Living at the top. Connectivity limitations and summit depth drive fish diversity patterns in an isolated seamount

Abstract

The fish assemblages of the Galicia Bank and the closest continental slope (northwest of Spain) were analysed using otter trawls to improve our understanding of how environmental drivers structure seamount fish communities in the deep sea. The effect of environmental drivers on these assemblages was studied using multivariate techniques together with the variation in α and β diversity across assemblages. Fish fauna in the study area was distributed in 5 different assemblages generated by the action of 3 main drivers: depth, distance to the coast and presence of cold-water corals. The observed differences in species composition among assemblages were mostly explained by species turnover across a depth gradient. The seamount summit and the continental slope showed important differences despite sharing similar depths, mainly because several species requiring shallow juvenile habitats were absent from the summit. These absences were observed in both summit assemblages inside and outside the cold-water coral reef. Our results show that in isolated seamounts with relatively deep summits, the lack of connectivity with shallower areas limits the presence of certain species, probably due to the impossibility for these species to migrate directly from shallow to deeper seabed areas. These species are replaced by species with preferences for deeper habitats, providing the fish assemblages located at the top of the summit with a deeper profile than observed in fish assemblages of the continental slope.