Effects of intertidal mussel cultivation on bird assemblages

MusselMytilus eduliscultivation on intertidal flats affects the invertebrate community, often adversely, and this may have detrimental consequences for shorebirds. Here we present the results of an experimental study to quantify the effects of intertidal mussel cultivation on shorebirds. A study area of 4.32 ha, comprising experimental plots and control plots, was laid out in summer 1999 on the mudflats of the Menai Strait in Wales. Regular counts throughout winter of 1999/2000 established pre-cultivation patterns of bird usage. Mussels were laid in the experimental plots in April 2000 and bird usage in these plots and the controls was monitored over the 2 subsequent winters. Although no species were lost from the experimental plots, the bird assemblage in them changed. This reflected variation in the distribution of the 5 most abundant species. However, none of these key species declined in abundance following the laying of mussels. CurlewNumenius arquataand redshankTringa totanusincreased in abundance, although, unexpectedly, oystercatchersHaematopus ostralegusdid not. At this study site, commercial mussel cultivation may have beneficial effects, not just for the birds that eat mussels, but also for other species that can take advantage of the associated changes to the benthic fauna and habitat complexity. However, features of conservation interest at other localities may mean that bottom cultivation of mussels will have detrimental rather than beneficial effects. The environmental effects of proposals to initiate or expand bottom cultivation of mussels need to be assessed on a case-by-case basis.