The stingray population at the sandbar – Cayman’s most popular tourist attraction – is starting to rebound after a sharp and unexplained decline in numbers.
Scientists are cautiously optimistic about the situation at the sandbar after counting 97 stingrays at the site during the latest population survey.
Ultrasounds conducted during the survey also revealed that around a third of the female rays were pregnant.
The headcount, a snapshot of how many rays are at the site in a week, is still short of the population estimates from a decade ago, when up to 130 separate animals were observed at the site.
But it represents something of a comeback from the worrying low of 57 stingrays counted in the summer 2012 survey.
The annual population census, which includes measuring, tagging and collecting blood samples for the rays that frequent the sandbar, has been extended this year to include four separate five-day surveys in January, April, July and October.
During the January survey, researchers counted 87 animals. In the April survey, they counted 97.
The figures show a steady increase, with 74 counted in the previous July 2013 survey.
Guy Harvey, whose Guy Harvey Research Institute has been heavily involved in the research over the past decade, said, “The numbers are still low, but they are starting to coming back.
“Ten years ago, we would stop tagging at 100. There was many more than that out there, at least 130.”
Between 2002 and 2003, researchers tagged 164 rays at the sandbar. Mr. Harvey said these numbers remained largely consistent until 2010, when operators at the tourist attraction reported a sharp decline. A survey in that year confirmed that numbers had dropped as low as 61.
Shark predators and natural mortality are counter-balanced by “recruitment,” either from the birth of new stingrays or from the arrival of new animals at the site. The reason for the sharp decline around 2010 remains unexplained.
The researchers intensified their monitoring of the site in response to the decline and continue to check on the health of the rays, which are individually estimated to be worth up to $10 million to the Cayman Island’s economy over the course of their lifetime.
Scientists from the Georgia Aquarium in Atlanta joined Guy Harvey’s team and researchers from the Department of Environment during the latest survey.
As well as tagging and measuring the animals, they took blood samples, collected DNA and conducted ultrasounds on the stingrays, which revealed that around a third were pregnant.
Mr. Harvey said this was consistent with previous surveys, but the increase in frequency of surveys is expected to help establish their reproductive patterns more clearly.
Each ray was caught by hand and lifted onto the work boat, then transferred into a kiddie paddling pool where researchers measured, tagged and took blood from the underside of the base of the tail.
Stingrays are typically solitary animals in the wild and the situation at the sandbar is unusual, the Department of Environment has said. Part of the research is designed to determine if there are any health impacts for the rays as a result of being fed by humans.
Mr. Harvey said Georgia Aquarium experts recommended they be fed a more varied diet than the handfuls of squid typically favored by tour operators, but he said the rays showed no obvious signs of ill health compared to rays foraging in more typically wild environments.
The April survey showed two new large females had taken up residence at the site, as well as five new males. There are only 15 males overall.
Mr. Harvey said there was some concern over the absence of two “regulars” at the site, known as Tripod 1 and Tripod 2, because of their short tails.
The six rays released from the Dolphin Discovery tourist attraction to comply with legislation making it illegal to take stingrays from Cayman’s waters are still at the sandbar and appeared to be thriving, according to Mr. Harvey.