There's a warning here, though. The first cited sentence says "no dumber than" and often "just as intelligent". I'm not sure about the author. But it looks as if measuring "intelligence" is a function of figuring out how to communicate with them. I thought this was fascinating.

"There is a lot of evidence now that fish are no dumber than birds or many mammals - and in many cases they are just as intelligent.
"Many fishes - such as minnows, sticklebacks, and guppies - are capable of the same intellectual feats as, for example, rats or mice.
"They can learn their way around mazes, they can learn to recognise other fish, and they can remember which individuals are better competitors.
"According to research at Plymouth University, goldfish have a memory span of up to three months - and can even tell the time."