Nothing makes human boundaries look sillier than having to draw lines through the sea. Last month, the English research yacht Forever Changes set sail from Bangor Marina to start counting the summer's basking sharks off Northern Ireland.
It carried the blessings of Dermot Nesbitt, the North's Environment Minister (whose public engagement with nature and wildlife could teach Dublin ministers a thing or two), as well as the support of the Ulster Wildlife Trust and a grant from the Environment and Heritage Service. Nesbitt declared himself delighted with "this exciting study of the basking sharks that use our shared seas".
The study is one notionally confined to Britain. It has protected this enormous fish within the 20-kilometre limit since 1998, has made it a priority species under the UK Biodiversity Action Plan and has even tried to get CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) to monitor the international trade in its bits and pieces (these days, mainly its fins). With such a stake in its conservation, Britain has funded a three-year research project on the species.
The Republic's indifference to the fate of Cetorhinus maximus is made more striking by its history in the island: the catching of the "sunfish" for the oil in its liver, prodigiously off the west coast in the days before paraffin and then much later and very locally, at Keem Bay, Achill, Co Mayo, for more hi-tech uses in the aerospace industry, and in cosmetics. More than 1,000 sharks a year were being netted and harpooned from currachs in the 1950s, but 20 years later only one crew remained.
The decline was blamed on over-fishing of a local population: basking sharks are long-lived with a very slow reproduction rate of one or two per cent a year. But now the cause is thought to lie in a shift of ocean currents and their concentrations of zooplankton, the principal food of Cetorhinus. The great fish cruises slowly at the surface, huge jaw lowered like the ramp of a cargo plane, its gill-rakers straining out tiny copepod crustaceans into what, arriving in the stomach, looks very like tomato ketchup.
A sighting scheme organised by Dr Simon Berrow and his colleagues in the Irish Whale and Dolphin Group, which recruited the help of yachtsmen and fishermen, found an entirely new pattern of distribution in 1993. It counted 425 sharks all round the coast, sometimes in groups of 20 or more, but they were scarce where they had once been plentiful (as at the "Sunfish Bank" off Co Mayo) and abundant in waters off the south-west of Ireland and off the east coast.
One year's sightings can be no more than a sample: last week a group of birders, watching the autumn passage of sooty shearwaters past Kilcummin Head in north Co Mayo, enjoyed the bonus of four basking sharks cruising close under the cliffs.
But a creature once indelibly associated with the western currach (as in O'Flaherty's Man of Aran) is now as likely to be sighted from the deck of a passenger ferry to Britain or France.
Indeed, the Isle of Man is now the creature's hot-spot, and the island's Basking Shark Society (www.isle-of-man.com/interests/shark) operates daily boat tours through the summer. In Cornwall, too, the quirks of plankton fronts carried on ocean eddies have sometimes attracted spectacular numbers - more than 200 sharks gathered to feed in Kennack Bay in 1998, their tall dorsal fins spiking the sea between shore and horizon.
At 10 metres long or more, and the weight of an elephant, the basking shark is the second-biggest fish (the biggest is the tropical whale shark, Rhincodon typus, a plankton-feeder that can reach 12 metres or more). Found on continental shelves in the temperate zones of both hemispheres, it has been hunted, on and off, for a very long time.
Yet almost nothing is known about its breeding ecology - or even where it spends the winter. In our corner of the Atlantic, the sharks begin to arrive "from nowhere" as the plankton blooms in April. They stop feeding in autumn and disappear southwards again, perhaps to wintering grounds off Morocco. There, in one hypothesis, they spend their time "cruising in some energy-efficient mode at great depths beyond the continental margins".
Along with summer counts, Britain's three-year project aims to solve the persistent mystery of the sharks' seasonal movements. In the 1980s, a shark fitted with a radio transmitter was tracked by satellite for 17 days off the west coast of Scotland, and this paved the way for the present study, led by British scientists David Sims and Julian Metcalfe.
Last summer, their team fitted 10 sharks with "pop-up archival transmitting tags", programmed to surface at intervals over a year and beam a signal to a satellite. Five of the sharks were tagged off Plymouth in the English Channel and the rest in the Firth of Clyde, since one of the objectives is to find out if Cetorhinus lives in separate populations. Another 10 sharks are being tagged this summer, and as their locations are tracked, the information will be mapped on the project's website (www.cefas.co.uk/sharks).
The threat to basking sharks has been spasmodic, as global markets for its products have waxed and waned. The worth of the oil in its liver, for example, has fallen away as the livers of more southerly sharks have been found to yield more squalene, a scarce hydrocarbon. The value of a set of fins, (weighing 70 or 80 kilos) has also been erratic. For a period in the 1990s, Norway was exporting the fins to Japan - and throwing the shark-meat back into the sea. But no organised hunt for the fish has left Norway for the past four years, perhaps because of poor catches. A purely arbitrary Norwegian quota for basking sharks fished in EU waters has been steadily reduced and never actually filled.
Thus, the work carried out on Forever Changes and her sister research vessels is dedicated to the precautionary principle. What little we know about the basking shark tells us nothing about its stocks and their population structure, only that reproduction comes late and sparingly and that this alone creates a vulnerable species. It is a pattern the shark shares with many deep-sea fish - orange roughy, for example - about which we are showing no caution at all.