| Sharks' role in the oceans |
Sharks play a very important role in the oceans in a way that an average fish does not. Sharks are at the top of the food chain in virtually every part of every ocean. In that role, they keep populations of other fish healthy and in proper proportion for their ecosystem. How do sharks keep the oceans healthy?Keep food webs in balanceSharks have evolved in a tight inter-dependency with their ecosystem. They tend to eat very efficiently, going after the old, sick, or slower fish in a population that they prey upon, keeping that population healthier. Sharks groom many populations of marine life to the right size so that those prey species don’t cause harm to the ecosystem by becoming too populous.The ocean ecosystem is made up of very intricate food webs. Sharks are at the top of these webs and are considered by scientists to be “keystone” species, meaning that removing them causes the whole structure to collapse. For this reason, the prospect of a food chain minus its apex predators may mean the end of the line for many more species. A number of scientific studies demonstrate that depletion of sharks results in the loss of commercially important fish and shellfish species down the food chain, including key fisheries such as tuna and other important fish species that maintain the health of coral reefs. Eliminating the apex predator affects the entire food chain.In a scientific study1 of the mid-Atlantic part of the United States, depicted in the above slide show, 11 species of sharks were virtually eliminated. Of the 14 species of marine life that those sharks used to eat, 12 became more plentiful and caused great damage to the ecosystem. For example, the cownose ray population was no longer kept limited by sharks and grew out of control. As a result, the rays destroyed the population of bay scallops, their favored food. (Click to view slide show.)
But, people like bay scallops, too! The scallop fishery, which had been thriving for over 100 years, was virtually gone, with scallop catch dropping to only 13% of its high point2. And, scallops were also no longer there to perform their function of filtering and cleaning the ocean water. Keeping prey populations healthyPredatory sharks prey on the sick and the weak members of their prey populations, and some also scavenge the sea floor to feed on dead carcasses. By removing the sick and the weak, they prevent the spread of disease and prevent outbreaks that could be devastating. Preying on the weakest individuals also strengthens the gene pools of the prey species. Since the largest, strongest, and healthiest fish generally reproduce in greater numbers, the outcome is larger numbers of healthier fish. Keep sea grass beds and other vital habitats healthyThrough intimidation, sharks regulate the behavior of prey species, and prevent them from overgrazing vital habitats. Some shark scientists believe that this intimidation factor may actually have more of an impact on the ecosystem than what sharks eat. For example, scientists in Hawaii found that tiger sharks had a positive impact on the health of sea grass beds. Turtles, which are the tiger sharks’ prey, graze on sea grass. In the absence of tiger sharks, the turtles spent all of their time grazing on the best quality, most nutritious sea grass, and these habitats were soon destroyed. When tiger sharks are in the area, however, turtles graze over a broader area and do not overgraze one region. SummaryWhere sharks are eliminated, the marine ecosystem loses its balance. In the parts of the ocean where sharks have been fished out of existence, we can see the dangerous result of removing the top predator from an ecosystem. The lesson is important. Sharks are being killed for their fins for shark fin soup, a food that has assumed cultural value but is not important for human survival or health. However, removing the sharks can result in the loss of important foods that we do depend upon for survival.Sharks have survived for 450 million years, but may be gone within the next decades. Life within the oceans, covering 2/3rds of our planet, has enjoyed a relationship with sharks for about 450 million years. Our growing demand for shark fin soup has increased the slaughter of sharks to such a great extent that many shark species are already nearing extinction. What will the health of oceans be like when such an important group of animals have been destroyed? Do we want the destruction of sharks and the oceans to be the legacy we leave for our children? References:1Ransom A. Myers, Julia K. Baum, Travis D. Shepherd, Sean P. Powers, Charles H. Peterson; Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean; SCIENCE, MARCH 2007 2Clyde L. MacKenzie, Jr., The Bay Scallop, Argopecten irradians, Massachusetts Through North Carolina: Its Biology and the History of Its Habitats and Fisheries, Marine Fisheries Review, US National Marine Fisheries Service, June, 2008 Additional References:Declines in large shark populations on the East Coast of the US led to the collapse of North Carolina's century-old bay scallop fishery. (Source: Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean - Ransom A. Myers - Dalhousie University - 2007). A study by Enric Sala of Scripps Institute of Oceanography suggests that the loss of sharks may have contributed to the decline of reefs in the Caribbean, most of which are now dominated by algae. (Source: New Scientist - April 23, 2005) Studies of remote, pristine ecosystems demonstrate the positive impacts of the presence of sharks, including greater biodiversity, larger numbers of fish and healthier sea grass beds in areas with healthy populations of sharks as compared to similar systems in which the sharks have been overfished. (Source: High apex predator biomass on remote Pacific Islands. Stanford University) “When a shark fishery in Tasmania experienced a population collapse, octopus, which they had fed on, exploded. Octopi feed on spiny lobsters, so lobsters crashed, and fishermen were put out of work.” (Helfman. 2007. Fish Conservation: A Guide to Understanding & Restoring Global Aquatic Biodiversity & Fishery Resources”) [From the removal of tiger sharks] “Some unexpected outcomes also occurred: a total and rapid crash in the abundance of tuna and jacks, and an increase in bottom fishes.” (The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems. – ICES Journal of Marine Science, 57: 476–494.See p. 488 for the tuna reference. Stevens, J. D., Bonfil, R., Dulvy, N. K., and Walker, P. A. 2000) Observations from Palau, which is now a Shark Sanctuary, indicate that biodiversity has improved since they began to protect sharks. (Based on a conversation with an official with the Palau Mission to the United Nations) Predicting ecological consequences of marine top predator declines. A Summary of New Scientific Analysis: Heithaus, M.r, Frid, a., Wirsing, a.j and Worm, B. 2008. Trends in Ecology and Evolution 23(4):202–210. The role of sharks in the ecosystem by Mike Bennett 2005 - School of Biomedical Sciences, The University of Queensland, St. Lucia, Queensland, 4072 Australia Coupled changes in structure and function in reef fish assemblages: how predators increase fisheries production: Sandin, S. A. Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0202 Ecologically justified charisma: preservation of top predators delivers biodiversity conservation High apex predator biomass on remote Pacific islands Ongoing Collapse of Coral-Reef Shark Populations Predators as Prey: Why Healthy Oceans Need Sharks Loss of top predators causing surge in smaller predators, ecosystem collapse Direct and indirect fishery effects on small coastal elasmobranchs in the northern Gulf of Mexico Status and Conservation of Oceanic Pelagic sharks Long-term declines in Two Apex Predators, Bull Sharks (Carcharhinus leucas) and Alligator Gar (Atractosteus spatula) in Lake Pontchartrain, an Oligohaline estuary in SE LA. Effects of Losing Top Predators - Predicting ecological consequences of marine top predator declines. |
