There is has been so much fish farm sewage for the past decade, that even in the early 2000s, it started causing world wide problems. Read this World Wildlife Fund report from circa 2000: http://www.wwf.org.uk/filelibrary/pdf/secret.pdf. It makes sobering reading of fish farm sewage and its role in toxic algal blooms.
Also see, fish farm sewage causes damaging algal blooms: http://www.eurocbc.org/page222.html.
"Salmon farm wastes may tip the ecological balance to such an extent that algal blooms become toxic. During the past decade, there has been a worldwide increase in marine microalgae that are harmful to finfish, shellfish and humans. Mass mortalities of farmed salmon have been recorded recently in the Chiloe area of Chile, Shetland in Scotland and in Norway where millions have died in their cages. Harmful algal blooms associated with intensive aquaculture operations have been recorded in Scotland, Ireland, Norway, Japan, Finland, Hong Kong, New Zealand, Canada and Chile.
Amnesic, Paralytic and Diarrhetic Shellfish Poisoning events have plagued the Scottish, Canadian, Irish, Chilean and Norwegian coasts. The international community have finally begun to tackle this issue. AQUATOXSAL, for example, is an EC-funded research project involving Chile, Argentina, France and Germany investigating the links between salmon farm wastes and toxic algal blooms. In Scotland, in response to a petition by marine toxicologist Allan Berry, the Scottish Executive have hired Professor Ted Smayda of the University of Rhode Island to assess "the impact of nutrient inputs from fish farms on the algal communities of the Scottish coastal zone". And in British Columbia the Pollution and Prevention and Remediation Branch of the BC Ministry of Environment hired consultants to "document emerging research with respect to plankton blooms and netcages". Other research has focused on the use of seaweeds to remove salmon farm waste, the addition of chemicals to 'neutralise' toxic wastes or the use of 'nappies' to collect wastes. In the final analysis, completely closed systems for the containment of contaminated wastes can be the only solution. The solution to pollution is not dilution."
The solution is getting fish farms out of our pristine oceans and putting them on land in closed containers. Next, the sewage post.