Getting to Know Sharks in St. Croix Waters

A tiger shark in the Atlantic Ocean (photo from Shutterstock)

Lemon, nurse, Caribbean reef and tiger sharks meander in and out of the marine protected area around Buck Island Reef National Park even during a Category 5 hurricane, a scientist told an audience that filled the room at the Guinea Company Warehouse Wednesday evening.

Grace Casselberry, a marine ecologist and PhD student at the University of Massachusetts, talked about a multi-year study of the apex predators and pointed out the reasons the protected area around Buck Island is so important, but understudied, as a stronghold for the population.

Beginning in 2011, sharks have been tagged with acoustic telemetry receptors and tracked by 134 receivers placed on the sea floor around Buck Island and Long Bank. The team catches the fish and inserts the tag inside a dime-wide cut in its abdomen. Then the sharks are released and swim away normally, Casselberry said. The shark would lose the tag if it was placed outside the body, she added. When the shark passes within 200 meters a ping is sounded.

Grace Casselberry talks about sharks living around Buck Island Wednesday. (Source photo by Susan Ellis)

Since 2013, Casselberry said more than 3 million pings were recorded from 48 tagged sharks. Scientists studied the data to determine how long the sharks stayed in any one area, what the area was used for and what paths they traveled to leave.

Other marine creatures in Virgin Islands waters are also being tracked by acoustic telemetry, including conch, lion fish and barracuda.

All shark species studied were found to use the Buck Island area year round. The nurse and reef sharks preferred the sandy bottoms but also spent time in the linear reef of Long Bank. The lemon sharks spent more time in the reef, sea grass and rocky bottom. The small (three foot) lemon sharks explored the area some but the six-foot lemons circle the island.

“All they do is swim around that island every day,” Casselberry said.

Caribbean reef sharks moved more though the area more than the others, research found. As they reached two years of age, they became territorial – not encountering each other often. Casselberry said it may be they were keeping away from each other to avoid conflict or they had just identified their feeding grounds.

Tiger sharks also moved around more than the others and separated themselves by age and size. The juveniles roamed the south side of the island and the 8 – 10 footers established their home on the north side.

The dozen nurse sharks followed by the scientists changed their movements as they matured. They expanded their territory, swam deeper and hung out on a shelf dropping off to deep water. Two mature female nurse sharks left in July and August and Casselberry wondered if they traveled to breeding grounds.

“Clearly Buck Island is their home because they come right back and they’re here every day,” she said.

Data has been analyzed to understand what the Buck Island sharks did during Hurricane Maria. None of them strayed far from the area but some went to places they had never been to before, according to Casselberry. All swam deeper to avoid disturbed water. The lemon sharks moved deeper and a little to the north but then returned to their usual haunt. Nurse sharks also went deeper during the worst of the storm and then explored Teague Bay reef afterwards, possibly looking for food. The tigers and reef sharks just went deeper and deeper.

Casselberry announced the sharks will be tracked by satellite in the future due to a grant from National Geographic. The sharks then can be followed wherever they go in the world with tags attached to the dorsal fin sending location signals to satellites.

One reason sharks are endangered, according to Casselberry, is that they only reproduce around 20 pups every other year whereas fish such as snappers can lay 1.5 million eggs a year. Also, their habitat has shrunk due to fishing practices and development of coastal areas. Sharks’ favorite nurseries are shallow areas along the coastlines to protect their young from predators and other dangers.