Another Life: Searching for sunfish where jellyfish gather

Where I'd quite like to be just now, to flee the hillside midges, is out there on a breezy island clifftop, couched among cushions of sea pinks and gazing down into a slow, crystal swirl of sea. Fulmars on stiff wings glide into stained-glass shadows, all Harry Clarke blues and greens. A grey seal spirals up from the depths to point its nose to the sky. And – who knows? – just once I might see Mola mola, that huge, improbable disc of a fish, flexing its dorsal fin through the swell or tilting itself, like a mirror, to the sun.

The biggest bony fish in the world (whale sharks are all cartilage), the ocean sunfish seems one big head, with long fins at top and bottom to act as paddles and a stiff frill of skin where its tail should be. Although many of those reaching Ireland in summer are smaller specimens of less than a metre across, one of the largest ever recorded, by divers in Brandon Bay, Co Kerry, in September 2003, was some 3.6 metres long.

Mola mola is far from a new southern migrant to Ireland, but climate change and the drift of jellyfish – a favourite if far from only food – may explain a recent surge in sightings. Analysing records since 1689, a Dingle marine biologist, Declan Quigley, found that more than two-thirds of sightings were in the past two decades, particularly during July, August and September, and mainly from the west and south coasts.

Tralee Bay and the north coast of the Dingle Peninsula seem to have a special draw for them: up to 20 have been seen in one day. Marine scientists now regard the bay as a possible nursery ground for the species, like those known in Bali, in Indonesia, and off the coast of California. Sunfish are notably the most fertile of all vertebrate animals, with one female containing 300 million eggs. Their rate of growth is colossal: for a 25mm larva to grow into a three-metre adult needs an increase in mass of 60 million times.

What suddenly brought the species to close scientific notice was partly its appetite for jellyfish – indeed, along with the leatherback turtle, as a leading predator on the adult medusae – and partly the great increase in its numbers as a bycatch in surface drift nets.

Long seen as a largely inactive fish, drifting passively at or near the surface, the sunfish actually travels fast enough on the ocean to be comparable to sharks or ocean-going salmon. Satellite tagging has shown that it is also much more vertically mobile than anyone had known.

In its forays down to deep ocean, to hunt small fish and zooplankton, the neutral buoyancy of its body, lined with unyielding gelatinous tissue, makes up for the lack of a swim bladder to cope with changing water pressures.

The first satellite tagging of sunfish in the northeast Atlantic took place off Dingle in 2007. An international expedition was organised by Dr Tom Doyle of University College Cork, who has been working for years on jellyfish and their predators, together with Dr Jon Houghton of the University of Wales at Swansea. (Their Ecojel website is at jellyfish.ie.) They used harnessing techniques developed in the same waters to tag leatherback turtles, the other prime munchers of jellyfish – if with a far bigger bite.

Just how the sunfish devours a jellyfish of any size is still somewhat mysterious: its small and prissy mouth looks scarcely fit for more than nibbling. Inside, its teeth are fused into a little beak of bony plates – all that remained of one sunfish, indeed, recovered from the stomach of a dead killer whale washed up at Doohoma, Co Mayo, in 2010.

The wider appetite of ocean sunfish, from seabed crustaceans and molluscs to midwater fishes and copepods, is further borne out by Quigley’s observation that nine of the Irish fish recorded, weighing between 9kg and 172kg, have been caught on rod and line in coastal waters.

Nonetheless, the main pattern to emerge from the Kerry satellite tagging of three sunfish was extensive travel through the water column, "in both horizontal and vertical dimensions, presumably in search of their patchily-distributed jellyfish prey". Among the jellyfish reported so far this summer have been more than 100 barrel jellyfish, Rhizostoma pulmo, counted by reader Des O'Hara on Duncannon beach, Co Wexford, the biggest 65cm across.

Also known as Rhizostoma octopus, it is the meaty species that brings leatherback turtles across the Atlantic to particular bays of Irish Sea coasts where the jellyfish sometimes crowd densely into tens of square kilometres. The turtles seem, however, to have no competition from Mola mola. Two years of aerial surveys by the Ecojel team over the Irish Sea and the Celtic Sea spotted 68 Mola mola and quite separate drifts of Rhizostoma. Perhaps the sunfish are wary of biting off more than they can chew.