The report is the first such comparative study to include aquaculture. About 800 farms, producing 450,000 tonnes of salmon and trout annually, release the nutrients in the form of fodder and excrement.
Fish farming is the only sector to show such relative increases.
Discharges from traditional sources such as agriculture, industry and municipal sewage have fallen dramatically. The industry has not acted illegally, however, and the SFT says there are no immediate plans to impose restrictions.