AV observes that there are four types of fish most likely to be lucrative, and it's no accident that three of them (the Musselback Sculpin, the Glacial Salmon, and the Nettlefish) are the main ingredients of the Fish Feast, a valuable (although generalized) raid buff. I spend roughly an hour a week fishing up the materials for these to cover our raids and had never even looked to see what they retailed for. After reading over AV's comments, I took a peek at my server's AH and had a /headdesk moment discovering that my main could have been a much wealthier character if I'd just spent a little time yanking up a few extra fish here and there.
So if your character is at or close to the Northrend fishing cap (and especially if you're still on the lookout for the Sea Turtle, which requires you to fish in Northrend pools anyway), give your AH a look and see how the fish are selling. You might be pleasantly surprised. And, as always, I highly recommend El's Extreme Anglin' as a great resource for the beginning and advanced fishermen out there.