For many years, Great Lakes fish populations have been subject to severe pressures from overfishing, pollution, and invasions by non-native species. Governments have sought to compensate by setting fishing quotas, reducing pollution, and trying to control invasive species. As a result, some native fish are making a comeback. Pike are once again swimming in Toronto Harbour now that pollution levels have dropped. An artificial reef off Detroit’s shoreline is being built to facilitate the return of the sturgeon, whose rocky habitat was previously dredged for use as building material.

What is happening?

Native fish species of the Great Lakes range from the little emerald shiner to the huge lake sturgeon, and telling their story is like trying to piece together a giant jigsaw puzzle. It is useful to look at how top predators such as the lake trout and walleye are doing. Like the wolf on land, these big fish are a major force in shaping the lives of smaller species around them. The health of the predators acts as a barometer of the stability and resilience of the whole community of Great Lakes fish.

The swift, sleek lake trout was virtually king of the waters for hundreds of years, but its numbers declined drastically during the 20th century due to a combination of predation by the sea lamprey and overfishing. After nearly being wiped out in much of the Great Lakes, and surviving only in pockets in Lake Superior and Georgian Bay, the lake trout is now able to survive through natural reproduction in Lake Superior and parts of Lake Huron. They were starting to reproduce again in Lake Ontario, but the young may now be falling prey to another invasive species, the round goby. In most other areas of the Great Lakes, hatchery-reared lake trout are needed to keep the species going. It still faces a range of threats, including declines in its prey fish, competition from non-native species that have invaded the lakes, and pressure from humans.

The big-finned, sharp-toothed walleye is another ruler of its waters. Walleye (also known as pickerel) are found in all five Great Lakes, but in Lake Erie they are the dominant predator. This species is also the basis of important sport and commercial fisheries. The population fluctuations of the Lake Erie walleye indicate that this species is in difficulty. After soaring to historic highs in the 1980s, the walleye population plummeted by more than half in a decade. The reasons for the decline are not fully known but include years of poor reproduction, due in part to weather patterns and temperature variations, and changes in the food supply caused by non-native species such as zebra mussels. Fisheries around the lake are being significantly scaled back in recognition of the reduced capability of the walleye population to support fishing.

Farther down the food chain are smaller fish - a mix of native and non-native species such as perch, shad, sculpin, bloater, herring, burbot, alewife, and rainbow smelt. Some are themselves sport or commercial fish and all serve as prey for bigger species. Many of these prey fish are in decline, so predators higher up in the food chain have difficulty surviving.

We can learn a lot from the fate of Diporeia, a tiny freshwater shrimp that swims sideways and that most people rarely see. These fat little crustaceans, also known as scud, feed on algae and other organic matter in deep waters and form an important part of the base of the food chain. Many species of forage fish in turn feed on scud. Scud populations are in dramatic decline in four of the five Great Lakes and have vanished from some regions. This species appears to be losing the competition for food to the invasive zebra and quagga mussels.

Figure 6:
Trends in abundance of lake trout in the Great Lakes. The graph uses a series of different measures but give a sense of the health of lake trout populations. Only Lake Superior and parts of Lake Huron have self-sustaining populations. In the other lakes, lake trout populations are not yet self-sustaining.

What is being done?

In the Great Lakes basin, states, Ontario, Tribes and First Nations, and two federal governments manage the fisheries to meet a wide range of needs, including food, recreation, cultural heritage, employment, and the health of the aquatic ecosystem. The United States and Canada cooperate on fisheries management through such organizations as the Great Lakes Fishery Commission. This group coordinates fisheries research, controls sea lamprey, and helps coordinate fishery management among the state, provincial, Tribal and First Nations, and federal agencies. Fishing rates must be adjusted to the ability of various species to reproduce. Habitat must also be considered. It is key to the survival of all species. Governments at all levels, often in collaboration with non-profit organizations and industry, have been working to protect and restore habitat, often by creating conservation areas, parks and protected areas, and even artificial reefs. Pollution reductions in recent decades have promoted the survival and reproduction of fish.

Controls on pollution and predators were rewarded by the comeback of native species. Once the lamprey was brought under control through the introduction of coho and chinook salmon (745 million salmon were released into the Great Lakes between 1966 and 1998), native lake trout were reintroduced to a number of areas. The constant arrival of non-native species, however, continues to disrupt the Great Lakes ecosystem and put new pressures on native species.

Figure 7:
Trends in abundance of Lake Erie walleye. This fish is the top predator in Lake Erie, and its reproduction and survival provides information about the health of that lake’s ecosystem.