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I've been wondering the same also. We'll be in Bali at the end of April, and are doing at least one day at Manta point. Like Beckerman, I have tried 2 trips to supposed bullet proof locations for Manta's, both times were failures . I read, and I'll look for the link, the other day on a site about Bali area dive sites that Manta's were most prevalent in April/May SO!!!! Here's hoping!!!!!
They are not seasonal in the area : you can see them year round.
Getting to Manta Point & Manta Bay when conditions are right (swell < 1.5m) grants 90% chances to see some manta rays. Chances highly decrease with swell : you can get lucky and see some manta with 2.5m swell but you will be sick on the boat. 100% guaranted.
Swell forecasts (5 days accurate) need to be checked here : Ceningan Surf Report, Surf Forecast and Live Surf Webcams
If you're lucky you can spend an hour surrounded by 20 of them.
But do not get too excited, honestly most of the time you will not be so lucky and may able to see 3 of them from 5 minutes to 1 hour depending of various factors : sea conditions, sun - they seems to love getting a nice tan - number of divers & snorkelers underwater + how they interact with the manta, time of the day (better experience in the morning), ...
Finally if you see some dolphins on the south tip of Nusa Lembongan on the way to Manta Bay / Manta Point, the trip to Manta Point might be not worth the time to get overthere : never saw manta with dolphins in the area. Don't ask me why
So, is this article/website incorrect?
are there any other sites that frequently are visited by Mantas in Bali?