterize and quantify the bycatch and discards of both types of dredge. The dredge tows were performed along sandy bottoms at depths ranging from 0 to 2 m between August 2011 and July 2012. A total of 29 species belonging to 20 families were collected from 137 tows during 67 fishing trips. The total catch in terms of biomass was 107,752.69 g for the MD and 94,257.40 g for the HD, of which 10.78% and 5.08%, respectively, comprised commercial bycatch species, discards, individuals of the target species smaller than the minimum legal size (< MLS), and mortality. Besides the target species (D. trunculus and C. gallina), 27 other species were caught as bycatch and partially discarded. The predominant commercially valuable species in the bycatch were Ruditapes philippinarum (0.94%) and Mytilus galloprovincialis (0.48%) for the HD, and Acanthocardia tuberculata (0.34%) and Rapana venosa (0.34%) for the MD. The rate in weight of discards including < MLS and mortality were 9.22% for the MD and 2.82% for the HD. The gastropods Cerithium vulgatum (58.28%) and Tritia neritea (13.80%) and the crustacean Liocarcinus depurator (5.57%) comprised 77.65% of the discards in weight. Overall, a lower commercial bycatch species + discards + < MLS + mortality rate in terms of weight (5.08%) was observed for the HD compared to the MD (10.78%). The findings of this study are important for the development of policies for the conservation and sustainable fishing of bivalves.