Effects of cockle abundance and cockle fishery on bivalve recruitment

Understanding possible effects of cockle dredging on bivalve dynamics should include studies of the influence of cockle abundance on bivalve recruitment and of effects of fishery on survival of recruits present at the time of fishing and on future recruitments. Numerical densities of adult cockles and of recruits of Cerastoderma edule, Limecola balthica and Mya arenaria were monitored by twice annual sampling for 21 years (1973-1993) along 7 transects in the fished part of Balgzand, a 50-km2 tidal flat area in the westernmost part of the Wadden Sea. Between-year variability in recruitment success in all 3 species was negatively correlated with adult-cockle abundance. Recruit densities as estimated in August were relatively low in years of cockle fishing in all of these species. This was so already well before fishing started in September. So these low densities were not an effect of fishery, but of the high cockle abundance that made cockle dredging profitable. The proportions of recruit numbers estimated in August (just before cockle dredging) that were still present half a year after fishing (in March) were not different between fishing and non-fishing years nor between fished and unfished areas in fishing years. Densities of new recruits in years following cockle-dredging somewhere in the area were not different from densities in years not preceded by any fishery nor were they lower at the fished sites in the year after fishing than in other years. The lack of significant influences of fishery in the studied area and period was observed in all 3 bivalve species. The above results were obtained in areas with relatively muddy sediments. Earlier reported negative effects of cockle-dredging on bivalve recruitment may be limited to the minority of tidal flats characterized by relatively coarse sediments with <1% of silt.