Dead zones can encompass areas of ocean 100,000 square kms in size where little can live because there is no oxygen left in the water. Nitrogen pollution, mainly from farm fertilisers and sewage, produces blooms of algae that absorb all of the oxygen in the water.

Unlike the United States and countries in the European Union, Canada has no national standards for sewage treatment for cities. Montreal dumps billions of litres of untreated sewage into the St. Lawrence River, while the postcard-perfect tourist city of Victoria, British Columbia dumps all of its waste directly into the Pacific Ocean.

Such waste can contain high levels of toxic chemicals, heavy metals and excreted pharmaceuticals. The latter pose risks that are only beginning to be understood. Emerging research shows negative impacts on marine life from residues of birth control and antidepressant drugs like Prozac even at extremely low concentrations of less than one part per billion.

"The big unknown" is what effect these pharmaceutical residues might have on chronically exposed plants, animals and people, Christian Daughton, chief of the environmental chemistry branch at the U.S. Environmental Protection Agency, has been reported as saying.