First, here are some basics:
– Fishing loot tables are assigned to zones and subzones, not to bodies of water. For example, there are four different loot tables distributed among five different parts of Southfury River, depending on whether a particular part of the river is considered part of Durotar, the Northern Barrens, Azshara, Ashenvale, or Winterspring.
– The fishing loot table that you get is determined by where your bobber lands, NOT where you’re standing! This makes it very easy to confuse addons like Fishing Buddy, which can only look at where you’re standing.
– The drop rates for some fish can change throughout the day and/or throughout the year. The most well-known examples are Nightfins, Sunscales, Summer Bass, and Winter Squid, but drop rates have also been seen to fluctuate for Redgills, Spotted Yellowfins, Stonescale Eels, Electric Eels, and Glossy Mightfish.
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For the most part, fishing loot tables in Azeroth are divided into Freshwater and Saltwater categories, and into six “tiers” that correspond to the level range of the zone in question. Thus, there’s a Freshwater 1-10 table, a Saltwater 1-10 table, a Freshwater 10-20 table, a Saltwater 10-20 table, et cetera. However, there are also some zones and subzones with more highly specialized loot tables, such as the Darkshore coastline and the Sludge Fen in the Northern Barrens. Fish with fluctuating drop rates are exclusively or almost exclusively found in the level 40+ tiers.