This decline in basking shark sightings is accompanied by an increase in the presence of other marine species, suggesting significant changes in the region's marine ecosystem. To try to understand this phenomenon, scientists have been observing the fauna of the Sea of the Hebrides, a marine protected area, for the past twenty years.
The alarming decline of basking sharks
Basking shark sightings in Scotland reached their lowest level in twenty years in 2023, according to reports published by the Hebridean Whale and Dolphin Trust (HWDT) and NatureScot, a Scottish nature agency. As a reminder, in 2010, a record year, 162 sharks were recorded, but only 7 were observed last year. A worrying drop that experts are calling for analysis to understand the causes.
These large fish, considered the second largest fish species in the world after the whale shark, tend to congregate on the west coast of Scotland between May and October. A good time to feed on plankton, their main food source, and to reproduce. But in recent years, basking sharks have been less visible.
The Sea of the Hebrides, a favourite haunt of basking sharks
In 2023, basking shark sightings occurred in the Sea of the Hebrides Nature Conservation Marine Protected Area (Hebridean Sea of the Atlantic Ocean located in the United Kingdom on the west coast of Scotland). This marine protected area, off the west coast of Scotland, was set up to maintain the region's marine biodiversity, with basking sharks being an iconic species. Despite conservation efforts, numbers remain in sharp decline.
Experts believe this is due to the availability of zooplankton, the basking sharks' staple food. Rona Sinclair, a marine mobile species monitoring advisor and NatureScot representative, said it is possible that the movement of plankton, caused by ocean warming, has had an impact on the sharks' presence at the surface.
She explained to the British media BBC that “They may also be present in depth, beyond the reach of observations, but we must continue our research to better understand this phenomenon.”.
Worrying signs in marine ecosystems
Alongside the decline in basking shark sightings, the Hebridean Whale and Dolphin Trust has recorded a record number of minke whales, 167. Scientists have suggested there is a link between the change in basking shark populations and the change in minke whale populations, although there is no clear explanation for this.
According to Dr Lauren Hartny-Mills, a scientist at HWDT, these trend reversals could be caused by significant environmental factors, such as climate change.
Indeed, these variation phenomena can be justified by modifications to the marine ecosystem.
According to her, a thorough study of the seas must be carried out. In the face of natural and climate emergencies, it is essential to better understand what is happening in Scotland's seas“In this context, she is launching a call for mobilization”We need to better protect these remarkable animals and this unique marine region”.
Conservation efforts needed
Long hunted for their liver oil, rich in squalene, a natural lipid, basking sharks had a difficult time until the practice was banned in 1994.
Although hunting pressure has decreased today, other threats remain significant for this species, which has been classified as vulnerable by the International Union for Conservation of Nature (IUCN) since an observation in 2018.
Along with hunting, pollution, collisions with boats and the reduction in the availability of their food are other causes of the fragility of basking sharks.
In order to find solutions to this disaster, HWDT and other organisations are continuing with international cooperation to study the impacts of climate change on Scotland's marine ecosystems.
The Basking Shark, the second largest fish in the world
Cetorhinus maximusits scientific name, the basking shark is the second largest fish in the world, after the whale shark. Indeed, it can reach more than 10 meters long. Although its size is impressive, it is harmless to humans, as recalled by the specialized site of the national inventory of natural heritage (INPN), it feeds only on plankton thanks to a filtration technique, by opening its mouth wide, according to Geo it filters approximately 3,000 m3 per hour, the equivalent of an Olympic swimming pool.
The latter lives mainly in temperate waters, mainly off the Scottish coast where it migrates every year from early to late summer to feed and reproduce.
Sources : BBC, NatureScot, Hwdt, INPN
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