Lethal Legacy

The lake trout is native only to North America. Many of these stocks are disappearing because of human activity. The lake trout persists in all five of the Great Lakes, although its presence in some of them has declined.

The lake trout (Salvelinus namaycush) is a predator, feeding mainly on other fish. In parts of the Great Lakes, its main prey is a fish called the alewife. The alewife (Alosa pseudoharengus) is much smaller than the lake trout and feeds on even smaller aquatic organisms called plankton. During certain seasons of the year and when alewives are less plentiful, lake trout will also feed on other small fish and even plankton. The lake trout’s dependency on the alewife, and, in turn, the alewife’s dependency on the plankton, is part of the aquatic ecosystem’s food web, or series of interconnected food chains.

In the late 1960s, the government became aware that harmful new substances were entering the food web. Synthetic substances such as PCBs (polychlorinated biphenyls, used as insulating fluid) and Mirex (dechlorane, a fire retardant and pesticide) were discovered to last a very long time in the environment; once they entered the Great Lakes, they found their way to plankton, then on through the food web to alewife, lake trout and finally to humans who consumed the lake trout.

The effects of these and other contaminants on fish are not clear. Studies conducted in the Great Lakes have found an increased rate of lip and skin growths on coarse fish taken in polluted areas, and fish with liver and intestinal tumours in areas of extreme pollution. These studies suggest that pollution may cause the tumours and may indirectly cause the skin growths. Deformities in fish and invertebrates also increase in polluted environments. Tracing a specific cause, however, is difficult. Organisms are affected by the combined effects of eutrophication, degraded habitat and toxic chemicals. The number of stresses and their interactions can be overwhelming. The small amount of data available on chemical combinations clearly shows that both synergistic and antagonistic effects occur, and it may be unrealistic to expect that science will unravel all the possible permutations.

Animal tests indicate that ingestion of PCBs and Mirex by people may, above certain amounts, cause cancer and birth defects. Other pollutants, such as lead and mercury, are more systemic, affecting, for example, the central nervous system.

Now there are laws preventing the emission of PCBs and Mirex. Virtual elimination of these contaminants from the environment is the goal of both Canada and the United States. Because of their long-lasting effects, however, they continue to travel through the food web to top predators like lake trout. Since lake trout grow and mature slowly, they collect these materials in their tissues, especially fat tissues, over a long period of time. The longer they live, the more contaminants they tend to collect.

Fish are regularly collected and tested by scientists to measure the amount of contaminants they contain. The Ontario Ministry of the Environment recommends that fish with PCB levels at 0.3 parts per million (ppm) or more should never be eaten by children or women of child-bearing age and only occasionally (not more than two meals per month) by everyone else.

The contaminant problem has improved as studies show a decline in the level of many harmful substances found in fish. The lessons learned from this “lethal legacy” may help us to ensure an improved aquatic environment in the future.