Shark Week! Tagging – a tale of Sharks, Fish, and Seals

Everybody is interested in Great White sharks, and scientists are no exception. One advantage of being credentialed is the opportunity to look a lot deeper into the subject using tools and equipment not readily available to the public. Satellite tagging for example, which about ten years ago revealed that Great White sharks can travel thousands of miles across the ocean.

Satellite tags can record information such as depth, temperature, light, and chemical conditions of the water over time – revealing a lot about where the tag has been. After some prescribed period they release from the subject, float to the surface, and upload the data to a satellite giving researchers information stored on the tag.

When researchers want to examine the finer points of a tagged animal’s movements they can use acoustical tags; ultrasonic coded transmitters (UCTs) which can be tracked in closer ranges by “listening stations” located within the study area. These commonly broadcast identification codes over a modulated 69kHz tone. They run on batteries and chirp every few seconds.

Of course one liability of a chirping tag in the ocean is that the sound can be heard by some marine animals. And while the a tagged fish is unlikely to hear its own tag due to limited high-frequency hearing, at 69kHz seals, sea lions, porpoises, and dolphins can hear it quite well. So some researchers have found that fish tagged in their studies end up “ringing the dinner bell” to these piscivores and the tag ends up logging the movements of a fish dinner, not of a living fish.

Tagged whale shark, photo by Phillip Colla

Tagged whale shark, photo by Phillip Colla

On the flip side of this; a tagged Great White shark will be constantly announcing its whereabouts to some of its favorite prey (the same piscivorian pinnepeds and odontocetes) potentially interfering with its feeding habits.

This may all read to be favorable to the seals, sea lions, porpoises, and dolphins that can hear the tags: In some settings they can easily hear their food, in other settings they can avoid predation. This would be the case unless researchers are tagging sharks and fish in the same area, in which case the piscivores will need to be very selective about the chirps they chase.

This oversight has just begun to come to the attention of a few researchers. The shark conundrum was brought to our attention by Kim Allen of Intellipulse. The seal issue was brought up at a recent Acoustics Society conference by Kane Cunningham (et.al) at the UC Santa Cruz Pinneped Lab.

Having an unaccounted feeding variable in animal tracking studies is not particularly useful to good science – and disruptive to the animals and their habitat.