Rope
If you take a thread and wrap up a bunch of it, so much it becomes a hill of thread, you're going to have a lot of thread. You can wrap some of that thread around other thread to create a much thicker thread. This thread is a rope.
Imagine yourself twisting this hill of thread up to make thicker ropes. They're strong!
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Congratulations, now you have a perfect rope for tuna (つな) fishing. Tunas are massive, so you need a strong rope to reel one in.
Imagine yourself going tuna fishing on a small boat with nothing but one long rope. This is how tuna fishing is done where you're from, so you're sticking to tradition.