The researchers haul the fishing net on board, maneuvering the large, writhing stingray into a child’s paddling pool, set up on deck.
Working quickly, they measure the animal, record its tag number and take a small DNA sample. Two minutes later they lift the net back overboard and the ray gently glides away across the clear blue water.
“One down, about 90 more to go,” shouts Jeremy Vaudo.
It is day one of the stingray census and the team, from the Guy Harvey Research Institute, is out early. The goal is to count, measure and record tag numbers for every ray at the Stingray City Sandbar.
The research aims to help support the interaction zone – Cayman’s most popular tourist attraction.
Stingrays are not endangered animals; the desire to study them is about the long-term viability of the sandbar as a tourist attraction, as well as the welfare of the rays that frequent the interaction zone.
The prediction turns out to be correct. After two days, the researchers put the final count at 91 rays currently on the sandbar.
The numbers are encouraging compared to a low of 57 in 2012, but they are still some way short of the 160 rays tagged in the first census in 2002.
Mr. Harvey believes that with the increase in tourists visiting the site over the past few years, the current population level is the “minimum” possible without diluting the tourist experience.
“I think we are right at the minimum number, given the number of customers that are coming here every day,” he said.
He believes a people census could be the next project required at the sandbar. He estimates more than a million people visit the site each year, with inevitable consequences both for the welfare of the animals and the level of interaction.
The sandbar is so important to Cayman’s economy that Mr. Harvey believes the government should dip into the Environmental Protection Fund and support the research, which his organization has been funding for more than a decade.
“We have to monitor the stingray population levels at the interaction zone. Without monitoring, we wouldn’t have known that the numbers had died off in 2012. We wouldn’t have known where we were and where we had come from.
“Because we had the data, when the numbers crashed we were able to go straight to the ministry and say, we’ve got a problem.”
For researchers like Mahmood Shivji and Bradley Wetherbee of the Guy Harvey Research Institute, the aggregation – an anomaly for stingrays which are normally solitary animals – represents a unique opportunity for study.
Beyond recording population numbers, the census has two key goals – to use the access to such large numbers of rays to get general biological data on the species and to determine what impact, if any, the interaction zone is having on their welfare.
“I started this project thinking there has got to be some kind of negative impact,” said Mr. Shivji. More than a decade on, he is not so sure.
He acknowledges the interaction zone has changed the behavior of the rays. Normally nocturnal foragers, the sandbar rays now feed during the day and rest at night. They may be growing at slightly faster rates than “wild” rays, but there is no evidence that any of these changes are negatively affecting the animals.
The researchers would like to see a more varied diet for the sandbar rays, and they hope to do genetic studies to see whether the aggregation has led to an increase in mating and potentially harmful in-breeding, but there is no physical evidence that this is happening.
The female to male ratio is another unusual quirk. Southern Stingrays, like most species, have an approximately 50-50 ratio of females to males. At the sandbar, it is more like 90-10.
The difference could simply be down to the fact that females are bigger and out-compete the smaller males for the available food.
But Mr. Vaudo has a more interesting theory.
“There could be a tourism aspect to it. People want to have their picture taken with the bigger rays, so they feed them more and the little ones get ignored,” he says.
Compelling evidence that the experience at the sandbar is not harming the rays is that they keep returning to the site day after day, year after year. Of the 91 animals counted on Monday, 19 carried tags from the first census in 2002.
Boat propeller injuries represent the greatest, unnatural risk to the rays at the sandbar. With as many as 30 boats crowding the site on busy days, the potential for accidents is increased.
“If there is no enforcement and the numbers of people continue to increase, you are potentially asking for trouble,” says Mr. Shivji.
“The key is management of the behavior of the people,”
Additional blood and ultrasound work involving visiting veterinarians from the Georgia Aquarium was done during four trips to the sandbar in 2014. The results were encouraging, according to Guy Harvey.
“Importantly, we found about a third of the mature females were reproductively active in all four surveys in 2014. This implies sustainability of the population in the face of loss through migration, fishing pressure, natural mortality, boat strikes and sabotage.”
Mr. Wetherbee believes the research is important because it provides tourists with the reassurance that the interaction is not harming the animals.
He acknowledges that could change if the population dips and a declining number of rays has to be shared among an ever-growing number of tourists.
But he says the stingrays always have the option to move on. The real losers would be the people.
“Stingrays are an icon of the Cayman Islands,” he said. “If this stingray aggregation ended tomorrow, it would affect hundreds of people all round the island.”