Tiger Beach in the Bahamas is renowned for its paradisiacal beauty and frequented by an animal that might scare most people but is actually an excellent diving tourist attraction: the tiger shark (Galliocerdo cuvier). The sea is crystal clear and is only 5 meters deep on average, so the sharks, which can exceed 3 meters in length, can be easily spotted. They are pulled 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 found that females of species that frequently visit the region are larger and have higher hormone levels than other individuals of the same species who spend less time there. spend.
An article on their study is published in Animal Behavior. The findings point to the potential impacts of mass tourism on these sharks. The authors are the first to describe the effect of physical condition on behavior and decision making in these sharks.
“The area was dominated by larger females, some of them pregnant. In general, hormone levels were higher in female sharks that frequented the area where they were fed, compared to others that were divers. In addition, the former had better nutritional status, and had more omega-3s in their blood,” said the article’s first author, Bianca Rangel.
An earlier study by the group showed that nurse sharks (Jinglimostoma serratum) living in urban areas had higher blood fat and bacteria in their stomach contents than individuals of the same species living in better protected areas. .
“We cannot say whether tourism is harming these animals, as we were unable to collect material for testing before and after interactions with divers, which would have been ideal. However, we now have a plethora of evidence. There is a body that will be helpful for future evaluations,” said Renata Guimaraes Moreira, the second author of the article and the study’s supervisor.
shark monitoring
Isolating sharks that spend more time in the diving tourism sector, which were frequent in other areas, thanks to a monitoring project in Florida (USA) since 2011 and the Bahamas by Neil Hammerslag, the last author of the article and Research Associate It was possible thanks to the prof. University of Miami.
US researchers caught 33 sharks in 2013 and 2014 in an area northwest of Grand Bahama Island. They recorded the sex and length of the animals and examined whether the females were gravid using a portable ultrasound system. They also collected blood samples, which were kept in cold storage for analysis.
Before releasing the sharks, he tagged them with tiny acoustic transmitters implanted under the skin. Two dozen receivers were installed on the ocean floor 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 over a 90-day period, the researchers were able to identify animals that spent more time in the diving area than they were fed. “The diving area was primarily driven by larger females, while juveniles, which are smaller, stayed outside. Female hunters saw the advantages of being in this area and were able to dominate smaller sharks,” Rangel said.
This use of space was reflected in the levels of fatty acids and saturated fat in their blood, as well as stable isotopes of carbon or nitrogen indicating where the food came from. Tour operators use tuna and grouper carcasses to attract these sharks, and blood samples from females who spend more time in the area have higher levels of omega-3 and other fatty acids as well as nitrogen isotopes. .
Hormone levels were also surprisingly high in the blood of women who frequented the diving zone compared to others: by a factor of three in the case of testosterone, four in estradiol and 16.4 in corticosteroids.
“We don’t know why this was so. The levels of these hormones may 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 have not reached reproductive age and naturally have fewer of these hormones.”
Although the article is not conclusive on the causes of the physiological changes, it does emphasize the importance of considering life cycle stages, hormone levels and nutritional states in assessing the impact of diving tourism on shark diet.
more information:
Bianca S. Rangel et al, Physiological State Predicts Space Use of Sharks at a Tourism Provisioning Site, Animal Behavior (2022). DOI: 10.1016/j.anbehav.2022.07.004
Citation: Tiger sharks interacting with tourists are larger and have higher hormone levels, study shows (2022, November 8) retrieved 8 November 2022
This document is subject to copyright. No part may be reproduced without written permission, except for any fair use for the purpose of personal study or research. The content is provided for information purposes only.