Sharks of the St. Lawrence: Dispelling Myths and Preserving the Ecological Balance

An ecological perspective

Sharks are present in all the world’s oceans as well as warm tropical waters, temperate seas and polar seas. They are found both near the surface and on the seabed, in nearshore waters and in the high seas. Sharks have been present on Earth for over 450 million years and have outlived five mass extinctions. The trade of shark-derived products, most notably their fins, is a serious threat and is exacerbated by the slow reproduction of these species. According to the International Union for Conservation of Nature, approximately 60% of pelagic sharks are currently endangered. The 11 shark species in the St. Lawrence are considered threatened, with the exception of the chain catshark (chain dogfish), whose population appears to be stable, though data are insufficient.

In Canada, the great white shark has been known for thousands of years by the Mi’kmaq people. Indigenous people recognize the importance of this animal through its relationships with the different species in the ecosystem. Indeed, sharks are key predators for the regulation of food webs, which is why their absence would have enormous consequences for marine ecosystems in all of the world’s oceans.

A nuanced look at sharks’ vital role

As keystone species, sharks play a crucial role in regulating marine ecosystems. As top predators, they maintain the balance of food webs by controlling mesopredator (the predators located in the middle of the food chain) populations, thus preventing their excessive proliferation, which could degrade habitats and deplete resources. Additionally, by targeting sick or week individuals, sharks promote the survival of more robust individuals, thus contributing to the overall health of marine ecosystems.

As they migrate between deep and shallow waters, sharks provide essential nutrients to oftentimes nutrient-poor coastal areas. For example, grey reef sharks, which are found in the eastern and western Atlantic and Indo-Pacific oceans as well as the Red and Mediterranean seas, carry nitrogen from the depths to shallow coral reefs, where their excrement enriches the ecosystem. This nitrogen feeds thousands of marine species, thereby contributing to reef biodiversity.

The absence of sharks might also impact nutrient cycling in that their excrement provides vital levels of nutrients such as nitrogen and phosphorus, which are fertilizers for marine plants and phytoplankton. Without these nutrients, these organisms would be at risk, leading to dramatic declines in marine ecosystems. When they die, their bodies sink into the ocean depths, trapping this carbon for thousands of years.