Great whites lurk in darkness of new moon
Great white sharks off Cape Cod are more likely to lurk along the shore at night during the new moon, as the lower light levels help the predators attack seals, according to Cape shark researchers.
The sharks that the researchers tagged were more likely to be in shallow water during the new moon and less likely during the full moon, they determined in a new report.
“Tagged sharks were most likely to occupy shallow depths along Cape Cod at night during the new moon, which suggests that white sharks may patrol nearshore areas when conditions favor the species’ predatory stealth,” wrote the researchers from the Atlantic White Shark Conservancy, Massachusetts Division of Marine Fisheries and Arizona State University.
“White sharks rely on visual cues to initiate predation events but must remain undetected to maximize the probability of prey capture,” they wrote. “It has been suggested that lunar illumination affects predatory success rates by increasing or decreasing ambient light levels.”
The researchers in the new report concluded that sharks along the Cape spend nearly half of their time at shallow depths. The sharks spent almost half (47%) of their time at depths of less than 4.5 meters, or about 15 feet.
Sharks were more likely to occupy shallow depths at night during the new moon.
“During full moon periods, higher ambient light levels may allow seals to detect stalking sharks from a greater distance,” the researchers wrote. “Conversely, during new moon periods, lower light levels may provide white sharks, which have a retinal structure suited for both daylight and low light conditions, with a visual advantage over pinniped prey.”
During the day, tagged sharks were more likely to be at shallow depths during the first and last quarter moon.
“Additional research on white shark predation rates and energy expenditure under various conditions along Cape Cod is needed to determine the influence of local environment on predatory behavior and nearshore presence,” the researchers wrote.
People at the beach should stay close to the shore and avoid being in the water at night and during periods of low ambient light levels, the researchers said.
That’s when “local environmental conditions reduce water clarity and visibility, and when seals and other potential prey species are present,” they wrote.
Overall, the risk of a shark-human interaction is greatest during the summer and fall, the researchers concluded. Previous research has shown that white sharks aggregate along Cape Cod to prey on seals from the summer into the fall, which coincides with peak tourist season.