World Ocean Census - Extract 24 - The demise of the great sharks

22 July 2010

Ecologists have long understood that a reduction in predators affects the entire food web, that complex network of interactions between plants and animals that tells us who is eating what or whom. Census researchers investigating this premise determined that loss of predator species from oceanic food webs can cause long-lasting changes in the ecosystem that may be irreversible.

Researchers specifically studied the 11 species of sharks that scientists call the “great sharks,” whose diets consist of other elasmobranchs (rays, skates and small sharks). Census research has shown that the populations of great sharks have been decimated, and the largest individuals of these top predators have also been lost, as indicated by declines in the mean length of blacktip, bull, dusky, sandbar and tiger sharks. These losses suggest that overexploitation has left few mature individuals in these populations.

With fewer predatory sharks in existence, there has been an explosion of their prey species in coastal northwest Atlantic ecosystems, and the effects of this community restructuring have affected the entire food web. For example, cownose rays have increased to the extent where one of their prey species, the bay scallop, has been so reduced in numbers that a century-long scallop fishery has come to an end.

The reduction of shark populations has caused a high level of international concern, and efforts are growing to conserve them. However, scientists are uncertain whether such initiatives might be too little, too late. The global demand for shark fins and meat has not ebbed. Shark fin soup is a Chinese delicacy that is in high demand for weddings and other special occasions as a symbol of wealth and abundance. The practice of fishing sharks for their fins is controversial and problematic; it is considered a contributing factor in the global shark decline. Another problem for sharks is bycatch – when commercial fishermen catch sharks in the process of going after their species of choice. The sharks are thrown back overboard, most often dying or injured. An estimated 50 million sharks are unintentionally caught this way each year.

Text and images reprinted with permission from World Ocean Census: A Global Survey of Marine Life by Darlene Trew Crist, Gail Scowcroft and James M. Harding Jr., Firefly Books, 2009.

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