I asked Sami about using harpoons back on the old forum somewhere. From memory, what he told me was that angons were used to immobilise seals, not as much by physically dragging them by a rope but because it's a large heavy object impaled in them. He also told me that since the game doesn't have a way to have animals impeded in this way, you can't use angons in the intended way in the game (and I can tell you from experience that in the game even crippled seals can crawl into the water and swim away, taking your missile weapons with them).
It's one of the things I would really like to see added in the game, because with the way I play seals are an important food source. Right now I use traps to catch them, which isn't really accurate but I don't have the accurate means for catching them available to me, so here we are. It is possible to catch seals by active hunting, but you normally have to have multiple goes at the same seal where you wound it, it goes underwater, it comes back and you wound it again, until you finally kill it. This is possible because they like to return to the same spot over and over, which also makes trapping them super effective.