The mammoth sunfish, a species classed as vulnerable and not eaten in Europe, was 3.2 metres long and 2.9 metres wide, Mr Ostale said in an interview on Thursday.
He had been called in to assess the find, a record for the area which, due to tides and sunfish migratory patterns, has no shortage of such encounters.
"We tried to put it on the 1,000kg scale but it was too heavy. It would've broken it," he said.
Mr Ostale heads Seville University's marine biology laboratory in the Spanish enclave of Ceuta, on the north coast of Africa.