Lake trout in the Upper Great Lakes

The adaptations which have enabled lake trout to live in the upper Great Lakes are the same traits that put them at risk to

human impacts.  Because they are long lived and late maturing, they are susceptible to overfishing: when many reproductive fish are removed from the population it takes years to replace them.  Because they have evolved in a cold pristine environment, they are vulnerable to pollution and invasive species.  Because they evolved in a species-poor environment where adult lake trout were at the top of the food chain with little competition or predation, they were poorly equipped to deal with the influx of non-native species to the upper Great Lakes, particularly the parasitic sea lamprey.

Photo 2: Sea lamprey attached to lake trout. Photo: Fisheries and Oceans Canada.

Sea lamprey predation
Sea lampreys (Photo 2) are an eel-like fish native to the Atlantic Ocean. In North America, the sea lampreys’ native distribution included rivers and lakes with unimpeded access to the Atlantic Ocean, including the St. Lawrence River and possibly Lake Ontario.  They were prevented from spreading into Lake Erie by Niagara Falls until the opening of the Welland Canal in 1829 and subsequent improvements to the canal in 1919.  They were first recorded in Lake Huron in 1932 and Lake Superior in 1938.  The St. Marys River, which joins the two lakes, is thought to support the largest spawning population of sea lamprey in the upper Great Lakes.

Photo 3: Mouth of sea lamprey with sharp, rasping teeth. Photo: Kevin McVeigh, Fisheries and Oceans Canada.

Adult sea lamprey are parasitic predators. They feed by attaching themselves to the bodies of other fish, using a sucking disc with sharp, rasping teeth to bore into the bodies of their prey and feed on blood and other body fluids (Photo 3).  They typically remain attached to a host fish for two to 10 days, sometimes with several lamprey attached to a host fish simultaneously.  It has been estimated that 40–60 per cent of host fish die as a result of their wounds (Photo 4), either through blood loss or secondary infection.  An adult sea lamprey will kill approximately 18 kilograms of fish during its parasitic stage.  Sea lamprey will prey on most large-bodied fish species in the upper Great Lakes, but show a preference for lake trout.

Photo 4: Wound on lake trout caused by sea lamprey. Photo: Arunas Liskauskas.

When sea lamprey first invaded the upper Great Lakes, no means existed for controlling their numbers, and lake trout fisheries were heavily impacted.   In Lake Huron, the commercial catch of lake trout dropped from 1.5 million kilograms in 1937 to virtually zero by 1947. In Lake Superior, the catch dropped from a pre-invasion average of just over 1 million kilograms to less than 50,000 kilograms in 1961.  While other factors also affected the lake trout populations during this time, the introduction of sea lamprey was the catalyst for the collapse and the primary barrier to rehabilitation.

Figure 4: Annual commercial harvest of lake trout from the Canadian waters of Lake Huron and Lake Superior.

Commercial fishing
Commercial fishing in the upper Great Lakes was established in the mid-1800s to provide lake whitefish to the growing population in the Great Lakes region and to compensate for the loss of jobs in the declining fur trade industry.  As the numbers of whitefish started to decline, lake trout became more important.  By the turn of the century, larger vessels and improvements in fishing gear allowed commercial fishermen to travel farther and access deeper waters, which lead to greater catches.  

The harvest of lake trout in Lake Huron peaked in the early 1900s and came to an end in the 1940s when lake trout populations collapsed.  Lake Superior experienced a similar decline in lake trout numbers by the late 1940s (Figure 4).  

Other factors influencing lake trout
Lake trout populations in the upper Great Lakes are exposed to a number of different 

stressors (Figure 5):

Figure 5: Factors affecting the growth and survival of lake trout in the upper Great Lakes.

In the 1960s, Great Lakes researchers were perplexed by the death of lake trout fry from unknown causes. Numerous studies on fry mortality, poor growth, and unusual behaviour of juvenile fish showed that these were all related to low levels of thiamine in eggs. The affliction is known as thiamine deficiency syndrome and is caused by a diet composed primarily of alewife.