Hotspots: Density and species richness for cetaceans in the oceanic eastern tropical Pacific

Background/Question/Methods Biodiversity hotspots have received much attention but relatively little is known about hotspots in oceanic systems (waters seaward of the continental shelf). We describe density and species richness hotspots for cetaceans in the eastern tropical Pacific, a geographic area of approximately 21 million km² encompassing the ocean from the US-Mexico border to central Peru and seaward to Hawaii. Data were collected aboard two NOAA research vessels during August – November (ca. 240 sea days) of 10 years spanning a 21-year period (1986 – 2006). Line-transect methods and 20-power, mounted binoculars were used to detect, identify, and count all cetaceans sighted. A helicopter or fixed-wing aircraft working in concert with the ships photographed a subset of cetacean schools each year in order to calibrate group size estimates made by ship-based observers. Twenty-eight species of cetaceans were recorded. These included 15 species of dolphins, 5 species of beaked whales, 3 species of sperm whales, and 5 species of baleen whales.

Results/Conclusions Richness hotspots were clearly evident in three distinct regions: the Equatorial Front, the Costa Rica Dome, and waters to the west and east of the Baja California, Mexico peninsula. Although these hotspots encompassed areas of highest density for a few species, the correlation between richness and density for any given species was generally low, ranging from 0.03 to 0.44 with a mean of only 0.25 across all 28 species. Of particular interest, the proportion of 1x1-degree cells where a particular species was present, and encompassed by a hotspot tended to be low, ranging from 4 to 56% with a mean of only 20% when all 28 species were considered together. Richness hotspots were clearly coincident with oceanographic variability; specifically, richness hotspots tended to be located along oceanographic edges, and consequently, along the edges of distribution for many species. This result contrasts sharply with results from terrestrial systems, where richness hotspots typically represent centers of endemism. Endemism is rare for cetaceans in general, and for oceanic cetaceans in particular. In our study, richness hotspots did not describe, or even include the most important areas for many species. Instead, richness hotspots encompassed species-specific distribution edges, and therefore, potentially marginal habitat for many species. Conservation efforts focused on the cetacean richness hotspots described here may therefore not facilitate their conservation. These results have implications for ecosystem-based management and development of marine protected areas in open ocean systems for large-bodied apex predators.