One of the nice features of World of Warcraft is the ability to create custom macros to simplify common tasks in the game. I have a nice little on that you can place on a button for fishing.

What it does: If you do not have a fishing rod equipped, clicking the button will equip one. If a fishing rod is equipped, it will cast. If you hold down any modifier key it will re-equip your weapons.

Code (insert your object names where indicated in italics):

/equip [noequipped:Fishing Poles, nomodifier] fishing rod name;
/equip [modifier] main weapon name;
/cast [equipped:Fishing Poles, nomodifier] Fishing;

For example:

/equip [noequipped:Fishing Poles, nomodifier] Nat Pagle's Extreme Angler FC-5000;
/equip [modifier] Staff of Dark Mending;
/cast [equipped:Fishing Poles, nomodifier] Fishing;

If you are not using two handed weapons, you will need to enter an equip command for your offhand weapon as well:

/equip [noequipped:Fishing Poles, nomodifier] fishing rod name;
/equip [modifier] main weapon name;
/equip [modifier] offhand weapon name;
/cast [equipped:Fishing Poles, nomodifier] Fishing;

Happy fishing!