Reburial time and indirect mortality of Spisula solida clams caused by dredging

Abstract

Clam-dredging results in the exposure of Spisula solida individuals not caught by the dredge. Subsequent survival depends on clam damage, reburial time, and the time needed by predators to reach the impacted area.We analyse these variables and discuss the importance of predation on exposed S. solida caused by dredge fishing. Sampling was performed in July 2000 off the southern coast of Portugal, at Vilamoura, a traditional S. solida sandy fishing ground.We compared the time needed for S. solida individuals to rebury themselves, relative to the abundance of potential predators. Bivalves collected by divers were placed on the seabed, and the times required for reburial were measured. These were compared with the times needed for reburial of the clams exposed by dredge impact. At each of three dredge tracks, we analysed the number of predators that entered three equal quadrats (0.0250 m2) per minute. These results were compared with a non-affected control area. Impact caused by the fishing dredge significantly increases the number of exposed S. solida clams ðp < 0:05Þ and the abundance of potential predatory species ðp < 0:05Þ. The brittle star Ophiura texturata was the most abundant and first species to reach the dredge track (less than 3 min after dredge impact). Other species reaching the dredge track were Pomatochistus spp. (6 min after impact), Diogenes pugilator, and Nassarius reticulatus (both 9 min after impact). Although predators reached the impacted area while S. solida bivalves were still exposed, our results suggest that predation on the non-buried clams in the dredge track is not a major factor for subsequent indirect mortality of S. solida.