While the principles of aquaculture are generally accepted, experts fiercely debate which types of fish farming are safe to pursue. Salmon farming can be a dirty business. A biologist who worked for 30 years for Canada's Department of Fisheries has stated that a large salmon farm can pour as much liquid waste into the sea as a small city. One can add to that the plague of destructive sea lice, which thrive in densely packed salmon pens, and the schools of farm-grown fish which inevitably escape to the open sea, spread disease and compete for food at the breeding grounds with wild stocks.