oa Association of adult female whale sharks with open ocean and coastal upwelling frontal systems in the Eastern Tropical Pacific

Background Although whale sharks are a mostly solitary pelagic species, most of our knowledge of their ecology is derived from around a dozen coastal feeding sites around the world where mainly immature males aggregate. Darwin Island in the Galapagos Marine Reserve is one of a few oceanic sites around the world where mostly large female whale sharks may be observed on a predictable basis. Since 2011, the Galapagos Whale Shark Project has tracked their movements from Darwin to the Equatorial Front, which is located to the north of the Galapagos Islands, extending zonally over several thousand kilometers, and is most pronounced from July through November. Towards the end of the year, the whale sharks move to the eastern boundary upwelling zone off the shelf break of Ecuador and northern Peru. Approach We placed SPOT-5 tags on 24 whale sharks in 2011 and 16 in 2012 at Darwin Island (Galapagos Marine Reserve) and tracked their seasonal movements west along the Equatorial Front then east to the eastern boundary upwelling off Ecuador and northern Peru. Using satellite data, we extracted sea surface temperature (SST) cross sections for each shark position, meridional for equatorial positions and zonal for eastern boundary positions, to emphasize habitat occupation in terms of thermal gradients relative to the sharks. We analyzed a meridional ship hydrographic section to examine habitat occupation for one shark at the EF in relation to recorded depth preferences. Results The whale sharks occupied a geographical range of 4060 km from east to west, and 2027 km from north to south. Preliminary results show that within this range, the seasonal presence of adult female whale sharks at the Equatorial Front and the eastern boundary upwelling occurs when the frontal zones are fully developed. Whale sharks were consistently located along the warm side of the Equatorial Front, occupying a narrow thermal band on average: 25–25.5 C. The depth profile of one shark in relation to in-situ vertical profiles also supports this. Similarly, they occupied the warm side of the primary coastal upwelling front, on average 21.7 C. Conclusions Although encounters with whale sharks along the coastal waters of Ecuador and Peru suggest that a larger population comprising adults and juveniles of both genders occupies the continental shelf year-round, the seasonal movements through Darwin and the Equatorial Front are mainly carried out by large, adult females. Many of these sharks display distended bellies, suggestive of pregnancy. The Equatorial Front is an important feeding ground for both planktivorous and piscivorous seabirds, while other marine filter feeders such as manta rays and southern ocean sunfish have also been tracked along the Front. We propose that Darwin is a navigational waypoint to this offshore feeding ground, which may also function as an open water pupping area.