Bottlenose dolphin (Tursiops truncatus) aggregations at Cocos Island National Park, Puntarenas, Costa Rica, between 2017-2022

The bottlenose dolphin (Tursiops truncatus) is a common cetacean along the eastern Pacific coast, spanning from Canada to Chile. While widely distributed, the species exhibits region-specific adaptations influenced by environmental and biological factors. In Costa Rica, bottlenose dolphins form a neritic population in the coastal waters of Golfo Dulce and Golfo de Nicoya, whereas Cocos Island hosts a pelagic population that has developed distinct morphological traits due to their diet. Although Cocos Island is geographically remote, threats such as bycatch still affect cetacean populations. While the feeding behavior of these dolphins is relatively well-documented, there is little information on their distribution patterns and social structures. This study aims to enhance conservation efforts in international waters of the Eastern Tropical Pacific, by investigating the spatiotemporal distribution of bottlenose dolphin aggregations at Cocos Island. Monitoring was conducted from 2017 to 2022 using the Protocol for the Ecological Monitoring (PRONAMEC) of Aggregations of Aquatic Mammals, developed by Costa Rica’s National System of Conservation Areas. The study area, Cocos Island National Park, was divided into four quadrants, each surveyed over a four-day monitoring effort. A total of 36 field trips were carried out: 20 during the rainy season and 16 in the dry season. For each dolphin sighting, observers recorded the group composition (adults, juveniles, pups), geographical location, date, and time. Statistical analysis was performed to identify significant differences in sightings per field trip across seasons, quadrants, and years, as well as variations in group size and composition. Results indicated a slight decrease in the number of sightings per field trip over the years but an increase in individuals per sighting. Although trends were observed, no significant differences were found between the first and last years of monitoring or between seasons. Aggregations generally comprised three to five individuals, with the northeastern quadrant showing the highest number of sightings per field trip, followed by the southwestern. A significant difference was found only between the northeastern and northwestern quadrants during the rainy season. Groups with only adults were the most frequently observed by statistical differences. The marked increase in individuals per sighting in 2020 and 2021 may be related to the suspension of fishing activities and reduced tourism during the COVID-19 pandemic. The consistent presence of juveniles and pups across all quadrants and seasons suggests that Cocos Island serves as a critical habitat for feeding, reproduction, and juvenile development. However, this pattern was not observed in 2022, when juveniles and pups were notably absent. The distribution pattern, especially in the northeastern and southwestern quadrants, suggests that dolphins may rely on nearby seamounts as feeding grounds, given their high productivity. Continued monitoring is recommended to corroborate these trends over time and to support conservation efforts in this vital marine area.