Changes in fish diversity with depth

Over the past three days we have had the opportunity to dive parts of a single reef that all lie at different depths. At 90 feet, the reef is a vertical wall of coral. The wall is made up of various Montastrea spp. corals.  Generally, small fish are absent at this depth and larger fish are present.  Oddly enough on our most recent deep dive we found mainly smaller fish.  At 60 feet, the reef is the top of the wall.  Here, fish diversity and abundance was greater than at 90 feet. At 30 feet is the very top of the reef, where sand meets the edge of the reef, and the fish abundance and diversity is greatest. The smaller fish are much more abundant than larger fish.  Also, most of the juveniles are found at around 30 feet.