HIGHLY DIRECTIONAL SWIMMING BY SCALLOPED HAMMERHEAD SHARKS, SPHYRNA-LEWINI, AND SUBSURFACE IRRADIANCE, TEMPERATURE, BATHYMETRY, AND GEOMAGNETIC-FIELD

The homing behavior of scalloped hammerhead sharks (Sphyrna lewini) to and fro between Espiritu Santo Seamount and Las Animas Island and the surrounding pelagic environment was studied to reveal their mechanism s of navigation in the oceanic environment. Four sharks were tagged wi th ultrasonic transmitters and tracked at the former location and one shark at the latter site during July, August, or September between 198 1 and 1988. Hammerhead swimming movements were highly oriented: the me an coefficient of concentration (r) for sets of ten consecutive swimmi ng directions recorded during eight homing movements by three hammerhe ad sharks ranged from 0.885 to 0.996. Drift within a current could not explain this directionality, since highly variable directions were re corded from a transmitter floating at the sea surface after becoming d etached from a shark. Forward swimming momentum was an unlikely explan ation, since highly directional swimming was maintained for a period o f 32 min with only a gradual change in course. To maintain directional ity over this period, an environmental property should be necessary fo r guidance. The hammerheads swam at night, with repeated vertical excu rsions ranging from 100 to 450 m deep, out of view of either the sea s urface or the sea floor. The sharks' vertical diving movements were co mpared to distributions of spectral irradiance (relative to elasmo-bra nch scotopic and photopic visual sensitivities), temperature, and curr ent-flow directions in the water column. No relationships were evident between these properties and the sharks' oriented swimming movements. Movements of scalloped hammerhead sharks to and from a seamount were compared to topographic features in bathymetry and geomagnetic field l eading away from the seamount. Sharks swam repeatedly over fixed geogr aphic paths, and these paths occurred less often along submarine ridge s and valleys than maxima and minima in the geomagnetic field. No sign ificant difference existed between the degree of association of points from the sharks' tracks and points from track simulations and greater -than-or-equal-to 20-degrees changes in the slope of the depth record. On the other hand, significantly more points from the sharks' tracks were associated with slope changes in the magnetic intensity record th an points from track simulations. A magnetic intensity gradient of 0.0 37 nanoteslas/m (nT/m) existed at 175 m depth, where a shark swam dire ctionally, and this gradient was three times steeper than that measure d at the sea surface and exceeded that recorded at a depth of 200 m. T he hammerheads are hypothesized to find the seamount using geomagnetic topotaxis. The shark could be attracted to and move back and forth al ong ridges and valleys, features in the relief of magnetic field inten sities occurring over a geographical area.