Navyth is a fishing enthusiast, and as such, you can expect all his quests to revolve around fishing. Find him, catch some local fish he's got an interest in and he'll reward you with some sort of fishing gear.
|Alstor Slough||Complete Chapter 2||1,500 EXP, Knife T. Tonberry|
You'll find Navyth for the first time in Duscae, so you'll have to complete Chapter 2's penultimate quest, "Declaration of War". Search for him at the Neeglyss Pond south of the Coernix Station - Alstor, near the Neeglyss Pond Fishing Spot, where he'll challenge you to catch a Crag Barrmundi from the nearby pond. You can catch them at the Neeglyss Pond Fishing Spot, just use the Sweet Jamming: Custard lure and aim for the yellow spots, which indicate game fish.
|River Wnnath Fishing Spot||Finish "Fishing Buddies"||2,000 EXP, Butterfly Edge|
Make your way to the Wennath Riverhead Parking Spot (south-west of Lestallum) and park the Regalia, then head south down some stairs to reach the wilderness by the Wennath River. Follow the river north to find River Wennath Fishing Spot, near which you'll find Navyth. This time he challenges you to catch a Cherrycomb Trout. The catch (pun intended)? They only bite at dawn (5:00 - 8:00 hours) and dusk (16:00 - 19:00 hours). Wait around until the appointed hours, then go fishing. Cherrycomb Trout provide edible materials when caught, so aim for the flashing yellow dots and use the Whiskers: Crystal lure. Once you have one, return to Navyth and he'll reward you with the Butterfly Edge - a superior rod to even the Hell Blaster, if you upgraded earlier.
|The Vesperpool - West Bank Fishing Spot||Finish "Fishing, Naturally", Complete Chapter 7||3,000 EXP, Invincible Iron Giant|
Navyth's next quest is a way off, so you'll need to finish Chapter 7, and Chapter 8's obligatory starting quest "A Precious Source of Power" to continue Navyth's questline. When you have access to The Vesperpool again, search the southern shore of that lake (between the Myrlwood dugenon and the Capitis Haven Campground) to find The Vesperpool - West Bank Fishing Spot, near which you'll find Navyth. This time he wants you to catch a Vesper Gar, a somewhat more difficult fish than the last two you went after.
To catch this fist, using Super Baleen line at the least is suggested. As for lures, Stinker: Malboro and Stinker: Malbodoom both worth well. Unlike many difficult fish, this one doesn't have a great amount of stamina, but it degrades line durability very quickly. As said earlier, aim for flashing yellow dots and use the aforementioned materials and you should haul one out of the water soon enough.
|Galdin Quay - Vannath Coast Fishing Spot||Finish "Navyth's Challenge"||5,000 EXP, Tranquility|
The last quest in Navyth's questline has you fishing for the "Devil of the Cygillan", and the more prepared you are, the easier this will be. Consider raising your Fishing skill to level ten and make sure you have at least the Death Spin rod and Nereid reel. Ideally, however, you'll save this quest until after Altissia, where you can pick up the Llymlaen reel from the Totomostro. When you're ready, talk to Navyth near Galdin Quay, then head over to the Vannath Coast Fishing Spot during the day. Make sure you have Dragon's Whisker line (2,000 durability) and use the Burrower: Abyss Worm lure, as you're after a Murk Grouper, which are indicated by flashing blue dots. Even with all this preparation, it'll probably take around ten minutes to reel this monster in. After you catch it, return to Navyth and he'll give you the Tranquility rod - the best rod in the game.