Project: Movements and behavior of white sharks near a seal colony at Guadalupe Island, Mexico

Fig. 1. M/V Ocean Odyssey

Objective:

Our overall objective is to describe the movements and behavior of the white shark (Carcharodon carcharias) near a seal colony in Guadalupe Island.

Methods:

Individual sharks will be tagged with acoustic transmitters so that they may be detected and tracked. We will be using both simple beacons, which transmit pulses at a fixed rate and are used to identify the presence of an animal, and telemetry transmitters, which can be equipped with sensors. We are going to use transmitters with combinations of two of three different sensors: depth, speed and temperature. The pings produced by these transmitters are detected, amplified and translated to an audible form for the human ear and the decoding system (computer) with the use of a receiver and a directional hydrophone. Sharks are tagged from cages and smaller boats deployed from both the M/V Ocean Odyssey (Fig. 1) and M/V Islander by Absolute Adventures - Shark Diver who have provided continuing logistical support for this project.

Fig. 2.White shark observed near Guadalupe Island

Progress:

The last year we placed an internal transmitter with depth and temperature sensors on a 4 m white shark female called “Scarboard” (Fig. 2). The shark was tagged < 100 m in front of “Campo Norte” at the northeast of the Island. The adjacent coastline is home to a large colony of northern elephant seals (Mirounga angustirostris). The attachment method consisted of inducing the shark to swallow a piece of bait in which a transmitter with a treble hook was hidden (Fig. 3). The internal transmitter location would let us know if the shark was feeding in this area (we anticipated an elevation of stomach temperature when the shark swallows a warm bodied seal).

Fig. 3. Inserting ultrasonic tag into bait fish

During the following days, Scarboard remained in the area, swimming within a 2 km radius of the beach. The shark was detected near the elephant seal beach during October 28 and November 4, 8, 9 and 16 swimming at an average depth of 19, 43, 6, 18 and 29 meters respectively. We found that the shark tend to move in a yo-yo pattern (up and down) reaching a maximum depth of 147 meters (Fig. 4).

On November 9 scarboard was found in front of Guadalupe fur seal (Arctocephalus townsendi) beach south of Campo Norte, where it spent more than one hour patrolling the zone near the beach. Although the shark was found a few meters in front of a seal colony we saw no indication of feeding.

On November 20, Scarboard was seen by the tourist boats feeding on bait during the morning and afternoon, but we did not detect any signal even when we were close to the boats. The sharks tend to evert their stomachs in order to get rid of indigestible parts such as bones and feathers so the tag may have been lost during this process. The tag was inside from October 28 until at least November 16. Maybe this is the period of time required for the digestion of the bait fish in which the tag was hidden, after which the shark spit out the hard parts including the transmitter.

Fig. 4. Telemetry data from transmitter inside "Scarboard"

The data show that the internal temperature of the shark was warm (between 25 and 26 ºC), confirming previous studies showing that white sharks use well developed retia miriabilia in the circulatory system as heat exchangers. The highest temperature recorded for the stomach was 28.9 ºC on November 16. This value was almost 3 degrees higher than that recorded on other days. This may have been a predatory event, though we cannot prove that without direct observation. Perhaps like the shortfin mako's (Isurus oxyrinchus), parts of the body are regionally warmer than others, most notably the intestinal valve and stomach, the lateral swimming muscles, and the brain.

Results to date:

Our principal findings so far are: 1) the shark spent a great amount of time in front of the elephant beach; 2) the shark swam in a yo-yo vertical pattern 3) white sharks may evert their stomach approximately 20 days after eating, 4) the internal temperature of the stomach is about 7 ºC higher than the surrounding water remaining between 25 and 26 ºC.

We will continue to manually track sharks in the coming year. Additionally, we plan to deploy an array of sonobuoys (Vemco Radio Acoustic Positioning System) in front of the elephant seal beach. This system will permit us to monitor up to 10 tags at the same time by triangulation (Fig. 5).