Improving management of overlapping bottlenose dolphin ecotypes through spatial analysis and genetics

In the Northwest Atlantic the distribution of coastal bottlenose dolphins (Tursiops truncatus) overlaps with that of the offshore ecotype. We hypothesized that the distribution of the two ecotypes could be delineated by depth and/or distance from shore, facilitating their identification during surveys. We obtained 304 skin biopsy samples and identified each as either coastal or offshore using analysis of mitochondrial DNA. We then interpreted the spatial distribution of coastal and offshore forms using spatial analysis. Using a Classification and Regression Tree (CART) analysis, we found a statistically significant break in ecotype distribution at 34 km from shore. In waters beyond 34 km from shore and deeper than 34 m, all bottlenose dolphins were of the offshore ecotype. Within 7.5 km of shore, all 65 samples were of the coastal ecotype. Between these two areas only nine samples were collected, so the genetic composition of bottlenose dolphins in this area remains poorly known. To enhance our understanding of the spatial distribution of the two ecotypes, future research should obtain more biopsy samples in this zone. Nevertheless, our results indicate that a conservative abundance estimate for the coastal ecotype could be generated from surveys of bottlenose dolphins within 7.5 km of shore.