Horizontal and vertical movements of the scalloped hammerhead shark, Sphyrna lewini, in the southern Gulf Of California, Mexico

Abstract

Horizontal and vertical movements are described for four scalloped hammerhead sharks (Sphyrna lewini) tagged with ultrasonic transmitters. The trackings were carried out during August 1986, 1988-1989, and July 1989 in the pelagic environment surrounding Espíritu Santo Seamount and Las Animas Island in the southem Gulf of California. The hammerheads generally remained above the seamount and near the island during the day. Fifteen sharks were tracked during homing movements away from and back to the seamount. Of the excursions, 13 were made into the pelagic region during the night and two during the day. The maximum distance moved away from the seamount ranged from 4 to 20 km. Departure and return movements were at times close to and parallel to each other, suggesting that the sharks followed particular routes to preferred feeding sites. The sharks made vertical excursions, ascending and descending in the water column (in yo-yo form). The diving oscillations occurred between 50 to 450 m. These vertical movements were usually below the surface mixed layer and well away from the bottom. The range in water temperatures (2º-6ºC) encountered during these vertical excursions was small in relation to that reported for other species of fishes, suggesting that for this species the function of the excursions may not be thermoregulation.