Menstruation and the Risk of Shark Attack
This subject is open to considerable debate and has not yet been settled by empirical inquiry. However, there are a number of issues that need to be considered in arriving at an informed understanding of the topic.
To being with, the smell and taste receptors of many carnivorous sharks routinely permit the recognition of one part of blood in 100 million parts of water. With some species under some conditions, e.g., food deprivation, this sensitivity can be very much higher. Those expert in the central nervous system of the shark have suggested that two-thirds or more of its brain may be devoted to smell. I, for one, believe that there is a sound evolutionary reason for this.
Although the blood lost during menstruation typically is small and occurs over several days, there simply is no argument but that, most especially on the days of heaviest discharge, it is more than enough to be detected by a creature that not only can sense one part of blood in 100 million parts of water or better, but also often can tell which of its nostrils that part is closest to.
The extensive research findings to date on fresh human, animal and fish blood, other human bodily fluids, e.g., urine, and a wide variety of water-borne organic chemicals found in human fluids (e.g., amino acids, amines & small fatty acids) indicate to me that the carnivorous sharks' exquisite sensory capabilities can in fact detect menstrual blood, and do so at a considerable distance. Whether this attracts, repels or does nothing to their behavior is unclear.
Unfortunately, no studies of which I am aware have specifically tested for detection of menstrual blood by sharks. For obvious reasons controlled studies would be difficult.
So much for detection and on to the possibility of increased risk of shark attack during menses.
There are a couple of studies, such as that by Johnsen, PB., and J.H. Teeter. 1985. Behavioral responses of bonnethead sharks (sphyrna tiburo) to controlled olfactory stimulation. Mar.Behav.Physiol., 11:283-91, which suggest that sharks may be repelled by "dead" (hemolytic) blood, but they used animal rather than human blood and only a single type of shark, and for these and other reasons cannot be considered as conclusive. In order to appreciate that more study is required, one only need consider that the long dead blood used in chumming for sharks is remarkably effective at attracting them.
In the 1992 edition of his book, Diving and Subaquatic Medicine, Dr. Carl Edmonds suggests that menstrual blood may have repellent effect, but based this conjecture solely on the fact that women are attacked by sharks less often than men. Clearly there a many possible reasons for this statistic having nothing to do with menstruation.
There are as well several experts who opine that risk may be increased. These include George Burgess, Director of the International Shark Attack File, and Divers Alert Network (DAN). An article on shark attack in the Feb '02 issue of their "Alert Diver" magazine states: "Although there is no positive evidence that menstruation will increase risk, it is best not to tempt a predator that finds its prey via smell" (p.15). As with Dr. Edmond's comment, this other position also is without direct research support.
Anecdotally, there are numerous reports of menstruating divers being hassled by sharks. These, of course, prove nothing, as do reports by female divers who have never been bothered by a shark during menstruation.
My personal opinion is that there may be a very small increased risk of shark attack related to menses. But, as the risk of shark attack on a diver is already so infinitesimally small, and even smaller for women, it would be extraordinarily difficult to prove the matter one way or the other even if controlled studies were easily done.
This having been said, if I were a woman and was otherwise feeling up to scuba I would not alter my dive plans due to menstruation.
Of course for you hardy ladies diving cold water in drysuits, the whole discussion lacks relevance.
© Doc Vikingo