A truly strange phenomenon has been occurring along North America’s northeastern coast. Aggregations of basking sharks ranging from 30 to 1,400 have been observed from Nova Scotia, Canada to Long Island, N.Y. according to the National Oceanic and Atmospheric Administration (NOAA) scientists. While they are not uncommon, seeing them in large groups was thought to be rare. Why these huge animals congregate is yet to be confirmed, however its though to be related to socializing, feeding or courtship based on what is known from other species
Basking sharks (Cetorhinus maximus) are the world’s second largest fish, weighing in at more than 5 tons and growing as long as 32 feet. Along with the larger whaleshark and the megamouth shark, they are the three shark species which feed on plankton. The planktivorous filter feeder is named for its characteristic behavior of ‘basking’ near the water surface. However, more accurately this is feeding behavior moving slowly with its large mouth agape filtering zooplankton from the sea water.
In a recent study published in the Journal of Fish Biology, NOAA scientists analyzed the large groupings of basking sharks that have been documented since the 1980’s. Between June 1980 and November 2013 ten large aggregations have been documented. The researchers compared this information with data collected from earth-orbiting satellite and oceanic databases and from NOAA’s Northeast Fisheries Science Center (NEFSC) ecosystem monitoring (EcoMon) cruises from the same region, to delve into the derivation of this behavior.
The largest aggregation of sharks ever recorded by aerial survey was estimated to be at least 1,398 animals on November 5, 2013 in southern New England waters. Estimates of the zooplankton community were also taken at this time. The primary author of this study and researcher at NEFSC's Woods Hole Laboratory in Woods Hole, Mass, Leah Crowe said, “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.” The data collected from this large aggregation, the presence of juveniles and the abundance of plankton on the shelf at the time, the authors suggest that foraging likely played a role in the resultant behavior. They also suggested that animals maybe grouping to draft each other to increase feeding efficiency, as swimming with their large mouths open creates huge amounts of drag.
The sighting and environmental data from the NEFSC study was in agreement with previous studies finding basking shark aggregations form in locations with high prey abundance, which fall along thermal fronts and areas with high chlorophyll concentrations. These conditions often occur around spring and fall zooplankton blooms before the sharks’ seasonal migration out of the region. Specifically, the data also confirmed that these sharks most prefer sea water temperatures between 13 to 20°C. Although, the data is correlative it is not necessarily causative, as perhaps there could also be courtship related reasons for the aggregations, as proposed in previous studies, Crowe states.
“Although the reason for these aggregations remains elusive, our ability to access a variety of survey data though 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.”
For the publication of this fascinating work please see:
Crowe LM, O’Brien O, Curtis TH, Leiter SM, Kenney RD, Duley P and Kraus SD (2018) Characterization of large basking shark Cetorhinus maximus aggregations in the western North Atlantic Ocean. Journal of Fish Biology. https://doi.org/10.1111/jfb.13592
https://onlinelibrary.wiley.com/doi/abs/10.1111/jfb.13592
Also see the NEFSC NOAA article:
https://www.nefsc.noaa.gov/press_release/pr2018/scispot/ss1802/
