Posidonia oceanica wrack beds as a fish habitat in the surf zone

Abstract

Drifting macrophyte detritus cast along the shore, often called wrack, can wash around, accumulate in the surf zone and constitute a habitat where fishes forage and/or hide. We compared fish assemblages associated with Posidonia oceanica wrack beds with those associated with bare sand in the surf zone of Sardinia Island (Italy), NW Mediterranean Sea. In July–August 2020, in situ non-destructive visual censuses were performed to count fishes and record their behaviour at 3 locations, each including 3 shores characterized by both P. oceanica wrack beds and bare sand. The assemblages were mainly composed of small and juvenile/sub-adult individuals and significantly differed between the two habitats. Nine taxa were shared by both habitat types, seven were exclusively associated with P. oceanica wrack and only one species was found exclusively on sand. Higher species richness and fish density (without considering the contribution of gregarious fish in the water column) were observed on P. oceanica wrack than on sand. Assemblages were numerically dominated by Diplodus sargus, D. vulgaris and Mullus surmuletus. These species and other fishes, mostly represented by invertivorous (e.g., labrids) and omnivorous species, were often observed feeding on P. oceanica wrack and using the resuspended dark-brown vegetal fragments to hide. These findings clearly suggest that P. oceanica wrack beds attract fishes from surrounding habitats, some of them of commercial importance, providing food and habitat for juvenile and sub-adult stages.