Trial of a tractor dredger for cockles in Burry Inlet, South Wales

The effects on cockle (Cerastoderma edule L.) populations in Burry Inlet, South Wales of mechanical harvesting using a tractor dredger were investigated with an experimental trial conducted on 29 October 1992. Previously, only hand gathering methods were used, and the trial was intended to assist a licensing decision for mechanical dredging. Six blocks of dredged and undredged (control) plots were set out in each of two areas, one having a low density of cockles, the other high. Approximately 82% of the dredged areas was lifted by the blade of the dredger. The catch consisted almost exclusively of adult cockles (greater than or equal to 2 years old) over 25 mm in length. Appreciable losses of spat and one-year-olds from the dredged plots were also observed even though they were not taken in the catch. Possible reasons are discussed. Counts of damaged individuals remaining on the plots on the day after dredging were generally low for all age groups. During the year following the trial, none of the year-classes showed further mortalities attributable to dredging, and changes to shell growth were either minor or absent. Spatfall success in 1993 was depressed by 11% on dredged plots compared to that on control plots in the low density area, but was increased slightly (not significant p > 0.05) in the high density area. It is concluded that delayed effects of the dredging on cockle stocks were negligible. (C) 1997 International Council for the Exploration of the Sea.