To catch the Mullet in Ice Fishing, you need to have a Yellow or Gray Fish on your hook. When it comes, put the fish in front of the Mullet. It will eat it, and you will get extra coins. It does not work if you only have a worm.
There is a strategy to make sure you still have a yellow fish left to use as bait. As soon as you see the Mullet in the background, keep the fish on a hook. As soon as a new fish appears, place the fish currently on the hook onto your pile, and repeat until no fish are left. This way, you can ensure you have a fish on your hook.
In Club Penguin: Elite Penguin Force, you try and catch the Mullet without getting any squids, and you use the big Fluffy to get the Mullet.