I cannot honestly say that any form of large-scale commercial fishing is environmentally friendly because it is not, but at the same time I realise that the fishing industry will always be supported and subsidised by governments despite the corruption and ecological damage involved. 

Demersal and pelagic trawling, and purse seining are especially damaging to the marine environment because of the high rate of bycatch. Purse seining uses a large purse-like net which floats in the water, a shoal of fish enter it and it is drawn in at the top and then dragged through the water and back up onto the boat. Pelagic trawling uses a long net dragged behind the boat in the top layers of the water, catching anything in its path. Demersal trawling drags a net held open by wooden boards along the sea floor picking up organisms living deeper in the water. Each of these is indiscriminate when it comes to caught species and the immense pressures of huge amounts of fish packed into a small space results in the caught organisms almost always being crushed to death by the time they are pulled back up onto the fishing boat for sorting. 

While in the UK there are minimums for mesh sizes depending on species, when there is such a large amount of fish in the net, these do not prevent juveniles from being caught and pulled up. Catching juveniles is devastating to fish populations as they never have a chance to breed and are often too small for sale anyway, causing them to be thrown back into the sea. Additionally, mesh sizes of nets do not help at all when it comes to bycatch of larger fish species and cetaceans. These animals too are often returned to the water dead when they are found among the intended fish. 

Policing activities while at sea has proven almost impossible and so there is no way for the consumer to know how much damage has been done by that food product. The standards for animal welfare in the UK (concerning land animals) is relatively high (though many abhorrent practices are still permitted which says something about how we think about non-human animals worldwide but I digress) and for many, buying British pork or beef is a way to ensure, to some level, the treatment of that animal. By banning these three fishing techniques, especially if used in conjunction with fishing tackle buy-back schemes, no-take zones, and restrictions on fishing certain species during breeding seasons the health Britain's marine life could be protected to a much greater extent than it is now. If we cannot end fishing, then at least we could make the statement "fished in British waters" stand for a higher level of care and concern for marine wildlife.