The location of this type of toxin is usually downstream from industrial sites and in areas where flooding of tailings ponds could occur. Heavy metal contaminants include compounds like lead and mercury. The only way to ensure safety of anglers that may consume the fish, is to have suspected waterways and their fish population tested often for the presence of these compounds. Many heavy metals bioaccumulate such that fish higher in the food chain are affected the most. Fish may not appear to be affected, however, fish in these environments may have a reported increase in the prevalence of skeletal deformities, and gill and fin abnormalities.

Humans Safety: Fish caught from waterways determined to be heavily contaminated should not be eaten. For further information regarding mercury, see "Mercury in Fish: Guideline for Consumption." Saskatchewan Parks and Renewable Resources, Fisheries Branch, 1992.

Insecticide pollution can be most apparent at the mouths of creeks that were initially contaminated. Numerous species of fish can be involved and there would be a conspicuous lack of water insects. Plant life initially appears normal. The toxic release of insecticides (chemical compounds, organophosphates, or carbamates) can have multiple effects on the fish. In high level exposure, the toxin affects the fish's brain and fish will die in large numbers. In low level of exposure, the food chain will be severely disrupted and cause slower, less obvious diseases due to inadequate nutrition.

Human Safety: Insecticide contamination usually causes death of the fish. Any area suspected of having an insecticide contamination should ban all fish consumption until the problem is resolved and insecticide levels are diluted to safe levels.

This type of pollutant is relatively uncommon in Saskatchewan but is suspect when large number of fish are afflicted with an undetermined disease. Many suspected toxic contaminations are difficult to research and prove. Many affected fish show up long after toxins have become diluted. Thorough investigations are required. Toxins cannot be tested for on a random basis. Suspected specific toxins must be determined for accurate evaluation of water and affected fish. Many toxins do not kill the fish but affect their growth, reproduction, immune system and other systems. The presence of liver tumors associated with low level toxins is a good example of the possible long term affects of industrial waste.

Human Safety: No fish should be eaten by humans or domestic animals if high levels of industrial toxins are suspected in the waterways.

This is a widespread occurrence in shallow lakes and ponds. It commonly occurs in summer and winter and outbreaks are referred to as "summerkill" and "winterkill". Oxygen levels drop below critical levels and cause suffocation of the fish.

• Summerkill
  Lakes and ponds which experience summerkill characteristically are shallow and have an abundant growth of vegetation and/or algae. In the presence of sunlight, this vegetation produces oxygen from carbon dioxide present in the water, through the process of photosynthesis. During hours of darkness, the process is reversed and respiration occurs, which requires oxygen. When vegetation is overly abundant, it begins to die and the process of decomposition also requires oxygen. The fish are left with inadequate amounts of oxygen to maintain their life. Warm water has a lower carrying capacity of oxygen than cool and still water has less than moving water. Aeration systems can help add oxygen to the water.

• Winterkill
  As with summerkill, fish mortalities generally occur in shallower lakes and ponds in response to lack of available oxygen in the water. When a lake freezes and becomes covered in snow, little light can penetrate. Vegetation dies and any available oxygen in the water is utilized in decomposition. The oxygen available for fish is substantially decreased. Winterkill can be minimized if pond suspected to develop the problem, are removed of snow.

Human Safety: Fish suffering from oxygen depletion are rarely found alive. Those caught live but in poor health should cause no ill effects to humans but may not be palatable.