As for shark activity on the manta dive - I have several hundred dives at the manta night dive both as a captain and a dive guide, and to the best of my knowledge, sharks (other than rays) have never come into the dive site on the manta dive. Other times of the day are a completely different story. There have been times where a monk seal has shown up as well as Hawaiian sting rays too.
Are there sharks in the area? I answer that question on the boat with this bit of sarcasm -- "you test for sharks by sticking your finger in the water for 6 seconds then remove your finger from the water and stick it in your mouth, if it tastes salty then there are sharks in the water". Also, Manta Rays are sharks.
From what I have been hearing, (I'm no scientist), the increase of shark activity off shore of Kona has much more to do with changing ocean currents, increased whale activity, and ocean temperature, rather than the fish farm. I did not say instead of the fish farm. Rarely, if ever, is one factor solely responsible for change. One could also conclude that an increase of shark activity could be directly or indirectly related to more game fish being caught, exhausted, then released, rather than killed and sold. There is no scientific data to back that claim either.
As you can tell by my response, this is a hot topic in Kona.
Is the fish farm responsible for the high number of mantas showing up lately? I doubt that too. Mantas come for the zooplankton, and zooplankton do not feed on the food used by the fish farm. As one of the dive guides that I work with says, "this is a dinner theater, the mantas are here for the dinner and we get the theater".
See you there every Sunday, Monday, and Thursday, nights.