While the number of stingrays counted at the Sandbar are down considerably on earlier years, the latest census shows the population size is making a comeback, according to scientists who took a census of the ray population there last week.
During the five-day census, vets from the Georgia Aquarium, Cayman Islands Department of Environment staff and conservationist Guy Harvey tallied 74 Southern stingrays at the Sandbar in Grand Cayman, the islands’ most popular tourist attraction.
This compares to the worryingly low number of 57 that was counted in the previous census, done in July last year. Censuses of the rays have been done since 2002 and until last year, each census had found approximately 100 rays.
Guy Harvey, whose Guy Harvey Research Institute, has been heavily involved in the censuses over the years, said the declining population numbers indicated a threat to the continued viability of the Sandbar as a tourism attraction.
“We need to understand the reasons for the recent decline in numbers of rays at the Sandbar, and the data we collect will be used to inform management interventions for this extremely important economic resource for the Cayman Islands,” he said.
This year, the census takers caught and tagged 75 rays, not including the six rays released at the Sandbar from Dolphin Discovery on Friday. The vet counted 64 females and 11 males. Of the six newly released rays, five were males. In the July 2012 census, of the 57 rays tagged, only five were male.
This year, 20 of the rays caught were newly tagged.
As well as doing a head count of the rays, the vets took blood samples of each stingray and also performed ultrasound checks of the animals, which determined that more than half the female rays examiend were pregnant.
Veterinarians Tonya Clauss and Alexa McDermott from Georgia Aquarium, along with the aquarium’s nutritionist Lisa Hoopes, began checking the rays last Tuesday and continued through Saturday. They also carried out some checks on rays at Rum Point and the Stingray City dive site, tagging six at Rum Point and two at Stingray City.
This is the second year that the Georgia Aquarium team has done assessments of the rays. They had earlier carried out health checks on the six stingrays at Dolphin Discovery to determine if they were healthy enough to be released.
Dr. Clauss, director of animal health at the Georgia Aquarium, said the team checked the health, nutrition and reproductive status of the Sandbar rays. In general, she found the rays healthy, although further analysis of the blood samples will be necessary to get a clearer picture.
What is clear is that rays at the Sandbar have a considerably different diet to stingrays living outside the Sandbar. “There are definitely differences in the nutritional parameters,” she said, “but we would expect that to be the case.”
Rays at the Sandbar are handfed squid from boat operators and visitors. Wild stingrays live on a diet of crustaceans, worms, molluscs and slugs.
“We’re not saying the tour operators are causing the animals any great harm by feeding them an abnormal diet, but we would suggest a more diverse diet and a more natural one would be better for them in the long run,” Dr. Clauss said.
The Sandbar rays also continue to forage for other foods outside the Sandbar, the scientists have found.
Exactly why the stingray population has declined in recent years is not known, said Gina Ebanks-Petrie, director of the Department of Environment.
While she acknowledges that Southern stingrays are not considered endangered, it is concerning that the population of the rays at the Sandbar has declined.
“Two to three years ago, the tour operators began to report to us a decline in the numbers of rays at the Sandbar. That was really when we started to become concerned about the numbers,” Ms Ebanks-Petrie said.
“Prior to that, the Department of Environment’s concern about rays at the Sandbar really centred around the fact that … the rays at the Sandbar were not in a natural situation. Southern Stingrays are solitary animals in the wild generally, they don’t aggregate in large numbers in the wild like they do at the Sandbar. We were concerned … that they were being impacted because they were being fed with food that was not part of their natural diet and were being handled by people,” she added.
In 2007, the Marine Conservation Law was amended to designate the Sandbar and Stingray City Wildlife Interaction Zones, which regulated how many boats and visitors could be at those sites at any given time and the amount and types of food the rays could be fed. The law also made it illegal to remove stingray from either of those sites.