When there is a flock of birds circling and diving down into the water it usually means there is a school of fish around. This may also mean that there may be a few dolphins around as well. Dolphins and birds often fish together.
I regularly see cormorants and Pelicans following the dolphins as they fish along the rocks and often see the cheeky Seagulls swooping in to steel a small garfish that a young dolphin is chasing.
Dolphins will often chase the cormorants and seagulls as well. They see them sitting on the surface of the water and will often sneak up behind them or jump near them. This appears to be a game that the young ones particularly like.