Do Satellite Tags 'Jam' a Shark's
Ampullae of Lorenzini?
Responding to a request for an explanation of exactly how satellite tags work and whether or not the electrical signature of these devices could interfere with a tagged White Shark's sensitive electroreceptors, the ampullae of Lorenzini, I responded as follows:
Exactly how a satellite tag works depends upon the exact model and internal programming. I'm a biologist (and a fairly low-tech one at that) not an engineer, so I don't really understand the in's and out's of the functioning of electronic gadgets (My friend Adam Summers is both a biologist and a skilled engineer, so he could probably shed further light on this matter — if he can spare the time.). In general, there are two types of satellite tags used for collecting data on movement patterns of marine animals. I will refer to these two types as 'Continuous' and 'Archival'.
Continuous satellite tags send data about a tagged animal's position, direction and rate of travel, and sometimes other environmental data such as water temperature encoded in the form of pulsed radio waves; the data are sent to a geosynchronous satellite and beamed to a pre-determined, usually land-based, receiver. There the signal is decoded and analyzed. Because they transmit continuously, or at least continually, Continuous satellite tags require substantial power cells and their energy output (and resultant electrical field) is relatively strong. Since radio waves cannot penetrate seawater, Continuous satellite tags can only broadcast while at the surface. As such, they are used primarily for air-breathing marine animals such as sea turtles and whales, which must return to the surface periodically to ventilate their lungs. However, testing a system designed to track whales, Scottish biologist I.G. 'Monty' Priede used a Continuous satellite tag to track a Basking Shark (Cetorhinus maximus) during summer months — when this species spends much of its time at or near the surface — in the Firth of Clyde, off western Scotland. You can read more about the equipment Priede used and the results he obtained in Fisheries Research, 2 (1984): 201-216.
Archival satellite tags generally work the same as Continuous tags and collect the same sorts of data, but transmit all collected data in relatively few — often only one — massive burst of pulsed radio waves. These tags typically have a magnesium strip that slowly corrodes in seawater so that the tags 'pop' off after a pre-determined period has passed, float to the surface, and then transmit their stored data en masse. For this reason, they are also called "pop-up tags". A modified Archival tag may include a pressure sensor that periodically transmits stored data but does not detach from the animal until a pre-determined duration or number of transmissions has been reached. Archival tags are the remote sensing tool of choice among biologists tracking the movement patterns of large pelagic fishes, such as tunas and billfishes, as well as sharks. Because the data is not broadcast continually or constantly, Archival tags require far less energy and thus can be expected to generate a relatively weak local electrical field. You can read a bit about Archival tags in a 1990 paper by Donald Nelson in NOAA Technical Report NMFS 90: 239-256. (If anyone knows of a more recent review paper on satellite telemetry or archival tags, please let me know.)
The question, "To what extent do satellite tags affect the behavior of the animal being tracked?" is a very important one and not easy to answer with any confidence. The larger Continuous tags may add considerably to the drag, and thus swimming effort, of the tagged animal — I seem to recall reading a short paper in Nature late last year (August 1999 or so) in which turtle biologists actually measured the drag added to a sea turtle's cost of propulsion; they found that the device added an incredible 30% to the turtle's overall energy consumption — a figure that must impact its ability to escape predators, heal wounds, grow and reproduce.
The extent to which such tags interfere with the ampullae of Lorenzini or affect the behavior of sharks is not understood. In a 1993 paper (Copeia, 1993: 1010-1017), Keinath and Musick reported a satellite-tagged Leatherback Turtle (Dermochelys coriacea) in the Caribbean region whose transmitter pack had been bitten by a large shark, probably a Tiger Shark (Galeocerdo cuvier). Although the authors suggest the localized flux in electrical field generated by the transmitter pack may have facilitated the attack by the shark, the possibility that the Leatherback was attacked because it was swimming awkwardly or somehow strangely cannot be ruled out (according to the paper, the turtle was abraded by the harness that held the transmitter pack to its carapace).
The electrosensory limits of the White Shark (Carcharodon carcharias) have not been determined quantitatively. In their 1996 paper on the structure of the White Shark brain and cranial nerves (pp 121-130 in the Klimley-Ainley volume, Great White Sharks), Demski and Northcutt suggest that — based on the thickness and arrangement of nerve fibres in the dorsal nucleus — White Sharks may not be particularly sensitive to electrical cues and that such cues may not be all that important to this species. Although Demski and Northcutt base their determination on the partial head of a single specimen, this jibes well with some — but not all — reports of the White Shark's reaction to the powerful local electric field generated by the Shark POD. However, there is a fair bit of anecdotal evidence — including McCosker's experience with Sandy at the Steinhardt Aquarium, where the animal apparently became quite disoriented by a 0.125-millivolt flux in the local electrical field and White Sharks in the wild mouthing metallic objects such as swim steps, propellers, anti-corrosion plates and shark cages — that suggests White Sharks are quite sensitive to electrical cues. Thus, we have contradictory evidence about the extent to which White Sharks respond to electrical fields generated by electronic devices. (As part of my forthcoming experiments with White Sharks in South Africa, I hope to be able to test the elecrosensory limits of this species with a specially designed and built piece of electronic equipment. If I am successful, the results will be published in a scientific journal and, eventually, in the popular press.)
It is, however, worth bearing in mind that the intensity of an electromagnetic field falls off very rapidly with distance (expressed mathematically, as an inverse square of distance) and the seawater medium is very rich in ions (charged particles). As a result, despite the phenomenal electrosensitivity some elasmobranchs have demonstrated under laboratory conditions, the functional distance of a White Shark's — or other elasmobranch's — ampullae of Lorenzini in the wild may be limited to very short distances (say, on the order of a foot [30 centimetres] or less). So, unless the power pack of an electronic tag is strapped to the underside of a shark's snout, it seems unlikely that the tiny energy output of an Archival satellite tag would significantly affect a shark's ampullae of Lorenzini or behavior.
Time and further research may eventually shed further light on the extent to which electronic tracking equipment interferes with the electrosensory mechanism or behavior of the Great White and other sharks.
In any case, I hope this helps.
— R. Aidan Martin