WoWDB

Items and Rare Fish by Zone

Highmountain

 Funky Sea Snail (1) >  Salmon Lure

 Salmon Lure >  Ancient Highmountain Salmon*

 Frost Worm >  Coldriver Carp*

 Swollen Murloc Egg (2) >  Mountain Puffer*

Val'sharah

 Rotten Fishbone (3) >  Ancient Mossgill*

 Nightmare Nightcrawler >  Terrorfin*

 Drowned Thistleleaf (4) >  Thorned Flounder*

Azsuna

 Skrog Toenail (5) >  Aromatic Murloc Slime

 Aromatic Murloc Slime >  Leyshimmer Blenny*

 Pearlescent Conch >  Nar'thalas Hermit*

 Rusty Queenfish Brooch (6) >  Ghostly Queenfish*

Suramar

 Enchanted Lure >  Magic-Eater Frog*

 Demonic Detritus >  Tainted Runescale Koi*

 Sleeping Murloc (7) >  Seerspine Puffer*

Stormheim

 Moosehorn Hook (8) >  Silverscale Minnow +  Soggy Drakescale

 Silverscale Minnow >  Thundering Stormray*

 Ancient Vrykul Ring (9) >  Oodelfjisk*

 Soggy Drakescale >  Graybelly Lobster*

All Coastal Legion Waters

 Message in a Beer Bottle (10) >  Axefish Lure

 Axefish Lure >  Axefish*

 Stunned, Angry Shark (11) >  Seabottom Squid*

 Decayed Whale Blubber (12) >  Ravenous Fly

 Ravenous Fly >  Ancient Black Barracuda*

Notes:
* Indicates a Rare fish
(1)  Funky Sea Snail is an item that will summon a Bitestone Fishbrul that drops a  Salmon Lure when killed.
(2)  Swollen Murloc Egg is an item that will spawn a Murloc NPC, Swamprock Tadpole, that grants the buff Blessing of the Murlocs, increasing your chance to fish up  Mountain Puffer.
(3)  Rotten Fishbone is an item that will attract a Druid NPC (Lorlathil Druid) that casts the The Cat's Meow buff on you, increasing your chance to fish up  Ancient Mossgill.
(4)  Drowned Thistleleaf is an item that will grant the buff Blessing of the Thistleleaf, increasing your chance to fish up  Thorned Flounder.
(5)  Skrog Toenail is an item that will summon a Salteye Skrog-Hunter that drops a  Aromatic Murloc Slime when killed.
(6)  Rusty Queenfish Brooch is an item that is required to see  Ghostly Queenfish pools in Azsuna.
(7) Using  Sleeping Murloc will awaken a Confused Seerspine Murloc, who will run around briefly and drop some  Seerspine Puffer into the water.
(8)  Moosehorn Hook is an item that increases your chances to catch rare bait in Stormheim, rather than fish.
(9)  Ancient Vrykul Ring is an item that is required to see  Oodelfjisk pools in Stormheim.
(10)  Message in a Beer Bottle contains  Axefish Lure when opened.
(11) Using  Stunned, Angry Shark will spawn a Landlocked Shark, which will yield  Seabottom Squid when killed. Using it on land makes it much less dangerous than in water (hits quite hard).
(12)  Decayed Whale Blubber is an item that attracts flies -  Ravenous Fly can be looted from these.

My personal experience with this achievement.

I've been reading in various places about the amount of fishing required for this achievement - guides say ~12 hours of fishing, and various posts say hundreds of casts/hours spent on a single rare.

If I had to give only one piece of golden advice: DO NOT FISH FROM OPEN WATER!

I obtained this achievement in a fraction of the time that has been previously noted when fishing from open water. Below I've listed the steps I took.

1. You need to obtain some  Arcane Lures. These are obtained from Conjurer Margoss on a floating island north of Dalaran. See my comment here for further details on getting there and more.

   • If you join a fishing group in group finder (try searching "fish", "margoss", or "drowned mana") you will net  Drowned Mana very quickly in order to buy  Arcane Lures.
   • It's hard to say how many you will need; I would estimate about 5 per zone, but your mileage may vary. Any extras you have can be sold on the Auction House, or can be very useful for unlocking traits on  Underlight Angler by catching rare fish for Artifact Power.
2. Using the chart below, you want to try and net all the lures you need for each fish. In each zone, find areas of water (usually streams or lakes) that have fish pools in them.

   • The ideal way to tackle this is to scout out multiple pools before you pop your lure, if possible. That way, after you have exhausted one pool you can head to the next.
   • In my experience, even if you need to trek quite a ways to find another pool, it is worth it! One may think that it is a waste of the time on the  Arcane Lure buff, but even factoring in travel time, the catch rate of lures from pools is so much higher, it is still worthwhile.
3. As soon as you get a lure for a rare fish you haven't caught yet, save it until you find another pool (or even better, multiple pools that are close to each other). This is simply so you can have a chance for fishing up more lures, with your  Arcane Lure active.

   • Since most of these lure buffs are only 2 minutes, once a pool is depleted simply keep fishing from open water until it expires rather than go searching for another pool (unless you know another pool is very close by).
   • It's ok, and even ideal, to have both a rare fish lure and the  Arcane Lure active at the same time - the buffs do not negate each other.

I hope this advice is helpful to you! My take home point is, fish from pools as much as possible - it saves a HUGE amount of time! The added bonus is that you'll end up with a lot of useful fish that can be sold on the Auction House or used in Cooking recipes.

If you are in not the biggest hurry (or even if you are) I find a great strategy is to set out to do World Quests, and fish from any pools you come across. You can cover most of the rare fish this way.

Bigger Fish to Fry is a requirement to obtain the Fishing artifact - the  Underlight Angler!

There are a number of Rare fish in Legion, and items (bait or otherwise) that will help you catch them.
The Rare fish and items to catch them are zone specific (although some are obtainable on any shoreline of the Broken Isles).

Rare Fish

There are 18 rare fish coming in Legion that can be fished up. Each rare fish grants +5 to Fishing skill when used, consuming it in the process.

• There are 3 Rare fish per zone, and 3 that can be caught on any coastal waters.
• These can be fished up from a Fishing skill of 1, so this can be an effective way to power level Fishing.
• Catching one of the rare fish listed below while fishing will grant the achievement A Cast Above the Rest. Catching them all will grant the achievement Bigger Fish to Fry.
• All Rare fish are Bind on Pickup.
• If you are fishing with the  Underlight Angler equipped, you will obtain a DIFFERENT version of each rare fish that grants Artifact Power instead of Fishing skill (e.g. you would fish up  Coldriver Carp instead of  Coldriver Carp)

  • Note that this does not retroactively apply to rare fish you obtained before getting the Artifact - the only use for these short of maxing fishing skill is vendoring for a decent chunk of change.
• The Rare fish in the chart below are marked with a star (*)

Fishing Bait and Items

Some of the items that help you catch Rare fish will spawn and NPC that grants a fishing buff; others contain bait, or spawn NPCs that drop bait.

• All fishing bait buffs last for 2 minutes once used (except for  Rusty Queenfish Brooch and  Ancient Vrykul Ring, which last 5 minutes each).
• All buffs last the full duration, except for Moosehorn Hook, which expires as soon as you catch a bait.
• Almost all baits can be bought/sold on the Auction House. Use this if you are having trouble with a particular fish!

  • The exception to this is the  Stunned, Angry Shark, which has a 1 minute duration in your inventory.
• All baits are Unique, meaning you can only carry one. The further caveat to this is that you WILL NOT catch any bait that is currently in your inventory!

  • This can be used to your advantage if you are looking for a particular bait in a zone - by having certain baits in your bags, you will ensure you do not catch them again.
  • If you are wanting to obtain multiples of the same bait, you can put baits on the Auction House, mail them to an alt, or put them into a Guild Bank or Guild Bank Tab that only you have access to. This will allow you to catch the same bait again.

    • A portable mailbox like  MOLL-E (Engineering) or  GUMM-E (from the Draenor garrison Engineering Workshop) is a huge boon when doing this out in the field!
  • It is worthwhile saving some rare fish bait, in order to level Artifact Power on your  Underlight Angler if and when you get it.

Each zones baits are now purchaseable from the fisher friends, for their respective currencies, with Impus selling the coastal baits.