Any bodily fluid probably is attractive to sharks. Blood, in any form, may be at the top of the list. The sharks’ ability to detect even minute amounts of blood and scents of other organic material is amazing.
Several years ago in the Bahamas we observed juvenile blacktip sharks cruising in waist-deep water. We placed a carcass of a filleted fish in the water and watched as several sharks caught scent of the fish and rapidly made a bee-line to the carcass from long distances away. Obviously no movements from the carcass were involved – only smell was used as the sharks weaved back and forth catching the scent of the small (less than a pound) carcass.
Menstrual blood almost certainly can be detected by a shark, and we’re sure urine can be as well. Do we have positive evidence that it is a factor in shark attack? No, and until some menstruating and non-menstruating divers volunteer to take part in a controlled test we’ll never prove it. In our opinion it likely is attractive to sharks in certain situations.
Certainly menstruating women are attractive to such smell-oriented animals as dogs. Sharks, with their extreme olfaction abilities, surely are capable of detecting at similar low levels. Does that mean a menstruating woman is setting herself up? No, but if one is attempting to maximize reduction of risks it is one thing that can be avoided.
As of this writing there have been a male:female ratio of 9.2:1, or more than 90% attacks have occurred on males. This reflects a historic pattern of more males engaged in marine aquatic activities, especially those that put humans most at risk, e.g. surfing, diving, long distance swimming, warfare. It in no way can be attributed to sharks “preferring” males over females. In recent years proportionately more females are being attacked because more females are engaging themselves in riskier, formerly males-only activities.
Our advice? Don’t worry about it. Lots of women safely dive while menstruating. Although we haven’t got solid scientific data on the subject, so far we haven’t seen any obvious pattern of increased attacks on menstruating women.