While working my way through leveling fisher, I've been keeping track of important fish to keep that can be used to gain exp or currency. I thought i'd share the list for others to use too. Its handy to know which fish have a use, even if you aren't up to that level yet, so you can save them for when they are needed, or you can sell them on the MB to people who want to buy their exp.
Fishing Class Quests
lvl 53 - 3x Icepick HQ (Unfrozen Pond, South Banepool)
lvl 55 - 3x Shadowhisker HQ (Hundred Throes)
lvl 58 - 1x Letter Puffer HQ (Landlord Colony)
lvl 60 - 1x Catkiller HQ (Eddies)
Used for Leves;
lvl 50 - Coerthan Crab (Riversmeet, Clearpool)
lvl 50 - Lake Urchin (South & West Banepool Ashpool)
lvl 50 - Blueclaw Shrimp (Hundred Throes, Iron Feast, Whilom River, Smoldering Wastes, Quickspil Delta)
lvl 52 - Three-lip Carp (Whilom River, Smoldering Wastes)
lvl 52 - Grass Carp (Hundred Throes)
lvl 52 - Cloudfish (Vor Sian Siran, Eddies, Eil Tohm, Weston Waters)
lvl 54 - Mahu Wai (Cloudtop)
lvl 54 - Blue Cloud Coral (Cloudtop)
lvl 54 - Bouyant Oviforms (Cloudtop, Blue Window)
lvl 56 - Pipira Pira (Hundred Throes, Iron Feast, Whilom River)
lvl 56 - Hundred Fin (Waston Waters)
lvl 56 - Brown Bolo (Tharl Oom Khash)
lvl 58 - Fountfish (Delta Quadrant, Pappus Tree)
lvl 58 - Yalm Lobster (Eddies, Quickspill Delta, Upper Thaliak River)
lvl 58 - Bullwhips (Alpha Quadrant, Aetherochemical Spill)
Collectibles:
lvl 51 - Glacier Core 310+ (Greytail Falls)
lvl 51 - Ice Pick 106+ (Unfrozen Pond, South Banepool)
lvl 52 - Whilom Catfish 459+ (Hundred Throes, Whilom River, Smouldering Wastes)
lvl 53 - Sorcerer Fish 646+ (Unfrozen Pond)
lvl 54 - Bubble Eye 162+ (Voor Sian Siran)
lvl 55 - Dravanian Squeaker 158+ (Weston Waters)
lvl 56 - Warmwater Bichir 683+ (Smouldering Wastes)
lvl 57 - Noontide Oscar 258+ (Eil Tohm)
lvl 58 - Weston Bowfin 376+ (Weston Waters)
lvl 59 - Illuminati Perch 826+ (Thaliak River)
lvl ?? - Manasail ???+ (Tharl Oom Khash)
lvl ?? - Pteranodon ???+ (Sohm Al Summit)
lvl ?? - Caiman ???+ (Voor Sian Siran, Iron Feast)
lvl ?? - Tupaxuara ???+ (Habisphere)
GC Provisioning Missions
Cloud Coral (Greytail Falls)
Seema Patrician (South Banepool)
Ammonite (South Banepool)
Mercy Staff (West Banepool)
Bubble Eye (Voor Sian Siran)
Marble Oscar (Smouldering Wastes)
Gobbie Mask (Sohm Al Summit)
Rudderfish (Churning Mists)
Black Magefish (West Mourn)
Cobrafish (Tharl Oom Khash)
Vampiric Tapestry (Mok Oogl Island)
Used in Crafting
Blueclaw Shrimp (Hundred Throes, Iron Feast, Whilom River, Smoldering Wastes, Quickspil Delta)
Coerthan Crab (Riversmeet, Clearpool)
Rock Mussel (Whilom River, Smoldering Wastes)
Spring Urchin (Dragonspit)
Lake Urchin (South & West Banepool Ashpool)
Unidentified Flying Biomass (Habisphere)
Fanged Clam (West Banepool, Ashpool)
Pipira Pira (Hundred Throes, Iron Feast, Whilom River)
Sweetfish (Upper, Middle and Thaliak River, Greensward)
Seema Patrician (South Banepool)
Mogpom (Greensward)
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I'm sure there might be more fish that are collectible that haven't been found or might need the Folklore books to access. The highest level ones (Pteranodon, Tupaxuara, Caiman etc) can be broken down into Dawnborne, Duskborne and Seaborne Aethersand using Aetherial Reduction. However, they may also be potential turn ins for Red Gathering scrips, so i've been keeping mine for now.
There are also probably more fish that can be turned in at your GC, but since I'm lvl 60 now I don't know what the other fish on the roster for lower levels might be. If you have any fish you can add to the list, please do so.