Behavioural response of Mullus surmuletus to habitat modification by the invasive macroalga Caulerpa taxifolia

Densities of red mullet Mullus surmuletus have declined since 1996 at Cap Martin, Alpes-Maritimes, France, following colonization by Caulerpa taxifolia. Foraging M. surmuletus were rarely observed over C. taxfolia or the seagrass Posidonia oceanica, but were observed over bare sand and endemic macroalgae. Within colonized sites (where the mean cover of C. taxifolia was 30-100%), fish were concentrated on areas with low cover of C. taxifolia (<21%). A difference in the frequency of observations of foraging groups (17%) was observed between sites with and without C. taxifolia. The observed patterns were probably due to a physical barrier to foraging that is presented by meadows of C. taxifolia. Changes to habitat structure that occur when the substratum is colonized by C. taxifolia influence the accessibility to benthic food resources, and consequently the foraging activities of fish that feed on benthic invertebrates. The decline in density of M. surmuletus at Cap Martin could be the result of fish emigration from colonized sites (unfavourable to the species) to uncolonized sites. (C) 2004 The Fisheries Society of the British Isles.