An Upstream Adventure: Salmon Run on the Humber River
— written by Thais Bernos, Gadfly Stratton, and Nicholas E. Mandrak, University of Toronto Scarborough
Every Fall, if you pay close enough attention, you might just spot salmon jumping over the dams dotting Toronto’s Humber River on a journey to their upstream spawning grounds. But the salmon seen here today are not the same species that were historically native to Lake Ontario. That honour goes to the Atlantic salmon, a species considered extinct in the lake as a result of overfishing and mill activity in the 1800s. Indeed, the Humber River alone once had 13 grist, lumber, and textile mills dotted alongside it. These mills released organic matter, like flour and sawdust, directly into the rivers, the decomposition of which led to anoxic conditions. And the associated mill dams prevented upstream salmon migration to spawning grounds.
Old Mill Road, bridge across the Humber River
Atlantic salmon is an anadromous species, meaning it usually requires both freshwater and seawater habitats to complete its life cycle. Occasionally, however, these salmon become trapped in lakes and adapt to live in freshwater environments for their entire lifecycle. In Lake Ontario, Atlantic salmon eggs hatched in streams, where the young fish spent the first few years of their lives. Later, they migrated to the lake where they grew to large sizes, feeding on aquatic insects and other fish. When ready to reproduce, they would return upstream to where they were born to spawn in the Fall, before returning to the lake once more. This migration between their natal streams and lake is repeated over several years before the fish died.
Juvenile Atlantic salmon. Credit: Lorenz Seebauer
Atlantic Salmon Life Cycle. Used with permission from T. Bernos.
Atlantic salmon have since been reintroduced into Lake Ontario, including the Humber River. However, in addition to the original threats of habitat degradation and barriers in spawning rivers, these fish also have to contend with invasive species, like sea lamprey. Despite these challenges, there’s hope for the population to rebound in the lake. The next time you spot a fish jumping over the Old Mill, it may actually be an Atlantic salmon returning to its native spawning grounds.
Invasive Species
Sea lamprey is an invasive species first reported in Lake Ontario in 1921, likely arriving from the Atlantic coast through the Erie Canal. Within 35 years, it had spread throughout all of the Great Lakes, severely impacting the ecosystem. By parasitizing and often killing top fish predators, the sea lamprey destabilized the food web and caused precipitous declines in populations of lake trout, lake whitefish, and walleye. With fewer larger fish remaining, sea lampreys switched to smaller prey, like ciscoes. The decline of top predators and smaller competitors created space for other invasive species, including alewife, to move in, leading to further deleterious changes to the food web.
The sea lamprey is able to complete its entire life cycle in fresh waters. In the Great Lakes, lamprey eggs initially hatch in streams where the larvae remain for three to ten years until they metamorphose into their parasitic form and migrate to the lake. Adult sea lamprey parasitize prey in the lake for a year or two before migrating back to streams to spawn and die.
Sea Lamprey Life Cycle. Used with permission by the Great Lakes Fishery Commission.
Taking Back Control
Understanding the sea lamprey life cycle is key to taking back control. Lampricides–chemicals capable of killing lamprey–can be applied to streams known to contain their larvae. For the lampreys that reach the Great Lakes, traps and barriers can be used to prevent them from migrating back to their spawning grounds. Traps have been effective in capturing large numbers of migrating adult lampreys, with a reported 953 sea lampreys caught in the Humber River and Dufferin’s Creek areas in 2021. Barriers, on the other hand, take advantage of the lamprey’s inability to jump as high as other migratory fishes. While sea lampreys can inch their way up vertical surfaces, these barriers are designed to break the suction that lampreys create with their mouths in order to accomplish this task.
Sea Lamprey Barrier. Used with permission by the Great Lakes Fishery Commission.
It is important to note that several species of lamprey are native to the Great Lakes ecosystem, including the American brook lamprey, chestnut lamprey, northern brook lamprey and silver lamprey. The American brook and northern brook lampreys are filter feeders, meaning they bury themselves in the bottom of streams and filter microscopic food suspended in the water. Chestnut and silver lampreys are parasitic, consuming the blood of other fishes, but rarely killing their hosts. Unlike the invasive sea lamprey, native lampreys play important roles as predators and prey in the Great Lakes food web.