FREUND, E.V.*; DEWAR, H.; CROLL, D.A.: Locomotor Tracking of the Spine-tailed Devil Ray, Mobula japanica

Three-dimensional movements and foraging ecology of a small manta ray species, the spine-tailed devil ray, Mobula japanica, were monitored using acoustic tracking (n = 2) and archival tagging techniques (n = 4) in the southern Sea of Cortez. The rays ranged in size from 65 – 112 cm disc length (mean = 87.6 cm, n = 6) with track duration ranging from 21 to 28 hours. Simultaneous biophysical oceanographic profiling allowed examination of temperature profiles and prey distribution in relation to ray movements. The primary prey of these rays is krill, Nyctiphanes simplex, which remains at depth (~150 m) in dense clouds during the day, then moves to the surface and disperses at night. The rays spent most of their time above the thermocline and appeared to forage at night when the krill swarms approach the surface. When krill were present the animals displayed a step-wise ascent pattern which may indicate foraging through clouds of krill. Maximum dive depth during one track reached 445 m. Consistent throughout the tracks was a dive and ascent pattern suggestive of burst and glide swimming. Tracking speed was typically 1 knot with bursts up to 4.5 knots. The rays traveled up to 30 miles in 24 hours and did not always remain in areas where krill density was high. Information on movements and habitat use is critical for understanding the impact of fisheries on manta ray populations. This impact is potentially high because of the mantas’ low fecundity rates (~1 pup per year). In addition to improving our understanding of their natural history, we hope this research will provide valuable information when management and conservation strategies are considered. Supported by National Geographic.