Project: Big Shark, Big Loss, Big Impact: Sharks are Apex Predators

Canada’s three oceans, Atlantic, Pacific and Arctic, are home to 41 species of sharks including the Shortfin Mako, Greenland, Sleeper, Basking, Smooth Hammerhead, and even the Great white shark! Sharks often fill the role of “apex” or top predators in their ecosystems because of their large size and few natural predators. As apex predators, sharks feed on animals below them in the food web, and help regulate and maintain the balance of marine ecosystems. Most sharks have diverse diets and switch prey species when certain populations are low. By switching to more abundant prey, they allow low populations to rebound and prevent abundant species from monopolizing a limited resource. Others scavenge the sea floor to feed on dead carcasses. Apex predators also influence spatial distribution of prey species through intimidation and keep them from overgrazing certain habitats.

The unfortunate reality of course is that humans are the ocean’s true apex predators, killing more than 100 million sharks each year. Fishing has reduced many large predatory shark populations worldwide by 90% over the past century. Sharks now represent the largest group of threatened marine species on the International Union for Conservation of Nature’s (IUCN) Red List of threatened species, yet only three of the 350 shark species (Basking, Whale and White), are protected from the pressures of international trade. The remaining species are ignored or seen as low priorities despite their vulnerability to overfishing and their important role in their ecosystems. In Canada there are ten species of sharks designated at risk by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). These include White, Brown Cat, Tope, Bluntnose Sixgill, Blue, Basking, Porbeagle, Spiny dogfish, Shortfin Mako, and Spotted Spiny Dogfish.

Some fisheries directly target sharks as their intended catch, but other fisheries accidentally capture sharks as “bycatch”, a term used for unintended catch. These unwanted sharks are thrown overboard, often dead or injured. It is estimated that tens of millions of sharks are caught as bycatch each year, which is nearly half of the total shark catch worldwide.

Between 26 and 73 million sharks are killed annually for their fins globally (caught both specifically and as bycatch). Once a delicacy and sign of prestige in Asian cultures, shark fin soup consumption is on the rise. A single bowl of soup can cost up to $100 making the fins the most commercially valuable part of a shark. Since the rest of the shark is less valuable and bulky, the fins are removed and the carcasses thrown overboard. This practice, known as “shark finning,” only uses between one and five percent of the shark. Shark finning is prohibited in Canadian waters since 1994.

In December 2011, Canadian MP Fin Donnelly introduced a private member’s bill that would ban the import of shark fins into Canada. Canada imports around 77 000 kilograms of fins each year. The practise of shark finning is already illegal in Canadian waters, but there is no law to prevent importation. If the ban is implemented, those in violation could face jail time or a fine of up to $100 000. Some Ontario municipalities have already taken steps in this direction – Toronto, Oakville, and Brantford have already banned the sale of shark fins. The bill was defeated in 2013 by a vote of 138 for and 143 against.

The overfishing of apex predators can lead to serious consequences for many other ocean species, in ripples called “cascades”. Comparisons of areas with and without apex predators have shown that apex predators maintain greater biodiversity and higher densities of individuals, while areas without apex predators experience species absences. Without apex predators there is the potential for unchecked predation by lower predatory species and overgrazing of vegetation by herbivorous prey species.

The following is a simplified Atlantic food web: Shortfin Mako sharks are apex predators and eat Bluefish (known in Canada as Pollock). Bluefish are medium sized piscivores weighing as much as 18 kg. They eat smaller fish like Atlantic Herring. Herring are small forage fish that eat plankton.

Reproductive rates vary widely between these different fish species. Shortfin Mako sharks bear between 4 and 18 young once every 3 years. Bluefish spawn once a year with females producing about one million eggs per year. Atlantic Herring can spawn from April to November in Canadian waters. A fully mature female can produce upwards of 260 000 eggs at a time.