Dynamics of the bottlenose dolphin, Tursiops truncatus, off the west and south coast of Puerto Rico: population, distribution, threats, and genetic structure

Abstract

To understand the dynamics of a species, information on the status, abundance, and relationship of contiguous populations is essential, along with information on its geographic range, habitat analysis, and needs. Although the bottlenose dolphin (Tursiops truncatus) is commonly found around Puerto Rico, no assessments have been done to determine the population’s size, extent, and distribution, and this presents management challenges as defined under the Marine Mammal Protection Act of 1972. To address these needs, a multidisciplinary approach was utilized to develop a comprehensive picture of the bottlenose dolphin population in Puerto Rico. This approach included a boat-based survey for distribution and abundance estimates and a skin biopsy sampling of free ranging individuals to determine ecotype composition. The study areas were the south and west coasts of Puerto Rico between the years 2002 and 2015. The distribution was modeled to determine the spatio-temporal distribution patterns of sighting location, group size, composition and behavior in terms of key features such as distance from shore, depth and habitat type. It was found that dolphins had a more prevalent nearshore distribution within 5 km of the coast, but they are also uniquely associated with edge and slope habitats of the platform and bank/islands off the west coast. Dolphin’s sightings were rare on the central portion of the insular shelf and on deep waters outside the shelf edge. From 2013 to 2015, mark-recapture methods of photographic surveys yielded a current abundance estimate of 127 dolphins within the study area. Using a catalog of 2,270 photographs, scarring patterns were used to determine factors affecting the population. Among all cataloged dolphins, 15% showed scar patterns attributed to natural causes, 28% had scars attributed to ii anthropogenic causes, and 9% had scars of unidentified origin. Over time, there were significant increases in the proportion of scars caused by anthropogenic interactions and the proportion of individuals having two or three such scars. The mitochondrial control region was sequenced from 27 live dolphins in the study area, plus from 11 stranded dolphins from around Puerto Rico and five stranded dolphins from Guadeloupe; the and results were combined with sequences available from the Atlantic and the Pacific Ocean. Although only one ecotype was observed morphologically, the genetic data showed the presence of two distinct genetic lineages in Puerto Rico. Given the largely coastal distribution of dolphins and the demonstration of increasing anthropogenic impacts, management should prioritize further studies on (1) population health, (2) mapping potential hotspots of human dolphin interaction, and (3) the movements of individuals that may affect population size and interactions between the two genetic lineages.