Spatio-temporal variability of ichthyophagous bird assemblage around western Mediterranean open-sea cage fish farms

Ichthyophagous birds aggregate at cage fish farms attracted by caged and associated wild fish. Spatio-temporal variability of such birds was studied for a year through seasonal visual counts at eight farms in the western Mediterranean. Correlation with farm and location descriptors was assessed. Considerable spatio-temporal variability in fish-eating bird density and assemblage structure was observed among farms and seasons. Bird density increased from autumn to winter, with the great cormorant being the most abundant species, also accounting largely for differences among farms. Grey heron and little egret were also numerous at certain farms during the coldest seasons. Cattle egret was only observed at one farm. No shags were observed during winter. During spring and summer, bird density decreased markedly and only shags and little egrets were observed at only a few farms. Season and distance from farms to bird breeding/wintering grounds helped to explain some of the spatio-temporal variability. •Spatio-temporal variability of ichthyophagous birds was assessed at eight Mediterranean cage fish farms.•Great cormorant, shag, grey heron, little egret and cattle egret density at farms was studied.•Bird density showed a marked seasonal pattern: highest density occurred in autumn and winter.•Differences between farms were related to the proximity of natural breeding/wintering areas.