I do not believe they migrated deep. I think we have hunted them out on the upper portions of the southern reef and over the last year we have now put a major dent in the population in the 110' - 150' range, now that that area is being hunted hard. I was on a ceviche run a week back in an area of the wall that normally produces well. Hit it long and hard to 150' and found 2 medium and 2 dinks, it had been picked pretty clean. A friend who is a DM found a cave at 160' with a nest in it and fill a zoo keeper 3/4 full. On two previous dives over the wall that day I found very few down to 150', these same areas were the go to areas only a year back.
When left alone I believe they mature due to a lack of any other predators but I think they tend to stay close to a specific area, when there is migration I would think it is due to a lack of food and they would migrate upwards to the higher food density. This explains the occasional mature fish I see in calm currents in the open around 100' between the top of the wall and the reef itself.