This quest is one of the challenges of week 6 of season 5 of Fortnite and you'll have to destroy sevent fishing rod barrels.
Blow up Fishing Holes at Sharky Shell, Sweaty Sands, or Flopper Pond
Fishing holes actually refer to groups of fish, which are displayed in-game by eddies on the surface of the water, and fish swimming. To succeed in this challenge you will have to detonate three. Among the different ways to fish in Fortnite, one of them is to simply use an explosive in the school of fish.
Succeeding in this challenge is not complicated: you just need to have some explosives in your inventory. We recommend that you land at Sweaty Sands to complete the challenge.
Once equipped with a few grenades, rush towards the water's edge (if you're in Sweaty Sands, go to the pink building), spot groups of fish and throw a grenade.