Characterization of pelagic assemblage on islands and seamounts of the Cocos Ridge: a regional approach for the Eastern Tropical Pacific

Oceanic islands and seamounts serve as aggregation hotspots for pelagic species. Their community structure and environmental drivers remain poorly understood in regions like the Cocos Ridge in the Eastern Tropical Pacific. In this study, using drifting stereo-baited remote underwater video stations (BRUVS), we characterize the pelagic assemblages across 60 deployments at 2 islands (Cocos, Darwin) and 5 seamounts (Las Gemelas, Medina, Paramount, West Cocos, Corcovado). We recorded 30 species from 15 families (16 teleosts, 11 elasmobranchs, 2 mammals, 1 reptile). Community structure varied across the ridge, with significant differences emerging between habitat types. At the habitat level, island assemblages were driven by 7 species: the silky shark Carcharhinus falciformis, yellowfin tuna Thunnus albacares, rainbow runner Elagatis bipinnulata, Galapagos shark Carcharhinus galapagensis, green turtle Chelonia mydas, wahoo Acanthocybium solandri, and bluestriped chub Sectator ocyurus. In contrast, seamount assemblages were characterized by the scalloped hammerhead Sphyrna lewini and the pelagic thresher Alopias pelagicus. Community variation aligned with environmental gradients, including current velocity and bottom depth, with additional influence of dissolved oxygen, thermocline depth, and fluorescence. Species-level responses tracked these gradients: S. lewini aligned with dissolved oxygen, T. albacares with bottom depth, common bottlenose dolphin Tursiops truncatus with current velocity, and C. falciformis with both bottom depth and thermocline depth. These findings demonstrate habitat-specific pelagic assemblages along the Cocos Ridge, driven by oceanographic and topographic factors. They underscore the complementary ecological roles of islands and seamounts in supporting threatened migratory species, providing critical evidence for ridge-scale spatial management and conservation planning in the Eastern Tropical Pacific.