Ocean Sightings As Giant Increase Temperatures Rise Irish Sunfish In Of In Waters

Increase in sightings of giant sunfish in Irish waters as ocean temperatures rise — A Island Off Sunfish Picture: Nick Year Pfeiffer An Mayo, Inishglora, Last Photographed Off

Fri, 14 Jan, 2022 - 07:00

Eoin English

Rising ocean temperatures have caused an enigmatic ocean giant to venture north into Irish waters and beyond, almost 50 years of observations off the Cork coast have helped confirm.

A study by a team of researchers from Ireland and the UK has found that ocean sunfish — the largest bony fish in the world — expanded their range in the 1990s and 2000s by more than 200km north.

The study, which was published yesterday in the international peer-reviewed journal Marine Biology, analysed sightings of sunfish made from the Cape Clear coastal bird observatory by thousands of experienced bird watchers over 4,600 days of sea watching.

The study’s lead author, Dr Olga Lyashevska, from Galway Mayo Institute of Technology, said the research team found there was a general absence of sunfish sightings before 1989, with three to four sightings annually after that, up until 1999.

But from 2002 to 2007, there was a five-fold increase in sunfish sightings, up to 21.3 per annum, she said.

Diving instructor Jane Sharp in September 2003 with the gigantic sunfish they found basking on the surface of Tralee Bay. Picture: Ronnie Fitzgibbon/Provision

The team noted the increase “was significantly correlated” with the position of the 13C isotherm, a line of equal sea surface temperature on ocean charts, which moved north by over 200km in that time.

The increase was linked with warming seas and coincided with observed increases in other species such as sea bass and plankton.

Senior study author, Dr Tom Doyle, from UCC, said not having any information on sunfish abundance is like not knowing how many zebra migrate through the Serengeti.

“For a long time now we have suspected that sunfish have increased in Irish waters, but it is only now, after retrospectively analysing 47 years of coastal sightings from the Cape Clear bird observatory, that we can say for sure that sunfish increased in the 1990s and 2000s,” he said.

The sunfish is very recognisable because of its unusual flattened shape and size. Measuring up to 3.2m long and weighing as much as 2.3 tonnes, they are found in all temperate and tropical waters and seasonally migrate to warmer waters. As adults, they feed almost exclusively on jellyfish.

They are excellent swimmers, capable of long-distance migrations and deep dives, and they are easily spotted when they swim on their side at the ocean’s surface — known as "basking".

Steve Wing, the head warden at Birdwatch Ireland’s Cape Clear bird observatory, said the study has demonstrated how sustained citizen science projects can provide unique insights on the historical abundance of enigmatic species.

For five decades, bird watchers have come to Cape Clear and made observations of bird migrations and other animals including sunfish, sea turtles, and basking sharks,” he said.

“Without this sustained effort and incredible expertise, we wouldn’t have the insights we now have."

The study was supported by the EPA.