Lake Trout in the upper Great Lakes

Lake trout were once the most important commercial fish in the upper Great Lakes.  Their numbers declined drastically during the 20th century due to overfishing and predation by sea lamprey.  After decades of rehabilitation and sea lamprey control, lake trout are now reproducing naturally in most of Lake Superior and parts of Lake Huron.

Figure 1: Location of upper Great Lakes -- Lake Huron and Lake Superior.

Biology of lake trout.

Lake trout (Salvelinus namaycush) are well adapted to the deep, clear waters of the upper Great Lakes: Lake Huron and Lake Superior (Figure 1).  They have inhabited the cold, low-productivity waters of northern North America for more than two million years.  Today, these fish are found primarily in the Great Lakes and in deep, cold lakes on the Canadian Shield.  Ontario is home to more than 25 per cent of the lake trout waters in the entire world.

Lake trout typically spawn in early fall in rocky, nearshore areas kept clean by wind, wave, and current action.  In the upper Great Lakes, some populations will spawn on deep submerged reefs and, in rare cases, may migrate up rivers to reproduce.

Figure 2: Life cycle of lake trout.

The fertilized eggs settle into spaces among the rocks and gravel, and develop under the ice through the winter.

The eggs hatch in mid-April and early May.  Newly hatched fry remain hidden in the gravel and survive on their yolk-sac for several weeks.  Once the fry become mobile, they seek out deeper waters to avoid predatory fish including larger lake trout.  Young lake trout feed initially on zooplankton, but soon shift to shrimp-like crustaceans and small fish.  As they grow, lake trout are opportunistic feeders and will eat whatever prey is available, including alewife, smelt, zooplankton, insects, freshwater sponges, and other lake trout. 

Adult lake trout tend to target prey species such as whitefish, lake cisco (formerly known as lake herring), and even small mammals, such as mice and shrews.
 

Figure 3: Three forms of lake trout found in the upper Great Lakes.

There are several forms of lake trout that differ in body size and shape, habitat, and prey preferences.  Biologists have described more than twelve of these forms, but three general groups are acknowledged in the upper Great Lakes (Figure 3):

• Lean lake trout are the dominant form in nearshore waters that are less than 80 metres deep.  Their slender appearance is the one most commonly associated with lake trout.
• Siscowet live in deep water and grow fat by eating lake cisco.  This form of lake trout is recognized as a distinct subspecies (Salvelinus namaycush siscowet).
• Humpers are the least abundant of the three forms and are found primarily on deep offshore reefs (or humps) and steep banks leading to deep water.

Lake trout have evolved several traits that allowed them to survive and thrive in the upper Great Lakes. They have a low metabolic rate, which allows growth and movement at low temperatures; they can survive long periods without food; and they grow slowly and mature late.  Sexual maturity is reached at about seven to twelve years of age in the upper Great Lakes, compared to three years of age for walleye and bass.  
 

Long-time Lake Huron commercial fishermen interviewed in the late 1970s reported some 16 different forms of lake trout.  These included redfin, yellowfin, sand, green, grey, black and lean lake trout.  Most of these forms likely no longer exist.