How many trout do these diving birds really eat?

[Gollum11]

I don't know if they are gannets or cormorants, but I saw 3 of them at Milton lake yesterday and word must spread quickly because I counted over 20 there today! That is insane. I always remember seeing a few this time of year, but never in numbers like this. Can they eat a fish a day? If so they can really put a dent on a small lake in a hurry. Was just wondering if anyone else has noticed similar numbers on other bodies of water.