Seamount aggregation effects for threatened species at Cocos Island National Park: Insights from a Long-Term Monitoring Framework in the Eastern Tropical Pacific

Seamounts serve as crucial aggregation sites for pelagic species in remote environments, which are often vulnerable to unregulated fishing. However, the high costs and logistical challenges of reaching these remote locations usually limit long-term survey efforts. The biological importance of seamounts along the Cocos Ridge, between the Galapagos and Cocos Islands, for threatened migratory species has been previously acknowledged, though past studies have been limited to short-term, spatially restricted surveys. From 2021 to 2024, we expanded biological and oceanographic surveys at the West Cocos and Gemelas seamounts, the shallowest seamounts within the Cocos Island National Park, using baited remote underwater video stations (BRUVS) and a CTD instrument to measure environmental variables along the water column to a depth of 100 meters. A total of 276 BRUVS were deployed in two distinct zones around each seamount: at the seamount’s peak and 10 nautical miles away, under the hypothesis that pelagic species abundance and diversity would be significantly higher closer to the seamounts. Our analysis revealed significant differences between survey sites, with higher abundance and diversity of pelagic species observed near the seamounts. The hammerhead shark showed the strongest association with the seamounts, although its abundance was negatively correlated with higher water temperatures, deeper thermoclines, and lower oxygen levels in the water column. Both seamounts were identified as biologically important areas, not only for hammerhead sharks but also for thresher sharks and silky sharks—all endangered species that are highly exposed to fishing pressure in the Eastern Tropical Pacific (ETP). Our findings provide a crucial baseline for evaluating the effectiveness of the 2021 expansion of Cocos Island National Park and demonstrate that the aggregation effect of seamounts for large, threatened pelagic species is concentrated within small spatial scales around each seamount. Based on this study, a regional initiative to monitor seamounts has been promoted through the MigraMar network of scientists, paving the way to expand this effort across the ETP in the coming years.