Ocean sunfish are found across tropical and temperate waters and they have been recorded swimming across the length of the Atlantic near countries ranging from Iceland to Chile, according to the Wildlife Trust.
Mat Westfield, Wales strandings co-ordinator at Marine Environmental Monitoring (MEM), said the incident was reported but it was unfortunately too late to analyse why it had washed ashore.
“We have to collect it when its still fresh because, once it starts decomposing, all the information is lost,” he added.
Westfield said they have seen incidents like this rise around Welsh and Cornish waters this summer as a result of the heatwaves that have blighted the UK in recent weeks.
He added: “We seem to be getting a lot of sightings of sunfish by the various tour operators and fishing vessels.”
Despite this, he stressed it was most common to see ocean sunfish in the summer months, as they hunt food.
Westfield said he had also seen more reports of jellyfish sightings in recent months, which the sunfish primarily feeds on.