In Old School RuneScape (OSRS), there are several methods to gain easy fishing experience (XP) quickly. Here are some of the most straightforward and effective methods:
 

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Fishing Trawler: Participating in the Fishing Trawler minigame can be a good source of Fishing XP. It requires little effort and can yield decent XP rates. The trawler runs approximately every 10 minutes, so you can hop worlds to find an active game.

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Barbarian Fishing: Once you have level 48 Fishing, you can start Barbarian Fishing at Otto's Grotto. This method combines Fishing, Agility, and Strength training, making it efficient for gaining XP in multiple skills at once. You can expect around 50,000 XP per hour at higher levels.

Fly Fishing: Fly fishing at the Barbarian Village offers decent XP rates with minimal effort. You can fish trout and salmon, which grant faster XP than lower-level fish. This method is ideal for low-level accounts.

Big Net Fishing: Located in Catherby and the Fishing Guild, big net fishing is an easy and AFKable method for gaining Fishing XP. Simply set your character to fish with a big net, and they will automatically catch fish without much player interaction.

Minnow Fishing: Once you reach level 82 Fishing, you can fish minnows in the Fishing Guild's minnow platform. Minnow fishing provides both Fishing and Fishing XP. It's a bit more intensive than some other methods but can be profitable if you exchange the minnows for sharks.

Lobsters/Swordfish/Tuna: Fishing for lobsters, swordfish, or tuna can be profitable and offer decent XP rates. Head to a popular fishing spot like Catherby and fish these high-level fish for XP and some profit.

3-Tick Fishing: This method is more advanced and requires precise timing. By using the 3-tick method with fly fishing or barb-tail harpoon fishing, you can achieve very high XP rates but at the cost of being more click-intensive.

Sharks: At level 76 Fishing, you can start fishing for sharks, which are highly profitable and provide good XP rates. The best spot for shark fishing is usually in the Fishing Guild.

Remember to use any available fishing equipment and boosts, such as the Fishing Outfit, fishing potions, and the Ardougne Cloak (for a chance to catch two fish at once). Also, consider training Fishing alongside other skills that complement it, like Cooking or Agility, for a more efficient leveling experience.
 

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