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article imageUnusually large groups of basking sharks seen off northeast coast

By Karen Graham     Apr 1, 2018 in Environment
Scientists say a species of large, but harmless, sharks have been gathering in atypically big groups off of the northeastern United States and Canada.
The National Oceanic and Atmospheric Administration (NOAA) is reporting groups of 30 to 1,400 basking sharks have been seen in waters from Nova Scotia, Canada to Long Island, New York, reports CBC News.
NOAA says that while seeing an individual basking shark is fairly common, such large groups coming together is abnormal. Why the animals congregate has not been clearly determined, although it is thought to be related to feeding, socializing, and/or courtship, given these behaviors in other shark species.
A basking shark  mouth open  filter feeding. Photo credit: Greg Skomal
A basking shark, mouth open, filter feeding. Photo credit: Greg Skomal
NEFSC/NOAA
Basking sharks study
In the recent study published in the Journal of Fish Biology March 7, 2018, researchers analyzed large groups of basking sharks (Cetorhinus maximus) recorded off the northeastern United States coast to learn more about the phenomenon.
The researchers looked into aggregations of basking sharks off the Northeastern U.S. coastline between 1980 and 2013 and included aggregations centering on sightings with group sizes of at least 30 individuals. These aggregations occurred in summer and autumn months and included aggregation sizes of up to 1,398 individuals, the largest aggregation ever reported for this species.
In almost 40 years of aerial surveys for right whales, only ten large basking shark aggregation events were opportunistically recorded and photographed. These observations were collated with imaging from a number of earth-orbiting satellites and oceanographic databases and the Northeast Fisheries Science Center's (NEFSC) ecosystem monitoring (EcoMon) cruises in the same region, allowing researchers to come up with some insight into the strange behavior.
A basking shark is a passive animal and no danger to humans
A basking shark is a passive animal and no danger to humans
NOAA
“Aerial surveys provide a valuable perspective on aggregations and their potential functions, especially when coupled with environmental satellite and ship-based survey data,” said Leah Crowe, a protected species researcher at NOAA’s Northeast Fisheries Science Center and lead author of the study.
The research team did note the groupings of basking sharks seemed to occur in summer and fall when sea surface temperatures ranged between 55 and 75 degrees F (13 to 24 degrees C). And during the biggest aggregation event, available data indicated there was a high concentration of zooplankton present.
The biggest aggregation of basking sharks was photographed on November 5, 2013, in southern New England waters. An aerial survey counted at least 1,398 of the animals. And it was pure luck that the NEFSC’s EcoMon survey sampled the same area on November 16 and 17, 2013, providing an estimate of the zooplankton in that area at that time of year.
“Photogrammetry, the use of photographs to measure objects, has provided estimated lengths of animals at the surface and allowed us to classify animals in the aggregation as likely juveniles or mature adults,” said Crowe, who works at the NEFSC's Woods Hole Laboratory in Woods Hole, Mass.
Picture was taken at Georgia Aquarium  USA on May 5  2006. Pictured is one of the two resident male ...
Picture was taken at Georgia Aquarium, USA on May 5, 2006. Pictured is one of the two resident male whale sharks.
ZacWolf (CC BY-SA 2.5)
The basking shark (Cetorhinus maximus)
Basking sharks are the world's second largest fish, after the whale shark, growing as long as 32 feet and weighing more than five tons. They are highly migratory, slow-moving animals often sighted close to the surface with their large mouths open to filter zooplankton from seawater.
Basking sharks are considered passive animals and pose no danger to humans. Basking sharks, along with the larger whale shark, and the megamouth shark, are the three shark species that eat plankton.
“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. “The study also highlights the value of opportunistic data collection.”
More about basking sharks, abnormally large numbers, Nova Scotia, northeast US, Environment
 
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