In the main game, there are 38 fishing spots. You need to visit all of them in order to get to the place where you catch the Fiendfish, Easter Island. Each spot (besides the first one of each continent) requires a certain number of unique fish to unlock it. I've done a few test runs and this is how many you need for each one.

Nagoya Port 0 Lake Yamanaka (only unlocks the day after visiting Nagoya Port) 0 Arashiyama (only unlocks the day after visiting Lake Yamanaka) Catch Key to progress Hawai'i 12 San Francisco (after catching 12, you need to enter and exit something, I use the shop for this) 16 Detroit 22 Campbell River 23 New York (only unlocks the day after visiting Campbell River) Win New York tourney to progress Sea of Okhotsk 28 Appi River (enter and exit shop after catching 28 to unlock) 30 Tokyo Bay (only unlocks the day after visiting Appi River) Mekong River 33 Indian Ocean 40 Yangtze River 42 Mongolia (only unlocks the day after visiting Yangtze River) Catch Giant Taimen to progress North Sea 45 Loch Ness 46 Venice 48 Caspian Sea 50 Mediterranean Sea 52 Paris (only unlocks the day after visiting Mediterranean Sea) Win Paris tourney to progress Nile River 55 Lake Tanganyika 58 Cape Town 60 Savanna (only unlocks the day after visiting Cape Town) Win Savanna tourney to progress Shimanto River 63 Inland Sea 67 Kanmon Straits 70 Yakushima (only unlocks the day after visiting Kanmon Straits) Win Yakushima tourney to progress Sydney 75 Tasmania 78 Tahiti 80 Antarctica Catch Emperor Squid to progress (good luck) Amazon River A 87 Lake Baccarac (enter and exit shop after catching 28 to unlock) 88 Caribbean Sea 90 Amazon River B 100 Easter Island Catch Fiendfish to finish!

Tips and Tricks

• Pre-reeling: When a fish bites your line, especially one that's tough to catch, reel in for as long as you think you can to bring it in closer. For certain very big fish, it's also useful to reel your line in a little once it starts swimming towards your bait, drawing it in closer to the shore. For fish that let you reel in a lot, this is slower. For fish that constantly have you in the "stop reeling" zone (most of the story fish, for example), it's faster and makes it easier to catch them. Use at your own discretion. Fair warning though, some fishing spots have certain cutoffs for how close to the shore the fish will bite your bait.
• Over-reeling: Most fish will let you keep reeling a bit past the "stop reeling" point if it's a continued reel from outside of that point. It's less than a second, often less than half a second for most fish, but it's pretty useful. This will NOT WORK on certain bigger fish, including most of the late-game big fish and the yellow-shadowed fish.
• Forcing unique fish: Pink-shadowed fish are very useful to catch because there's usually only one per spot, so if you haven't caught it yet, you know it'll add to your unique fish count. This can also work with the size of the fish shadows; if there's only one super-big fish in a certain spot, you can wait until it bites your line and let any other fish that bites it get away. This strategy is very useful in all four of the tourneys, where most or all of the fish at the spot bite the same bait. This can be used in junction with the next trick...
• Tackle Box resetting: If you're looking for a specific pink-shadowed or big-shadowed fish that isn't showing up in the water, open your menu and enter and exit the tackle box (or any of the other options), and it'll refresh the fish in the water.
• What to buy: Spend nearly all your money on bait. Buy lots of different kinds and plan ahead with your buys so you don't have to go to the shop as often. The only rod I'd recommend buying is the Angler's Pole. It's the 2000 point orange one and it can catch any fish in the game. You'll need it for the Mega Telescope Eye, and for the metal fish in Detroit if you plan on catching those, because the Starter Rod can't reach them. The only fish I've had trouble reaching with the Angler's Pole is the Treasure Chest in the Caribbean Sea.

Finally, the most important part of this run, where to find unique fish and what bait you need to catch them. The guide I'm linking here is one I have open on my computer at all times during the run. https://gamefaqs.gamespot.com/wii/950919-fishing-master-world-tour/faqs/55908

Good luck to whomever reads this!