Reef manta rays in New Caledonia dive up to 672 meters deep at night. The first data collected on the diving behavior of reef manta rays in New Caledonia considerably extend the known depth range for this vulnerable species in decline, according to a new study. Source 8c.
I could understand that there may not be enough resources or food for these amount of manta rays but at the same time there's not much options - they can't afford to the deeper waters with predators surrounding in there and the other waters that are also being exploited but by man!