This achievement has 3 great guides already. I'm writing an updated guide that covers everything you can do to speed up this achievement, including some items added after prior guides were published. [Current through U39 Base-Game Patch with Necrom companion bonus.]
This achievement is tied to the in-game Master Angler achievement -- found in the menu under Journal > Achievements > Exploration > Fishing section (very bottom) -- which cascades to all other base game fishing achievements
except Craglorn Angler. When fishing, you have a small chance of receiving a rare fish (name in green or blue), which will then be checked off that zone's Angler achievement in this section.
So the name of the game is: How do we maximize the chance of a rare fish, and speed up this achievement?
=== RARE FISH CHANCE BONUSES ===Over the years, ESO has added many ways to increase your chance of catching rare fish. All of these stack, so the more you implement, the better your chances of catching rare fish. I finished this achievement fishing solo with bonuses 1-3, and saw a notable increase in efficiency from when I was only using one or two bonuses at a time.
The full list of bonuses is:
[1] Rare BaitFishing requires bait, and you will pick up common bait (Crawlers, Insect Parts, Guts, and Worms) as you play the game. When fishing, you may catch a rare bait that is used in another type of water. Using rare bait will give you a rare fish bonus. The rare bait are:
• Chub: Caught in foul water, bait for saltwater.
• Fish Roe: Caught in river water, bait for foul water.
• Minnow: Caught in saltwater, bait for lake water.
• Shad: Caught in lake water, bait for river water.
Always use this bait first when you have it, preferably stacked with other bonuses below.
[2] Artaeum Pickled Fish BowlThis is a food that, alongside stat boosts, grants you a bonus to catching rare fish as if you were fishing with someone else. The effect lasts for 2 hours. If you want to make your own, the recipe can be found while fishing in Artaeum, rarely inside a Waterlogged Psijic Satchel, and requires 5x fish, 3x rice, 3x clam gall, and 3x honey to prepare. Between the fishing requirement for the recipe, the rarity of the satchels and the recipe in the satchel, and the rarity of clam gall, I found it much easier to just buy the food from a guild seller. They tend to go for about 3500 gold each.
To maximize the value of this food, make sure you can devote 2 hours of in-game time to fishing. The timer will pause when you sign out, but will keep running if you decide to do something else in game. As Invincible666 points out in a comment on the top guide, you can add 50 minutes of run time for the fish bowl by maxing out the Provisioning passive ability
Gourmand (+20 mins) and the CP bonus
Rationer (+30 mins).
[3] Champion Point BonusesIn Update 29, released in early 2021, the champion point system was overhauled to three separate constellations: Craft, Warfare, and Fitness. The Craft constellation contains a fishing bonus.
At CP435 with 145 Craft points, you can unlock
Anger's Instinct which will give a bonus to your rare fish chance:
• 10 points into Breakfall.
• 15 points into Wanderer.
• 10 points into Steadfast Enchantment.
• 10 points into Rationer. (Can go up to 30 points to increase active time for Artaeum Pickled Fish Bowl.)
• 75 points into Liquid Efficiency.
• 25 points into Angler's Instinct.
I recommend waiting until CP585 with 195 total Craft points to tackle this achievement, so you can also unlock
Reel Technique for an additional 50 points (it's just to the left of Angler's Instinct). Reel Technique reduces the amount of time it takes a fish to bite by 25%, which will add up significantly while trying to complete this achievement.
[4] Fishing With Companion Sharp-as-NightAll companions come with a perk that is active when they are with you. This perk becomes permanent when the companions hit level 20.
In Update 38 (Necrom), two new companions were added, including Sharp-as-Night. Sharp's recruitment quest can be found just west of the entrance to Necrom in Telvanni Penninsula. His perk is Sharp's Patience, which will occasionally replace your catch with a trophy fish from the current zone (you'll see the companion perk icon in the loot notification). Once Sharp hits level 20, you will receive the Upgrade item Sharp's Handmade Fishing Lure, which grants this perk permanently for all characters in your account on the current megaserver.
[5] Fishing With Other PlayersYou also get a rare fish chance bonus for each other player that is fishing at a particular fishing hole.
Whether this bonus actually saves time is debatable, and personally I don't recommend it. Each fishing hole has the same number of total fish (14-16), before becoming inactive until respawn. Fishing solo, you get 14-16 chances all to yourself. With one partner, only 7-8. With two partners, 4-5. So even if you get rare fish more often, you will inevitably spend more time running from one fishing hole to the next to get the same number of chances. If I found myself fishing with someone else sometimes, great -- but I don't recommend planning fishing meetups unless you really want to go after this with a friend.
=== FISHING EFFICIENTLY ===Once we have all of our bonuses stacked, the name of the game is to go as quickly as possible, and maximize your time with the Artaeum Pickled Fish Bowl in effect.
I recommend using a ESO Fishing Map site (for example:
https://fishingintamriel.com/fishing-maps-for-eso/) and planning the fishing holes you want to hit up in four zones at a time. This way, you aren't wasting time waiting for a fishing hole to respawn if it's not there -- just move on to the next zone. Also, make sure you're rotating between water types if possible, to keep a steady intake of rare bait across all types.
Coldharbour should
always be in rotation until you've unlocked Coldharbour Angler. This zone is particularly hard to finish, since all water here is foul water; each time you catch a rare fish, there are 12 options you might get (vs. 3-4 in other zones). It's common for folks to get stuck on 1-2 fish in Coldharbour for a long time -- I went through 300+ crawlers, even with bonuses 2 and 3 stacked, to get my last 2 -- so keep it in rotation, fish if the spots are available, then go on to whatever other zones you're working on in parallel.
Even with the new bonuses, this achievement will still take time, but it's a lot more approachable now than it was in the early days of the game. Good luck, and may the RNG be in your favor!