From what I hear and see, manta sightings have not significantly changed. They are up and down, one year better than the other, one year a bit earlier than the other. Whale sharks sightings have decreased, especially at Similans and Surin national parks. I would say in the Hin Daeng area it has not changed so much, with sightings year round. 10 years and longer ago sightings at Richelieu Rock and Koh Tachai were in February-March-April reasonably common, these days they unfortunately very much are an exception.