But there’s something that might be just as scary as meeting up with a shark—at least from an environmental perspective. It’s the thought of what might happen if sharks disappeared from the oceans. That’s because sharks are important players in delicate food webs, suggests a new study out of Canada.

Fishing companies have been killing large sharks for decades. Sometimes they’ve done it on purpose, and sometimes they’ve done it by mistake. Because of these kills, the animals that sharks eat have boomed. And that’s bad news for the creatures even lower on the food web.

Along the East Coast of the United States, only sharks that are at least 2 meters (6.6 feet) long are tough enough to eat a lot of the medium-size sharks, rays, and skates living in those waters. Eleven large shark species in the region fit into that category.

Researchers led by Ransom Myers in Nova Scotia reviewed 17 surveys that counted big sharks and their prey during the past 35 years. They found that numbers of all 11 species have dropped since 1972.

As the big sharks disappear, most of the smaller sharks, rays, and skates have increased in number. Surveys have shown increases in 12 of 14 species of these sea creatures over the past 30 years. The populations of some of these species are 10 times as high as they were three decades ago.

Cownose rays travel in a dense horde. Each fall, such hordes from the bays of the New Jersey and Delaware region swim south along the coast for the winter.

W.S. Otwell

Researcher Charles H. Peterson recently heard fishermen in North Carolina complaining that cownose rays were eating up all the region’s bay scallops. He and his colleagues at the University of North Carolina’s Institute of Marine Sciences at Morehead City decided to test whether this was really happening.

To keep rays from eating scallops in certain areas, the scientists put a protective ring of poles around the scallops. Rays are wider than most sea creatures and won’t usually swim between poles that are spaced closely together. (The rays could turn sideways and fit through, but they don’t usually do this.) Other animals, however, swim easily through the gaps between poles.

In 2002 and 2003, at the beginning of the fall season, researchers found populations of bay scallops that were healthy and dense. But after rays migrated through, the scallops nearly disappeared in areas that were not surrounded by poles. Within protected areas, only half of the scallops were gone. It’s not even certain that the missing ones got eaten, Peterson says, since they might just have swum away.

The study suggests that efforts to replace declining populations of shellfish, such as scallops and oysters, might require extra levels of protection against predators.

The findings reinforce the message from a 1998 study of a food web in Alaska. In that area, killer whales can normally eat otters. Otters eat sea urchins. And sea urchins eat kelp. When the whales ate more otters, the study found, sea urchins thrived, and the kelp suffered.

In food webs, balance is key.—E. Sohn

Going Deeper:

Milius, Susan. 2007. Too few jaws: Shark declines let rays overgraze scallops. Science News 171(March 31):197. Available at http://www.sciencenews.org/articles/20070331/fob5.asp .

Sohn, Emily. 2003. Swimming with sharks and stingrays. Science News for Kids (Sept. 17). Available at http://www.sciencenewsforkids.org/articles/20030917/Feature1.asp .

This tiny European mysid shrimp, about a half-inch long, could spell big problems for the Great Lakes, where it’s just been spotted.

Steven Pothoven, NOAA, Great Lakes Environmental Research Laboratory

These crustaceans, called mysid shrimp, normally live in rivers near the western coast of the Caspian Sea in eastern Europe. In November, researchers from the National Oceanic and Atmospheric Administration (NOAA) found invading mysid shrimp in a channel in Lake Michigan. Now, NOAA reports that large numbers of the animal, called Hemimysis anomala, are living in southeastern Lake Ontario.

“I predict it will be a highly disruptive species,” says Anthony Ricciardi of McGill University in Montreal.

Mysid shrimp devour microscopic animals, which many young fish in the Great Lakes also eat. A heavy dose of new competition for food may harm fish populations.

The invasion is not a total surprise. From the 1970s through the 1990s, many creatures from the same area of eastern Europe entered the Great Lakes in ships that carry seawater to help keep them stable. This water is called ballast water. It’s often dumped as ships load and unload cargo wherever they happen to be.

In 1998, Ricciardi and a colleague came up with a list of 17 more European species that would probably arrive next. Hemimysis was on this list.

Although most European ships entering the Great Lakes now dump nearly all the water (and the critters living in it) from their ballast tanks before leaving Europe, a small amount still remains on board. That’s still enough water to harbor species such as Hemimysis anomala. This water can end up in the Great Lakes while ships unload and take on cargo.

New guidelines recommend that ships flush their tanks with saltwater before entering the Great Lakes. This “swish and spit” technique should be enough to kill any freshwater species that have come along for the ride.

It’s not yet clear how Hemimysis will affect Great Lakes ecosystems. Larger fish might find the shrimp appetizing, but these fish could then become more toxic to people. Mysid shrimp are fatty, so they can collect high doses of pollutants in their bodies. These pollutants then work their way up the food chain.

H. anomala look small and innocent, but don’t underestimate their ability to shake things up. “This is not,” Ricciardi says, “a species to ignore.”—E. Sohn

Going Deeper:

Raloff, Janet. 2007. Alien alert: Shrimpy invader raises big concerns. Science News 171(Jan. 13):20. Available at http://www.sciencenews.org/articles/20070113/fob3.asp .

You can learn more about the mysid shrimp known as Hemimysis anomala at www.glerl.noaa.gov/pubs/photogallery/Hemimysis.html (NOAA) and www.grid.unep.ch/bsein/redbook/txt/hemimysa.htm (United National Environment Programme).

Sohn, Emily. 2005. Smelly traps for lampreys. Science News for Kids (Nov. 16). Available at http://www.sciencenewsforkids.org/articles/20051116/Note2.asp .

______. 2004. Alien invasions. Science News for Kids (May 12). Available at http://www.sciencenewsforkids.org/articles/20040512/Feature1.asp .

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