Basking Sharks Are Gathering By The Thousands Off The US Coast

According to a study published in the Journal of Fish Biology, basking sharks are joining by the hundreds for the summer and fall season off the northeastern US coast and marine biologist are trying to figure out why.

Schools of the second largest fish in the world are ranging from 30 to nearly 1,400 individual basking sharks. The discovery began in 1980 when scientist were performing aerial surveys of the endangered North Atlantic right whale and they discovered an aggregation of the basking sharks. Since that point, nine more large grouping events have been documented.

Basking sharks are a filter feeding fish with enormous gill raker mouths that feed on zooplankton by slowly swimming through surface waters of both coastal and open-ocean areas. Adult basking sharks range from 20-26 feet, making a spotting relatively easy while the leisurely feed at the top of the water.

Aerial view of basking shark. Via NOAA/Corey Accardo

When the sharks are not congregating in huge numbers, they are typically diving into deep water and migrating long distances making the study of the large fish challenging.

Shark researchers are taking advantage data gathered by right whale scientists tp uncover clues to discover  why the sharks are displaying certain behaviors such as large-scale groupings.

“Aerial surveys provide a valuable perspective on aggregations and their potential functions, especially when coupled with environmental satellite and ship-based survey data,” said lead author Leah Crowe, a protected species researcher at NOAA’s Northeast Fisheries Science Center, in a statement.

Crowe and her team determined that the aggregation events occur during periods of warm sea surface temperatures (13-24 °C, or 55-75°F) and high concentrations of chlorophyll – meaning that large quantities of photosynthetic plankton are in the water.

During the largest of the 10 documented aggregations, in November 2013, at least 1,398 individuals were photographed swimming within an 18.5-kilometer (11.5-mile) radius area.

The findings are in line with those from previous studies on basking sharks that suggest the gatherings are related to seasonal zooplankton blooms.

Unfortunately, the researchers were unable to determine whether or not the sharks use these gatherings to engage in mating.

“Although the reason for these aggregations remains elusive, our ability to access a variety of survey data through the North Atlantic Right Whale Consortium Database and to compare information has provided new insight into the potential biological function of these rare events,” Crowe said.

To help scientists like Crowe gather data, anyone can submit their basking shark observations to the “Spot a Basking Shark” program run by the Pacific Shark Research Center.

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