Cute, but it kinda looks like he's sitting on a big wooly mammoth or something.

My first thought was a pile of poo.

But several comments:
I think this is cute, but the perspective seems off... if he/she is sitting on a rock on the side of the lake/river, fishing directly below seems odd. Also, there seems to be a shiny window type reflection in the hat, which throws me off a bit.
Finally, I would stick with one gimmick... I love the catfish (and wish they were a little bigger, infact), but the computer mouse is a bit much. It is almost distracting. My advice would be to change it to a regular mouse.

I think you have a good beginning and I would really like to see it reworked and resubmitted. Good luck!

^ Agree with SemantiTheft all the way, except change the mouse for a ball of yarn.

Very good idea. But ugly tshirt. :/

Double agree with SemantiTheft

Maybe just the catfish itself? it works as a lone image. laying it out everywhere is sort of distracting.

thought he was sitting on a walrus...

catfish by itself, definitely! Possibly with a mouse-shaped lure in front of it without the guy and the island. But not a computer mouse. A real mouse. The computer mouse is just a visual pun with no payoff.

lol love the idea but i don't really like the way it's drawn

Personally i feel lack of focus.

The catfish on the top left is stealing attention from the boy.

Your eyes tends to bounce top left, top right and bottom.
Lack of center of focus.

But probably thatz me.