The article says "otters aren't stupid". This is very true, and they don't like people around so keep the people on the bank and that is as good a deterrent as any. They have been so persecuted for so long, and much maligned as regards fish-taking habits in the wild, that they have a healthy respect for human presence. Commercial fisheries are a different matter from wild fisheries however, and the financial losses otters could cause to relatively captive and confined fish must be horrendous for owners. I don't however see if a fish is to cost £25,000 to replace, that otter-proof fencing would be all that 'prohibitive' in terms of cost if it saves that sort of money.