© Eli Martinez. Photo taken by Pat ford.
Tonic immobility, by definition, is a state of apparent paralysis that animals enter usually in response to a threat, although it has also been correlated with mating in sharks. Virtually nothing is known about its underlying neurological and physiological processes in animals. It is also known that some sharks can be placed in a state of tonic immobility where the shark remains in that particular state for an average of fifteen minutes, before coming out of it. (Henningsen) Due to this knowledge, a lot of scientists have used it to their advantage, particularly for tag insertion and measurements of the sharks.
It has been very beneficial for scientists to have the animals in this state, but it is known that sharks may not always respond to tonic immobility by inversion of the animal. A good example of forced tonic immobility is on Tiger sharks (particularly measuring 3-4 meters); hands can be placed lightly on the sides of the animal's face near the area surrounding its' eyes. Great White sharks, however, have been shown to be less responsive than other species when tonic immobility has been tried. Also, scientists believe that tonic immobility can be linked with being an act of defense, due to the fact that females seem much more responsive to it than males. (Sharkman)
Quite a few studies have been conducted on tonic immobility and its' effect on sharks. "Tonic immobility was induced in black tipped reef sharks (Carcharhinus melanoptera) and heart rate and ventral aortic blood pressure recorded." (Davie, et al.) Their findings suggested that tonic immobility (without branchial irrigation) was correlated with a "...significant depression in blood pressure and heart rate irrespective of the sharks being in air or in water." (Davie, et al.) In comparison, tonic immobility with branchial irrigation resulted in an increase in blood pressure in sharks in air, but not in water. (Davie, et al.)
© Eli Martinez. Photo taken by Paul Spielvogel.
After their results were examined, they came to the conclusion that during tonic immobility sharks are able to receive information from the ventilatory system. (Davie, et al.) The results of this particular study can only shed light to the specific sharks observed and due to the fact that sharks are oceanic creatures it would make sense for the part of the experiment conducted in air with branchial irrigation to cause more stress than the ones in the water with branchial irrigation.
In another study conducted on juvenile Lemon sharks, "Eight juvenile lemon sharks were subjected to four, three-hour treatments: (Brooks, et al.) under tonic immobility. Their results were shocking, to say the least. Tonic immobility, under this experiment, turned out to be a very stressful experience for the sharks. Amongst their findings, the most important ones were the ones that mentioned that tonic immobility "...induced a short term reduction in ventilatory efficiency..." and "Animals maintained in TI were significantly hyperglycemic compared to those that were not." (Brooks, et al.)
This particular study is vastly different to the first study due to many different factors and variables. One of them, according to many visitors to research laboratories similar to the one this study was conducted in, is that that sharks are "...confined to an enclosure..." to which then the researchers have to "...chase them down, catch them in a net, grab them and flip them..." It would make perfect sense for the sharks to be overly stressed due to being placed in this scenario.
There haven't been a great deal of scientific studies done on the effects of tonic immobility on sharks but one thing can definitely be stated, proper care should be taken when using the technique on the animals and it should not be done for long periods of time unless there are no better options; like while tagging a shark, taking measurements of a shark for research purposes or possibly removing hooks in order to facilitate things for the animal.
Written by Susana Navajas - US Coordinator, Shark Aid International. 11th March 2014.