Basking sharks stay in Irish waters year-round, scientists discover
Photo Cork, Peter Red Lettice April Sharks 2021 Strand, Along West Kayaker, Dan Basking Cleary Side At Swim In
Irish scientists have discovered that many basking sharks, thought to leave our coastal waters each winter, actually remain below the surface all year round.
Hundreds of the huge docile creatures visit Irish coastal waters each year and survive on a diet of plankton and other microscopic life.
The sharks, which can measure up to eight metres long and weigh over 5,000 kgs, are often filmed momentarily breaking the surface. Until recently, it was thought that the sharks migrated to warmer subtropical waters in places such as Africa in winter.
However, an international research team from Queen’s University Belfast and Western University in Canada have discovered that not all sharks leave.
The findings come from four basking sharks that were equipped with pop-off archival satellite tags off Malin Head. The tags were placed on the sharks to record water temperature, depth and location over a six-month period.
While two of the sharks travelled vast distances to the subtropical and tropical waters off Africa, the other two remained in Irish coastal waters throughout the winter.
First author on the study, Donegal-based Dr Emmett Johnston, said: “Our findings challenge the idea of temperature as the main reason for winter dispersal from Ireland.
"Previously we understood basking sharks departed Irish coastal waters for more southern latitudes in the Autumn in response to falling water temperatures in the northeast Atlantic.
"The sharks that remained in our coastal waters over the winter moved away from the surface but remained in relatively shallow waters (60m-100m) on the continental shelf when compared to the deep waters (400m- 600m or even 1,000m deep) that the sharks that moved into offshore waters occupied."
The study was prompted by frequent sightings of aggregating sharks along Ireland’s western coast during the summer months. The team wanted to know whether the seasonal disappearance of sharks from these hotspots in the autumn was driven by changes in water temperature.
The researchers were amazed that sharks off the coast of Africa experienced colder temperatures on a daily basis than the sharks that resided in Ireland, suggesting they didn’t move south simply to find warmer conditions.
The cooler temperatures experienced off Africa resulted from the sharks diving each day to depths of up to 600m, most likely in search of prey.
Co-author Dr Jonathan Houghton from Queen’s said: "The study tempts us to think about basking sharks as an oceanic species that aggregates in coastal hotspots for several months of the year, most likely for reproduction, rather than a coastal species that reluctantly heads out into the ocean when decreasing water temperatures force them to”.
The research team are still collaborating on basking sharks as part of the major EU SeaMonitor due to end in early 2023.
With partners from across Ireland, Northern Ireland, Scotland, Canada and the US the project is developing a collective conservation strategy for wide-ranging marine species that have inhabited our waters for thousands of years.