Another Life: Searching for sunfish where jellyfish gather
Climate change and the drift of jellyfish may explain a recent surge in sightings of nature’s biggest bony fish
Satellite studies: the ocean sunfish, or Mola mola. Illustration: Michael Viney
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.