The trawling is a fishing technique that is very dangerous for the marine life. The reason for that is that with this technique of fishing, by throwing very large nets and moving them around in the open waters, lot of species are caught, both the ones that were intended to be fished but also ones that haven't been intended for fishing.
That makes a very big problem, and even if the fishermen let back into the water the species that haven't been intended for fishing, very often some of them are already dead, or more commonly injured. When returned to the water, if returned at all, because of their injuries they can not function properly and die.
This method causes a big loss of biodiversity, which is why it is forbidden in numerous countries.
