Aloha Keith,
I've been noticing this for years off the Waianae coast of Oahu. Insularum
seem to "herd" from Oct-Jan. I know they migrate from deeper waters. This
was surmised by the thick orange sponge growth. The longer the egg-laying
process continues the external sponge covering starts dissolving from
exposure to higher levels of UV light. By the time they start to leave no
sponge is evident. This species prefer the first major reef drops
connecting to open sandy streaches that drop fast to deeper waters(usually
70-90ft). Divers off Kauai have noticed this as well. Sometimes you might
chance upon a dead Porites coral head covered in purple egg clusters.
Means the mamas have already left. It's good not to reveal these locations
as insularum tend to frequent the same spots as the previous years. Last
dive partner i showed this to wiped-out the whole massive colony.  CHRIS:)

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