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The Influence of Barnacle Cover on the Numbers, Growth and Behaviour of Patella Vulgata on a Vertical Pier

The effects of experimentally manipulated barnacle cover on the population size, mortality and growth of Patela vulgata L. were investigated on a vertical, barnaclecovered pier at Port Erin in the Isle of Man. One area was left untouched as a control, half of the barnacles on a second were scraped off in a chequered pattern, and on the third the barnacle cover was reduced to less than 2 °0. Subsequent growth rates were inversely proportional to barnacle cover. Mortality was highest on the control. Recruitment of limpets was markedly enhanced by the mosaic of 50 % barnacle cover. Behaviour on this vertical face differed from that on nearby more gently sloping shores, with foraging during emersion being more prevalent: however, barnacle removal reduced this foraging when the tide was out. The foraging of Patella vulgata whilst emersed was similarly shown to be more prevalent on vertical than horizontal surfaces at other localities in Britain. Emersed foraging on the pier was dependent on high humidity. It is suggested that on vertical surfaces, particularly if barnacle-covered, P. vulgata are more dependent on foraging whilst emersed because of the greater risk of dislodgement by waves when feeding whilst the tide is in.