I'd like to speak about the use of Acoustic Harassment Devices (AHDs). These are devices used in fish farms to deter seals from preying on the fish within the farm. This is all very well until you see what effect it has on cetacean life. Alexandra Morton, a marine biologist who is best known for her 30 year study on Orcas, who has since shifted the focus of her research to the impact of salmon farming; has written an article on the impact of AHDs. The article outlines the use of AHDs in the Broughton Archipelago in British Columbia, Canada. The use of AHDs seems to has little effect on the seals as the sluggish farmed fish are too easy prey; however it has a profound effect on the Orcas in the archipelago. As cetaceans both navigate and communicate acoustically, these devices have been claimed to be causes of mass strandings of whales in Scotland (see The Times article 'Man-made Noise is Blamed for Driving Whales to Their Deaths' by Frank Pope).
AHDs have the effect of putting cetaceans into an extremely noisy and disorientating environment and this has been to blame for a significant number of strandings. Not only does this seem unethical but it is also illegal; many cetaceans are protected or endangered and thus the dispersal of these species is prohibited by EU law as well as the Canadian Fisheries Act.
In summary, I don't think that the use of such devices can be justified to improve yield from fish farms.