TRIAL OF A TRACTOR DREDGER FOR COCKLES IN BURRY INLET, SOUTH WALES

The effects on cockle (Cerastoderma edule L.) populations in Burry Inl et, 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 i ntended to assist a licensing decision for mechanical dredging. Six bl ocks of dredged and undredged (control) plots were set out in each of two areas, one having a low density of cockles, the other high. Approx imately 82% of the dredged areas was lifted by the blade of the dredge r. The catch consisted almost exclusively of adult cockles (greater th an or equal to 2 years old) over 25 mm in length. Appreciable losses o f spat and one-year-olds from the dredged plots were also observed eve n though they were not taken in the catch. Possible reasons are discus sed. Counts of damaged individuals remaining on the plots on the day a fter dredging were generally low for all age groups. During the year f ollowing the trial, none of the year-classes showed further mortalitie s attributable to dredging, and changes to shell growth were either mi nor or absent. Spatfall success in 1993 was depressed by 11% on dredge d 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 Explora tion of the Sea.