It's just an anecdote, but
When I was working with the White Marline Porpoise Circus in Port Aransas, Texas, I watched a dolphin there, named Pete (a bottlenose from Florida) do a similar thing. There was a pelican that would steal his fish if we threw them in the wrong direction, so it seemed Pete was tired of this. One day between shows, we noticed about 8 fish, about 2-6 inches under the surface, in a circular ring, fairly evenly spaced. As they would sink, Pete went around to each one, pushing each one in turn, to the surface. The pelican appeared interested and wary. After about 10 minutes of this, the pelican flew and dove for one of the fish -- Pete grabbed him, and took him to the bottom and drowned him. First time I had thought of this as "tool use". This shows that the behavior occurs in tursiops as well.
The author seems to be an unusual person as well (and a PhD).