The game itself explains how to fish but i am pretty sure that you didn't read the instructions (neither did i until i had to catch the tentacles )
Fishing is a synchronization mini-game that requires your focus and timing. Once you get the handle of that it will be pretty easy!

To deepen a little to the mechanics once you catch a fish it will automatically start pulling away from you.To be able to capture it, you have to, time the small waves around the the center of the bait to be as aligned as you can to the stable circle of the bait.The game lets you know how good was the timing of your pull with stars.When there 's an exclamation mark above the fish it means that it change pattern so be ready to follow the new timing.
Give it a little time before you pull to really see the waves around it and you'll figure it out when you have to click.

There are 3 tier of fish in the game depending on the difficulty of them to catch and they distinguished by the color they appear on the map.

Tier One : (Blue) Can be found on the sandbank and main island.

Bluefish
Clam
Mackeler

Tier Two : (Yellow) Can be found on every of the 3 Islands to the west, southwest and south.

White Marlin
Crab
Flounder

Tier Three: (Red) Can be found on the northeastern island.

Red Snapper
Seabass
Lobster

This the last boss of fishing.



Now, tentacles are not a food but an artifact that is needed to complete the furniture collection .There are 8 total Writhing Tentacle in the game 2 for every island ,one visible one is hidden and can be found as RED fishing spots.These spots are accessible by boat or the canoe so you will have to travel some distance.
Also keep in mind that the golden weapons are at same spots with the visible tentacles on the map.


Also here the map with all the locations .
(this image is from FANDOM )

There are no tricks for this challenge but to get used to the mini-game and have patience.
I can recommend to take some fishing buff food with you ( the ones with a small fish to the corner of the food ) but it is not necessary .DO NOT spam the click button because you will lose energy!!!
You has to be as precise as you can in order to catch them.

If you read the guide thank you and have fun!

I like the game and i got frustrated because , i spammed the click button the whole game and it worked but , when it came to the Writhing Tentacle i almost gave up cause there were no real guides to help you. The most valuable info was the in-game tutorial and i figured that many people might get the same mistake as me.

Excuse my English if the grammar is not correct but its not my mother language :)


GOOD LUCK TO ALL !!!