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

Abstract

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 (d2 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. 1997 International Council for the Exploration of the Sea

Recent advances in understanding the environmental footprint of trawling on the seabed

“…In addition to financial constraints, early experimental studies were confronted with a lack of access to sufficiently accurate global positioning systems (GPS) that were necessary to enable the accurate positioning of research platforms used for the collection of samples from within experimentally trawled areas. For this reason, intertidal fishing impact studies most often were associated with the clearest outcomes (Cotter et al 1997). The development of differential GPS and a greater access to accurate positioning was the precursor to the eventual introduction of vessel monitoring systems (VMS) on board fishing vessels.…”

A quantitative analysis of fishing impacts on shelf‐sea benthos

“…An asterisk in the regime column denotes a study on a ®shing ground, where the level of disturbance was unknown. For a description of the other variables see Table 2 Reference Brown & Wilson (1997) x Inter-tidal dredging Mud East North America 1 0 4 Inter-tidal dredging Mud Northern Europe 2 0 1 210 Kaiser et al (1998) x (x) Inter-tidal dredging Muddy sand Northern Europe 1Á5 0 1 365 Wynberg & Branch (1994) Inter-tidal dredging Muddy sand South Africa 3 0 1 606 Peterson et al (1987) Inter-tidal dredging Sand East North America 35 1 2 730 Hall & Harding (1997) x (x) Inter-tidal dredging Sand Northern Europe 45 0 1 56 Cryer et al (1987) Inter-tidal dredging Sand Northern Europe 5 0 1 180 McLusky et al (1983) x (x) Inter-tidal dredging Sand Northern Europe 1 0 1 140 Heiligenberg (1987) x (x) Inter-tidal dredging Sand Northern Europe 7 0 1 140 Heiligenberg (1987) x (x) Inter-tidal dredging Sand Northern Europe 1 0 1 180 Heiligenberg (1987) x (x) Inter-tidal dredging Sand Northern Europe 1 0 1 140 Hall et al (1990) Inter-tidal dredging Sand Northern Europe 50 7 1 40 Brown & Wilson (1997) x Inter-tidal raking Mud East North America 1 0 2 Cotter et al (1997) x (x) Inter-tidal raking Sand Northern Europe 20 0 1 400 Hall & Harding (1997) x (x) Inter-tidal raking Sand Northern Europe 45 0 1 56 Kaiser et al (1998) x Otter trawling Biogenic East Australia 40 25 1 Kaiser et al (1998) x Otter trawling Biogenic East Australia 40 50 1 Van Dolah et al (1987) x (x) Otter trawling Biogenic East North America 20 1 365 Bradstock & Gordon (1983) Otter trawling Biogenic New Zealand 10 * Magorrian et al (1995) Otter trawling Biogenic Northern Europe * Sainsbury (1987) Otter trawling Biogenic North-western Australia 50 * Guillen et al (1994) Otter trawling Biogenic Southern Europe * 1095 Otter trawling Gravel East North America 94 * Freese et al (1999) Otter trawling Gravel West North America 5 206 1 Lindeboon & de Groot (1998) x Currie & Parry (1996) x (x) Scallop dredging Muddy sand South Australia 600 12 2 420 Thrush et al (1995) x (x) Scallop dredging Sand New Zealand 20 24 1 90 Thrush et al (1995) x (x) Scallop dredging Sand Ne...…”

Intertidal clam harvesting: benthic community change and recovery

“…Physical disturbance of intertidal sediments by invasive commercial bivalve harvesting activities is of concern to fisheries managers because of direct effects on populations of target species, causing non-catch mortality, and to nature conservationists because of the interference with the feeding behaviour of wading birds (Shepherd & Clark 1994), habitat degradation or the alteration of infaunal invertebrate community structure. The environmental effects of harvesting natural populations of intertidal and shallow sublittoral bivalves have received considerable attention because of the scale and intensity of operation, particularly with respect to tractor and suction dredging intertidally for cockles (Franklin & Pickett 1978;Moore 1991;Allen 1995;Rees 1996;Cotter et al 1997;Hall & Harding 1997). In contrast, Manila clam cultivation is a relatively new and emerging shellfish aquaculture sector in Europe and North America, and its associated environmental effects have been little studied to date.…”

“…However, in other systems, where bivalves are grown in direct contact with the substratum, mechanical harvesting is required which unavoidably leads to physical disturbance of the sediment habitat. The environmental consequences for the target species and the associated benthic infaunal community of suction/mechanical harvesting in intertidal areas have been studied for cockles, Cerastoderma edule (L.), and clams, Tapes philippinarum Adams & Reeve, (Moore 1991;Allen 1995;Cotter, Walker, Coates, Cook & Dare 1997;Hall & Harding 1997). In most cases, the immediate effects of harvesting were dramatic, reducing the number of species present and their abundance.…”