13-5 Protecting, Sustaining, and Restoring Lakes and Rivers

Case Study: Can the Great Lakes Survive Repeated Invasions by Invasive Species? They Keep Coming

Invasions by non-native species are a major threat to the biodiversity and ecological functioning of lakes, as illustrated by what has happened to the Great Lakes.

One of the biggest threats, sea lampreys, reached the western lakes through the Welland Canal as early as 1920. This parasite attaches itself to almost any kind of fish and kills the victim by sucking out its blood (Figure 13-19). Over the years it has depleted populations of many important sport fish species such as lake trout.

Canada and the United States keep the lamprey population down by applying a chemical that kills their larvae in their spawning streams—at a cost of about $15 million a year.

How Can Freshwater Fisheries Be Managed and Sustained?

encouraging populations of commercial and sport fish species and preventing such species from being overfished. Ways to do this include regulating the time and length of fishing seasons and the number and size of fish that can be taken, ensuring enforcement of regulations, and educating the public and politicians about the importance of effective management.

Maintaining habitat quality is also fundamental. This can involve preserving water quality by taking steps to prevent pollutants, sediments from soil erosion, and excess heat from degrading water bodies on which fish depend.

Case Study: Managing a Freshwater Fishery

Lake Nipissing is a mesotrophic (p. 151) freshwater lake, with a surface area of 873 square kilometres and an average depth of 4.5 metres, in central Ontario. The lake has more than 40 species of fish; however, it is known as a famous walleye-fishing lake and fisheries managers are mindful of the intense pressure on walleye populations as they plan and implement their harvesting and conservation strategies for the lake.

Despite the best efforts of fisheries biologists, the walleye harvest has often exceeded the maximum sustainable yield that biologists felt the population could sustain. After collecting data on the walleye population, the fisheries biologists identified their management challenges and used a variety of strategies and techniques to bring the situation under control.