Pretty fun glitch, makes for fishing lobsters go faster.
I don't know if it can work everywhere, but I do it at the Fishing Guild.
Minimum Requirements:
70 Fishing, 50 Strength -- Ability to actually Barbarian fish Swordfish!
What you do is, you need to have a
lobster pot in the first slot of your inventory.
Go to a fishing hole that lets you cage fish, and harpoon (not all that hard to find in the Fishing Guild)
Once there, START to Barbarian Fish your Swordfish.
Right click over the watering hole and select Harpoon Fishing spot.
As soon as your player starts to fish, wait until he catches one.
When you see him pull the fish out of the water, click on the watering hole.
Since you have the lobster cage already in your inventory, you don't need to right click. You will begin fishing.
Instead of using the cage, you fish them like you just fished the Swordfish, like a Barbarian!

You might have to restart the process a few times, but trust me it works.
It's a fun little glitch I actually discovered not more than ten minutes ago from this point!
I hope you all enjoy it, and happy fishing!
--
Jacob.