Effects of mechanical cockle harvesting on intertidal communities

1. Shellfish of marketable size can be harvested much more quickly and effi ciently using mechanical methods such as tractor-powered harvesters and suc tion dredgers than by traditional methods. The adverse effects of such mach ines on non-target organisms need to be considered carefully before licensi ng such activities. 2. A tractor-towed cockle harvester was used to extract cockles from intert idal plots of muddy sand and clean sand in order to investigate the effects on other benthic invertebrates and their predators. 3. 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 (Polychaeta) (initial density 120 m(-2)) to 83% of Pygos pio elegans (Polychaeta) (initial density 1850 m(-2)). Significant effects could not be detected in most populations with a density of less than 100 m (-2). 4. Populations of Pygospio elegans and Hydrobia ulvae (Gastropoda) remained significantly depleted in the area of muddy sand for more than 100 days af ter harvesting, and Nephtys hombergi (Polychaeta), Scoloplos armiger and Ba thyporeia pilosa (Amphipoda) for more than 50 days. 5. Invertebrate populations in clean sand with relatively few cockles Ceras toderma edule (Pelecypoda) recovered more quickly than those in muddy sand with a more structured community, which included several tube-dwelling spec ies such as Pygospio elegans and Lanice conchilega (Polychaeta). 6. Bird feeding activity increased at first on the harvested areas, with gu lls and waders taking advantage of invertebrates made available by harvesti ng. Subsequently, in the area of muddy sand, the level of bird activity dec lined compared with control areas. It remained significantly reduced in cur lews Numenius arquata and gulls for more than 80 days after harvesting and in oystercatchers Haematopus ostralegus for more than 50 days. 7. It is concluded from this study that tractor dredging for cockles in hig h density areas causes a sufficiently large mortality of non-target inverte brates that harvesters should be excluded from areas of conservation import ance for intertidal communities such as invertebrates, fish and birds.