Effects of mechanical cockle harvesting on intertidal communities.
A tractor-towed cockle harvester was used to extract cockles from intertidal plots of muddy sand and clean sand in order to investigate the effects on other benthic invertebrates and their predators. Harvesting resulted in the loss of a significant proportion of the most common invertebrates from both areas, ranging in the muddy sand from 31% of Scoloplos armiger (initial density 120 m-2) to 83% of Pygospio elegans (initial density 1850 m-2). Significant effects could not be detected in most populations with a density of <100 m-2. Populations of Pygospio elegans and Hydrobia ulvae remained significantly depleted in the area of muddy sand for >100 days after harvesting, and Nephtys hombergi, S. armiger and Bathyporeia pilosa for >50 days. Invertebrate populations in clean sand with relatively few cockles Cerastoderma edule recovered more quickly than those in muddy sand with a more structured community, which included several tube-dwelling species such as Pygospio elegans and Lanice conchilega. Bird feeding activity increased at first on the harvested areas, with gulls and waders taking advantage of invertebrates made available by harvesting. Subsequently, in the area of muddy sand, the level of bird activity declined compared with control areas. It remained significantly reduced in curlews Numenius arquata and gulls for >80 days after harvesting and in oystercatchers Haematopus ostralegus for >50 days. It is concluded from this study that tractor dredging for cockles in high density areas causes a sufficiently large mortality of non-target invertebrates that harvesters should be excluded from areas of conservation importance for intertidal communities such as invertebrates, fish and birds.