Characterisation of rhodolith beds in circalittoral waters off the southeastern coast of the Maltese islands

Abstract

ROV surveys were carried out at 56 stations off the southeastern coast of Malta as part of the LIFE BaĦAR for N2K project, primarily at depths of 50-100m, with a view to characterise the rhodolith bed present. Mean percentage cover of live rhodoliths ranged between 0% and 73.6%. High density-patches were found close to the coast and towards the northeast of the study area close to the Fisheries Management Zone boundary, with patches where rhodoliths were absent in between. Conversely, most of the dives in the south, southeast and northwest of the area had low rhodolith densities (<10%), or none at all. The majority of rhodoliths recorded (96.8%) were considered to be alive, with those having finger-like branches being the commonest morphotype in the overall area. Rhodoliths were mostly present at depths of 60-95m, with peak live rhodolith densities recorded at 78.5m. The shallowest records, with relatively high densities of rhodoliths, were at a depth of 53.7m, whilst the deepest record was at 106.9m albeit at very low densities. In terms of associated biota, a total of 169 different species and 18911 individuals of megabiota were identified throughout the entire ROV dives; Actinopterygii and Echinoderms were the most abundant taxonomic groups. Stylocidaris affinis, Centrostephanus longispinus, and Antedon mediterranea were the species found to be associated with rhodoliths the most. In addition, Flabellia petiolata was observed to grow profusely in areas where rhodoliths were present. No direct correlation between rhodolith density and species richness or abundance was observed. With respect to anthropogenic impacts, a total of 1047 anchored vessels were recorded in the area over a one year period, with the majority anchored where rhodoliths were present. An aquaculture zone was also overlapping with a dense patch of rhodoliths. The majority of the rhodoliths were recorded within one large active trawling zone, and trawl furrows were observed in areas with rhodoliths. Since the present study indicates that large parts of the rhodolith bed falls under the definition of ‘maerl’ as defined by Basso et al. (2016), and trawling on this habitat is illegal, locations of the trawl zones need to be revised. Besides its ecological interest, the present characterisiation of the rhodolith accumulations and maerl beds located off the southeastern coast of Malta will also serve to inform better management of this habitat.