Love love love this kind of stuff!
Here in Fiji we may be possibly witnessing something similar - at least, that is our working hypothesis although we're still debating about how to test it. Maybe you remember this paper, and this picture.
It is an old graph of Bull Shark sightings.
In essence, it shows how the Bulls disappear at year end for pupping and mating, and how they then return for the Running of the Bulls in late December/January.
But there's also another detail - see it?
Every March/April, an this usually around the Full Moon, we get a notable temporary dip in the number of Bull Sharks. Our hypothesis is that they are being drawn away by some event that is likely linked to predation - and it just so happens that many Fishes like incidentally those Surgeonfish form spawning aggregations around the Full Moon, this because the associated strong tides favor maximum dispersion of the fertilized eggs.
Obviously, so far, this is only a correlation and thus merely speculative - but it is a testable hypothesis and if certain things come to pass like we plan them to, we may soon be nearer to solving that riddle!
So keep watching this space! :)
Anway, real interesting observation from Moorea.