Train your Fishing level to 18. Use a fishing rod and bait to fish for herring. Fishing spots containing herring are located in Catherby, Draynor Village and the southern shore of Al Kharid. Sell herring at the runescape gold Grand Exchange for about 42 gold.
Train your Fishing level to 35. Use a small fishing net to fish anchovies. Fishing spots containing anchovies are located in the same places as sardines and herring: Draynor Village, southern shore of Al Kharid and Catherby. Sell anchovies at the Grand Exchange for about 41 rs gold.
Train your Fishing level to 40. Use a harpoon to fish for tuna. Fishing spots containing tuna are located in Catherby and the Fishing Guild. Sell tuna at the Grand Exchange for about 104 cheap rs gold.