This is also a side effect of the new fishing dailies as well. As fishers head to places like Wintergrasp (terrorfish daily), Nettlefish, Salmon, and Sculpin supply has exploded. The Monsterbelly, formerly one of the best selling fish on the market, has dropped through the floor. You might in some cases even be better off vending some fish, depending on how flooded your market is, or opening up a bank alt, loading it up with frostweave bags, and stocking for later.
Take a look at this chart from WoWEcon of the Monsterbelly market, as marked. Once patch 3.1 hit, the market tanked into oblivion.
The only thing propping up the fish markets in any way right now is the fact that feasts are tradeable and Ulduar's running, meaning that raids are using Fish Feasts. Once a sizeable number of guilds finish exploring Ulduar and get it on farm, expect the market for fish feast mats to degrade further.