he rapid decline of great sharks in the world’s oceans is disrupting the marine ecosystem by allowing more lowly fish to thrive, scientists warn today.
Overfishing of the ancient predators has lead to a sudden uprising of species they prey on, causing an abundance of skates, rays and smaller sharks, which are steadily devastating populations of shellfish, including scallops, oysters and clams, the researchers claim.
The findings suggest that the demise of the great sharks, whose primitive ancestors cruised the seas long before the rise and fall of the dinosaurs, may have unforeseen knock-on effects on marine life lower down the food chain.
Records from fisheries and research vessels dating from the 1970s to 2005 have revealed a dramatic nosedive in great shark populations. Tiger sharks and scalloped hammerheads may have declined more than 97% since the mid-1980s, while numbers of smooth hammerheads and bull sharks are believed to have fallen by 99% off the east coast of the US.
Writing in the journal Science, a team of marine biologists led by Ran Myers at Dalhousie University in Halifax, Nova Scotia, analyse fish research surveys over the past 16 to 35 years. The records show that while the abundance of 11 great shark species fell dramatically over the past 35 years, 12 of the 14 fish species they prey on had increased sharply.
In the waters along the US Atlantic coast, numbers of cownose rays, a staple of the great shark diet that can grow to four feet across, jumped 8% a year to an estimated population of around 40 million.
The explosion of the cownose population coincides with an almost complete collapse of scallops in the waters, leaving only those protected behind marine fences for local fisheries to take.
Without sharks to keep their numbers in check, researchers fear the migrating rays will drive down shellfish populations as they swim through, to the point where they are unable to recover .
Julia Baum, a co-author of the paper, said: “With fewer sharks around, the species they prey upon, like cownose rays, have increased in numbers, and in turn, hordes of cownose rays dining on scallops, have wiped the scallops out.”
Sharks are targeted by fisheries for their fins and meat, but are also taken as by-catch by fleets fishing for tuna and swordfish. As many as 73 million sharks are killed each year around the globe for the finning trade.
Ellen Pikitch, executive director of the Pew Institute for Ocean Science in Miami, said: “This is the first published field experiment to demonstrate that the loss of sharks is cascading through ocean ecosystems and inflicting collateral damage on food fisheries such as scallops. These unforeseen and devastating impacts underscore the need to take a more holistic, ecosystem-based approach to fisheries management.”
Charles Peterson, a researcher on the paper and marine biologist at the University of North Carolina in Chapel Hill, said the study highlighted the importance of maintaining populations of the ocean’s top predators. “Despite the vastness of the oceans, its organisms are interconnected, meaning that changes at one level have implications several steps removed. Through our work, the ocean is not so unfathomable, and we know better now why sharks matter,” he added.
In British waters, historic overfishing has seen the common ray decline to the point that surveys in the western channel have failed to spot any since the 1930s. More recently, numbers of blue and porbeagle sharks are believed to have fallen. The porbeagles are believed to be taken by Danish and French fleets, while Spanish long-line vessels take blue sharks migrating into British waters.
Last year, a team lead by David Sims of the Marine Biological Association in Plymouth tagged six blue sharks off the coast of Portugal to investigate their fate. Two were landed by fisheries within three months. “The ones that get here may be the survivors,” he said.
Dr Sims said the lack of hard data makes it extremely difficult to produce reliable assessments of fish populations, adding that many predators have such varied diets that cascade effects through ecosystems are complex and often difficult to pinpoint.
“There’s no doubt the fisheries are having an impact on the big shark populations, but what we really don’t know is what the ecosystem effects of that will be. There could be other factors involved that haven’t been measured,” he added.
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There is something unique about sharks’ teeth! A shark without teeth could not survive; it would starve. Therefore, unlike many other animals like dogs and canines, sharks continuously get new teeth to replace those that fall out. A shark’s mouth generally contains five or more rows of teeth, one behind the other. All rows, with the exception of the first, lay flat in the animal’s mouth. The next row rises up to replace any teeth that have fallen out or were broken. Sharks always make new teeth and have always spare rows of teeth. Sharks’ teeth are adapted to what they eat. Unlike humans, sharks do not chew. They are not omnivores, but carnivores. They use their teeth to grasp prey and, if needed, tear the prey into smaller chunks they can swallow. Most shark teeth are very sharp. Sharks’ jaws are powerful and the sharp teeth are capable of cutting through bone and even thin steel chains.
Shark teeth vary from being ferocious-looking curved spikes to flat triangular points, to points that are so small that they are not used for anything at all. The larger sharks, like the great white and the tiger shark, have triangular teeth with jagged edges. This helps to keep hold of large fish and animals so as to tear chunks of meat from their bodies or slice through a turtle’s shell. A sand tiger’s teeth, on the other hand, are long and narrow which make them look frightening, but in fact this type of shark is not very aggressive. The shape of its teeth is ideal for grabbing hold of slippery prey, like fish and squid. However, the whale shark, one of the biggest sharks on earth, has very small teeth. Whale sharks don’t use their teeth for biting because they simply filter their food.
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