In just 45 years, the amount of farmed salmon reared in Scottish waters alone has risen from zero to around 150,000 tonnes. Scotland is now a world-leader in the industry. The total value of Scottish farmed salmon is now around £500m, annually. And with demand growing in emerging markets such as China, the future of fish-farming looks bright. But this extraordinary growth has given rise to environmental challenges around the Scottish coast, and in the waters off Chile, Ireland and Norway.