Easy bio toxin: Just find one near the shore, aggro it and swim to the shore, making shore its following you. Once your on the shore wait for jellyfish to be on the edge of shore and just stab it with your spear / pike. You get like 20 - 30 bio toxin, but 1 spoils in about 40 minutes. I find it really useful to just go harvest bio toxin than use narcotics to keep a tamping Dino asleep, so you can use all your narcotics on tranq arrows or tranq darts. Oh and finally once you reach a high enough level (I think around level 90) you can learn the shock dart which uses the bio toxins, and 1 - 3 shots most Dinos.
While they are dangerous they are the greatest source of narcotics in the game. They give what’s called bio toxin and this will make torpor rise faster. If you want to kill them safely lure them to a raft and then crossbow them down. They are fairly weak so you won’t need many shots to slay them. Your greatest worry will be the sharks but if you can kite them into these guys you will find it works to your benefit.