Vertical niche overlap by two ocean giants with similar diets: Ocean sunfish and leatherback turtles

We used satellite tags to record the patterns of depth utilisation for four ocean sunfish (Mola mola) and two leatherback turtles (Dermochelys coriacea) moving in broadly the same area off South Africa. Individuals were tracked for between 2 and 8 months and dive data relayed via satellite. For all the sunfish and one of the turtles, we received binned data on depth distribution, while for the second turtle we received individual dive profiles along with the proportion of time spent diving. Leatherback turtles dived almost exclusively within the upper 200 m, spending only 0.6 and 0.2% of their time > 200 m. There were times when sunfish likewise occupied these relatively shallow depths. However, there were also protracted periods when sunfish spent the majority of their time much deeper, with one individual remaining around 500 m for many hours at a time. These results suggest that sunfish sometimes exploit deeply distributed prey which is beyond the foraging range of leatherback turtles. We conclude that while both species are believed to feed predominantly on gelatinous zooplankton, the fact that sunfish do not need to come to the surface to breathe means that they can occupy an expanded vertical niche compared to the leatherback turtle.