Project: Big Shark, Big Loss, Big Impact: Sharks Change Prey Behaviour

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 following is a simplified Pacific food chain. The Harbour Seal has a wide diet which includes Pacific Herring and Alaska Pollock. Pacific herring are fatty fish, congregate near the surface of the water, and are widely dispersed. Alaska Pollock are larger, found in deeper water, and have a more continuous distribution. Which fish species the seals prey upon depends on the presence or absence of Sleeper sharks. Even though Sleeper sharks consume few seals, their mere presence alters seal behaviour. Sleeper sharks prefer deep water and therefore when sharks are present seals prefer to hunt in shallow water. This increases herring mortality and decreases pollock mortality.

When sharks are absent, seals increase their consumption of pollock and decrease their consumption of herring. This causes herring numbers to rise and pollock numbers to drop. Other animals connected to these fish through the food web (either as predators or prey) are affected by these changes leading to a cascade response.