To answer Sweetdr0ps, the reason is because the Red Carp drops from Spinefish pools, and of all the useable fish in Pandaria, Spinefish are the LEAST valuable. And yet the pools (and the open water surrounding them) are in places that require the highest skill. So, if you are fishing in open water, there's more of a chance of trash fish instead of good fish, or the Red Carp. And since fewer people farm spinefish, there are fewer Red Carp on the AH, despite having the same drop rate.
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To answer Sweetdr0ps, the reason is because the Red Carp drops from Spinefish pools, and of all the useable fish in Pandaria, Spinefish are the LEAST valuable. And yet the pools (and the open water surrounding them) are in places that require the highest skill. So, if you are fishing in open water, there's more of a chance of trash fish instead of good fish, or the Red Carp. And since fewer people farm spinefish, there are fewer Red Carp on the AH, despite having the same drop rate.