//Hot Topic – Tracking Great White Sharks

Hot Topic – Tracking Great White Sharks

Acoustic Tracking Technologies Shed Light on the Feeding Behavior of Great White Sharks

(left) Specially modified REMUS (remote environmental monitoring unit) autonomous underwater vehicle (AUV) used to locate, follow, and record the behavior, habitat use, and feeding ecology of great white sharks off the coast of Mexico. Note the bite marks, scrapes, and scars on the right end (stern) of the AUV, damage caused by shark attacks on the vehicle. Image credit: Amy Kukulya, Woods Hole Oceanographic Institution. (right) Camera view of a great white shark biting the AUV while in the water column. Image credit: Oceanographic Systems Lab/Woods Hole Oceanographic Institution.

Acoustic fish tags are used to monitor and track fish movements. Small tags are surgically implanted, injected, fed (via baited fish), or externally attached to a fish. The tags transmit a high frequency signal, and acoustic receivers are strategically placed around the area of interest to listen for these sounds. The receivers can be placed on anchored buoys, cabled underwater arrays, or existing oil and gas platforms, as well as integrated into ocean gliders, or even carried by other marine animals. As acoustic signals from tags are detected and recorded by the receiver array, travel time can be calculated, and a fish’s location pinpointed. Tagged individuals can be tracked for periods ranging from days to years.

A common application of acoustic telemetry is to investigate animal behavior and survival. Large pelagic fishes, such as sharks, are difficult to directly observe, as much of their behavior occurs under water. Acoustic tags have been used to track and monitor the behavior of several shark species, including tiger sharks, blue sharks, basking sharks, and leopard sharks. A recent publication describes research that used acoustic tracking technologies and a specially modified REMUS (remote environmental monitoring units) autonomous underwater vehicle (AUV) to locate, follow, and record the behavior, habitat use, and feeding ecology of great white sharks (Carcharodon carcharias) near Guadalupe Island off the coast of Mexico. The AUV was equipped with a variety of ocean sensors, an acoustic navigation system, and 6 high-definition underwater cameras. Sharks were tagged with a 25 kHz acoustic transponder. (Two-way sound speed calculations are computed on board the AUV’s computers and an exact position of the tagged animal is determined.) After the animal was tagged (note: no animal was ever captured), the REMUS vehicle could orientate itself underwater in the direction of the shark, ping the transponder, listen for a reply, and then localize the shark’s position via data received. Once the vehicle localized the shark’s position, it estimated the animal’s position, course, depth and predicted speed. The AUV was designed to send all this data back to the ship, allowing operators to monitor the positions of both the shark and AUV in real time. This new tracking method is the first of its kind to be able to track a tagged animal in three-dimensional underwater space in order to record the environment in which it lives as well as record video of its behaviors and interactions.

Four sharks were tracked during six AUV missions, which yielded over 13 hours of high-resolution tracking data and underwater video footage. Tracking data revealed that the four sharks spent 80 percent of their (tracked) time at depths greater than 91 m (300 ft) and only 5 percent in water shallower than 24 m (80 ft). However, the AUV robot used in this study was only rated to 100 m (328 ft) and could not follow tagged animals below that depth. The sharks dived as deep as 161.5 m (530 ft), which left the AUV traveling overhead. Unexpectedly, the AUV’s video cameras captured 10 different sharks approaching, bumping, and/or biting the AUV from below. These were the first observations of great white shark subsurface predatory behavior ever recorded. Previous acoustic tracking data suggest white sharks take advantage of reduced visibility at depth, ambushing potential prey from below. Although it is important to know where sharks and large pelagic creatures spend their lives underwater, it is essential for conservation and management officials to also know what the animals are actually doing when underwater.

Great white sharks are also being actively tracked in the waters off California through the use of acoustic technologies. Dr. Barbara Block and other scientists involved in the Tagging of Pacific Predators (TOPP) Project, have placed Iridium enabled buoys and wave gliders (autonomous vehicles that use wave energy to propel themselves through the water while also using solar panels to provide power for onboard scientific equipment) in Pacific waters, to detect the pings associated with white sharks carrying acoustic tags. Each tag sends out a coded signal that is detected by the buoys and/or gliders, relayed to orbiting satellites, and then displayed online in real time. Live data feeds can also be accessed by iPad or mobile phone users. The objective of the TOPP Project is to understand the factors that influence animal behavior and build the tools required to make informed conservation and management decisions. Moreover, the animals carrying tags become mobile ocean sensors that contribute valuable ocean and climate science data.

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References

  • Christopher M. Clark, Christina Forney, Esfandiar Manii, Dylan Shinzaki, Chris Gage, Michael Farris, Christopher G. Lowe, and Mark Moline. 2013, “Tracking and following a tagged leopard shark with an Autonomous Underwater Vehicle.” Journal of Field Robotics. 30(3): 309-322.
  • Skomal, Gregory B., E. Mauricio Hoyos-Padilla, Amy L. Kukulya, and Roger P. Stokey. 2015, “Subsurface observations of white shark Carcharodon carcharias predatory behavior using autonomous underwater vehicle.” Journal of Fish Biology. 87: 1293-1312.
2017-09-07T13:49:59+00:00