Tiger sharks that interact with tourists are larger and have higher hormone levels, study shows

Sharks that frequent diving tourism sites where food is provided have higher levels of testosterone and corticosteroids than others that spend less time in these areas, as well as a better nutritional state. The effects of shark-human interaction are poorly understood.

By André Julião  |  Agência FAPESP – Tiger Beach in the Bahamas is famous for its paradisiacal beauty and for being frequented by an animal that might scare most people away but is actually an outstanding diving tourism attraction: the Tiger shark (Galeocerdo cuvier). The sea is crystal clear and only 5 m deep on average, so the sharks, which can surpass 3 m in length, can easily be seen. They are drawn to the site by local tour operators, who throw fish and other food into the water.

A group of scientists in Brazil and the United States, including researchers supported by FAPESP, have discovered that females of the species that frequently visit the area are larger and have higher hormone levels than other individuals of the same species that spend less time there.

An article on their study is published in Animal Behaviour. The findings point to possible effects of mass tourism on these sharks. The authors are the first to describe the influence of physiological state on behavior and decision-making in these sharks.

“The area was dominated by large females, some of them pregnant. Generally speaking, hormone levels were higher in female sharks that frequented the area, where they were fed, than in others that didn’t interact much with divers. Moreover, the former’s nutritional state was better, and they had more omega-3 in their blood,” said Bianca Rangel, first author of the article. Rangel conducted the study as part of her PhD research at the University of São Paulo’s Institute of Biosciences (IB-USP) with a scholarship from FAPESP. 

An earlier study by the group showed that Nurse sharks (Ginglymostoma cirratum) living in an urban area had more fat in their blood and bacteria in their stomach contents than individuals of the same species living in better-conserved areas (read more at: agencia.fapesp.br/38882/). 

“We can’t say whether tourism is or isn’t harming these animals, as we were unable to collect material for testing before and after interaction with divers, which would have been ideal. However, we now have a body of evidence that will be helpful for future evaluations,” said Renata Guimarães Moreira, second author of the article and supervisor of the study. Moreira is a professor at IB-USP and is supported by FAPESP.

Shark monitoring

Distinguishing the sharks that spent more time in the diving tourism area from those that frequented other areas was possible thanks to a monitoring project led since 2011 in Florida (USA) and the Bahamas by Neil Hammerschlag, last author of the article and Research Associate Professor at the University of Miami.

The American researchers caught 33 sharks in an area to the northwest of Grand Bahama Island in 2013 and 2014. They recorded the animals’ sex and length and checked whether females were gravid using a portable ultrasound system. They also collected blood samples, which were kept in cold storage pending analysis.

Before releasing the sharks, they tagged them with tiny acoustic transmitters implanted under the skin. Two dozen receivers were installed on the seabed to record the presence of sharks in the study area. Only 22 sharks were detected by the receiver array and analyzed in the study.

Based on spatial use data collected during a period of 90 days, the researchers were able to identify the animals that spent more time in the diving area, to which they were attracted by being fed. “The diving area was mainly frequented by large females, while juveniles, which are smaller, remained outside. The female predators saw the advantages of being in this territory and were able to dominate the smaller sharks,” Rangel said.

This use of space was reflected in the levels of fatty acid and saturated fat in their blood, as well as carbon or nitrogen stable isotopes indicating where the food came from. Tour operators use tuna and grouper carcasses to attract these sharks, and blood samples from the females that spent more time in the area contained higher levels of omega-3 and other fatty acids, as well as nitrogen isotopes.

Hormone levels were also strikingly higher in the blood of females that frequented the diving area than in others that did not: by a factor of three in the case of testosterone, four in that of estradiol, and 16.4 in that of corticosteroids.

“We don’t know exactly why this was. Levels of these hormones might have been higher as part of their more aggressive social dominance behavior while they were swimming with many other sharks,” Moreira said. “Another hypothesis is that they were at a stage in their life cycle when they were ready to reproduce. Juveniles haven't reached reproductive age and naturally have less of these hormones.”

Although the article is not conclusive on the reasons for the physiological alterations, it stresses the importance of considering life cycle stages, hormone levels, and nutritional states in assessing the impact of diving tourism on shark diet.

The article “Physiological state predicts space use of sharks at a tourism provisioning site” is at: www.sciencedirect.com/science/article/abs/pii/S0003347222001865.