Any bodily fluid released into the water is likely detectable by sharks.
A shark’s sense of smell is powerful – it allows them to find prey from hundreds of yards away. Menstrual blood in the water could be detected by a shark, just like any urine or other bodily fluids. However, there is no positive evidence that menstruation is a factor in shark attacks. Additionally, evidence suggests when swimming the water pressure will temporarily stop the flow of menstrual blood decreasing the chances any blood is released into the water.
Currently there is a gender ratio of 8 men for every 2 women bitten. Meaning, more than 80% of our recorded shark bites in history happened to men. This reflects a historic pattern of more men engaging in marine aquatic activities, especially those that put humans most at risk (e.g. surfing, diving, long distance swimming, spear fishing, etc.). Sharks do not “prefer” to bite males rather than females; rather men had a greater historical chance of being around sharks. According to recent data, more females are being bitten, attributable to more women gaining equality and engaging in water sports.
If someone is attempting to maximize reduction of risks, staying out of the water during menstruation is one step that can be taken. However, many people safely dive while menstruating, and we have continued to see no obvious pattern of increased attacks.