Long-line fishing is when a bunch of really long fishing lines are put out in the water, and they have lots of little hooks hanging off of them. It's like a big fishing net, but instead of catching a bunch of fish all at once, long-line fishing catches one fish at a time.
First, the people doing the fishing put a big boat out on the water and drop a really long line into the ocean. The line can be several miles long! On this line, there are lots of little lines, called "branch lines," that hang down with hooks on them.
The hooks are baited with things that fish like to eat, like squid or mackerel. The bait smells really good to the fish, so they come swimming over to have a bite.
Once a fish takes a bite, its mouth gets hooked onto the little hook on the branch line. This won't hurt the fish right away, but it can get very tiring for the fish because it has to swim with this hook and line attached to it.
Eventually, the fisherman will pull the line back up to the boat and they'll take all the fish off the hooks. This can hurt the fish quite a bit, so it's not a very good thing for the fish if they get caught in a long-line fishing net.
Long-line fishing is used to catch a lot of different kinds of fish, like tuna, swordfish, and sharks. But sometimes, it can be bad for the ocean because it can catch too many fish at once and hurt different kinds of animals that live in the ocean. That's why it's important to be careful when using this kind of fishing method, and to make sure we're not hurting the environment too much.