Scientists have confirmed local anecdotal reports that the number of stingrays at the Sandbar are decreasing.
Brad Wetherbee from the Department of Biological Sciences at the University of Rhode Island, working in conjunction with the Guy Harvey Institute and the Cayman Islands Department of Environment, has been carrying out censuses on the local stingray population since 2002. In his last check in 2008, he found more than 100 stingrays at the Sandbar at any given time, but on his most recent trip this year, he counted or tagged just 61.
The numbers he found during similar censuses in 2002, 2003, 2004, 2005 and 2008 showed consistent results, but when he returned to Grand Cayman in January this year, he was able to confirm that the reports of declining numbers were correct.
“In January this year, we went out … to catch them and tag them. In previous years, we easily caught 100. This January, we caught 61,” said Mr. Wetherbee. This month, when he carried out another census, he found 57, he said.
“People were saying that the numbers were lower, but they said the same thing back in 2002 when we first came here. They said that after Hurricane Ivan, but there were as many after Ivan as there had been before. Now, they are saying that in the last 18 months or so, there are not as many as there used to be … There are still a lot of rays, people can still got out there and interact with rays and find them there. There is definitely not as many as there were,” Mr. Wetherbee said.
The reason why there are fewer stingrays at the Sandbar is unknown, although there are plenty of hypotheses.
“Different people have different theories as to why the numbers are declining. Some think it is diet. They say that 30 years of interacting with tourists has finally taken a toll on their health. Some say sharks are eating the rays. Some think there are people out there at night fishing for rays. There might be fewer because they died or they left. Nobody knows,” he said.
Reports of the declining numbers prompted a team from Georgia Aquarium in the United States to come to Grand Cayman this month to carry out extensive health and nutrition checks on the local southern stingrays.
Tonya Clauss, director of animal health, at Georgia Aquarium, led a three-person team which consisted of herself, nutritionist Lisa Hoopes and veterinary technician Nicole Montgomery, to carry out a week-long health survey on stingrays in a variety of locations earlier this month.
“Because the population decreased, we thought it would be a good idea to take blood and tissue samples from the stingrays … to be able to assess their nutritional status and health in general,” Ms Clauss said.
Conservationist Guy Harvey of the Guy Harvey Institute invited the team to carry out the study. Other team members included the Guy Harvey Research Institute Director Mahmood Shivji, Mr. Wetherbee, Edinburgh University student Louisa Gibson and Mr. Harvey’s adult children, Jessica and Alex Harvey.
“We took samples from animals in Stingray City and the Sandbar,” Ms Clauss said. “We also wanted to get some samples from animals that… have less tourist interaction, so we went to the original Stingray City in deeper water and got samples from animals there. We also took samples from rays at Rum Point, which have very little contact with people,” Ms Clauss said.
The team, assisted by Department of Environment staff, caught the stingrays and transferred them into a little wading pool on board Mr. Harvey’s boat, where they tagged the stingrays with microchips, took measurements to assess the size and age of the animals and obtained tissue and blood samples.
One of the theories relating to the declining numbers is that the squid they are fed every day at the Sandbar by tourists is bad for them. That’s one of the issues nutritionist Ms Hoopes is looking into.
“So many of the animals at the Sandbar are dependent on the food resources provided by the tour boats. The tourists feed them squid, but that’s not a natural food for the stingrays. It is not something they would eat in the wild,” Ms Hoopes said.
In the wild, stingrays forage for molluscs, she said, but added that until the results of the health checks are complete, the team cannot yet determine the impact of a squid diet on the stingrays. “That’s something the data will show us. They may do quite well on squid. We’ll need to see those results in the blood work,” Ms Hoopes said.
The team ran a full gamut of checks, including a complete blood work-up, hormone levels, nutriments, vitamin levels, heavy metals and fatty acids, which will help compile a complete picture of health and nutritional information about the animals.
They found no evidence that the stingrays were being overfed and did not find any obese animals. They also did not find any rays with obvious shark bites.
Although the numbers may be down, the team found several pregnant stingrays and also young stingrays.
Mr. Wetherbee has also been tracking the movements of the stingrays with transmitters.
“The Sandbar is dominated by big bossy females. They’re a lot bigger than the males. When we track them with the transmitters and follow them around, the females were going around all day from one tour group to the next during the day. At night, they would swim 100 yards away and stay there at night … They did that every day for a year. The males are a lot smaller and probably not as competitive as the female in trying to get food so they forage for food at night and move further away,” he said.
He said he plans to return to Grand Cayman next year to do another census.
The Georgia Aquarium team have returned to Atlanta with their samples and are collating and analysing the data. Results should be known in coming months.
“It will take some time. We have been sharing information with Guy. We hope to publish some of the information in scientific journal. It is a rare opportunity to do a study like this on the southern stingray,” Ms Clauss said.