To get there open your spell book, choose skilling locations and Piscatoris Fishing Colony.
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To get all the supplys you need for fishing talk to Harry.
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Here's list of things you can catch from spots out there
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Green square - Shrimps and Tuna
Yellow square - Cod/Bass and Herring/Sardine
Red squares - Lobsters and Swordfish/Tuna
Levels you need for different fishes:
Shrimp 1
Sardine 5
Herring 10
Cod 23
Tuna 35
Lobster 40
Bass 46
Swordfish 50
Cooking :
There's cooking range just inside bank where you can cook different fishes and here are levels you need to cook fishes:
Shrimp 1
Sardine 5 <-- can't cook
Herring 10 <-- can't cook
Cod 23 <-- can't cook
Tuna 35
Lobster 40
Bass 46 <-- can't cook
Swordfish 50