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Not long ago, my encounters with basking sharks were pretty close up. When a scientist researching them off the Isle of Man needed help, with my experience of driving inflatable ribs, I was tasked with steering his small boat up to a 10-tonne shark, closely and carefully enough for him to jab a small tracking device into its dorsal fin.

It was hair-raising work – they are solid muscle, capable of smashing our hull. Nevertheless, I guided the boat gently alongside, and the device was placed. Like many others before it, the shark swam off unconcerned.

Now I observe them from further away: either I respond to alerts on social media or visit likely headlands – a good number of recorded sightings are from people on land.

Basking sharks have an interesting recent history here. They migrate north through island waters from May to August and may even breed here, then are rarely seen in autumn. Their migration routes are astonishing, with tagged sharks recorded off Morocco to the south and Newfoundland to the west.

But the research was stopped in 2022 as numbers dropped from 885 sightings in 2009 to just a single individual. Lack of food, warmer seas or more likely underwater electricity cables were blamed. It was suggested that electromagnetic fields emitted by the cables repulsed migrating sharks.

That theory has been turned on its head this summer, with more than 100 sightings so far. The south-west corner of the island is a hotspot. My second this year was from Bradda Head, where a particularly large shark was seen with two smaller ones at the entrance to Port Erin Bay. The larger shark was well across the bay by the time I arrived, but its huge rounded dorsal fin was glinting in bright sunlight – a magnificent sight.

Experts in our fisheries department say that this year there has been a good bloom of microscopic crustaceans called copepods, the primary food of these sharks. Wildlife sightings in Britain do not get much bigger than basking sharks, and local spotters – including me – are thrilled.

• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024, is available now at guardianbookshop.com