So what can you catch from them?

Dunno, I've never tried fishing in one. So what bait would you use? Maybe this?

THE CRABFISH

Oh, there was a little man and he had a little horse
And he saddled it, bridled it and threw his leg across.
Mister raddle-um, faddle-um
Oh, Mister raddle-um, faddle-um a-day.

He rode, he rode till he came to brook
And there he saw a fisherman, a-fishing with his hook.

"Fisherman, oh fisherman, oh fisherman," says he,
"Have you got a lobster that you can sell to me?"

"No sir, no sir, no sir," says he,
"But I've got a crabfish that I will sell to thee."

Well he grabbed the crabfish by the backbone,
Put it o'er his shoulder and galloped off home.

Well when he got home he couldn't find a dish,
So he put it in the pot where his missus used to pish.

In the middle of the night, she got up to squat
And the crabfish grabbed her by the glory-be-to-God!

"Oh, husband, oh husband, oh husband come hither,
The devil's in the pish pot and got me on his tether."

Well, she grabbed the brush and he grabbed the broom
And the beat the little crabfish all around the room.

They beat him on the head and they beat him in the side
And they beat him in the bollix till the poor old bugger died.

Now the moral of this story is very plain to see:
Take a good look in the pish pot before you take a pee.