Catch sea turtle
When you reach level 79 fishing, you might want to try out The Fishing Trawler minigame. buy rohan gold can be very fun and profitable. At level 79 fishing you can catch sea turtle and at level 81 fishing you can catch manta ray. Both fish can sell for over 3k each.
When harpooning fish, I suggest to hand fish them if you have completed the Barbarian Training for fishing and have the required levels. This way you will gain fishing XP, plus strength XP.