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 .

Since then, the Great Lakes Fishery Commission has spent many millions of dollars on attempts to control lampreys in the Great Lakes. The fish are a hearty bunch. Now, scientists from the University of Minnesota have found a new way to keep the destructive creatures at bay. They’re tapping into a lamprey’s sense of smell.

Two sea lampreys cling to the glass wall of a laboratory tank.

University of Minnesota

As larvae, sea lampreys spend up to 20 years in freshwater streams, where they eat and grow. Then, nearing adulthood, they move to a larger body of water, where they feed off one fish after another, for about a year. Finally, they swim to streams to mate. After a few weeks of breeding and laying eggs, they die.

Scientists have long suspected that lampreys follow the scent of each other’s pheromones, or body chemicals, to find suitable streams for mating.

An adult sea lamprey, about 1 meter long, swims to a spawning site.

University of Minnesota

To sort out which particular chemicals attract lampreys, the University of Minnesota researchers began with 8,000 liters (210 gallons) of water that had contained about 35,000 baby lamprey larvae. The scientists concentrated the liquid until they had just a few grams of gunk. They separated the gunk into individual chemical components, or compounds. Then, they tested each compound to see how lampreys responded to it.

The study turned up three compounds that both affected the lamprey’s sense of smell and attracted the fish. It was the first time that scientists have identified pheromones that affect migration in any vertebrate (animal with a backbone).

University of Minnesota researcher Peter Sorensen holds a sea lamprey.

David Hansen

After the researchers figured out which chemicals matter, they were able to create one of the compounds in the lab. If they can find a way to make the compound in large quantities, the scientists hope they’ll be able to use it to attract lampreys away from their breeding grounds. This would prevent the lampreys from reproducing, which would reduce their numbers and their impact.

Mimicking the lamprey’s scent could be a cheap and efficient way to reclaim the Great Lakes from a pesky parasite.—E. Sohn

Going Deeper:

Cunningham, Aimee. 2005. Whiff weapon: Pheromone might control invasive sea lampreys. Science News 168(Nov. 12):308-309. Available at http://www.sciencenews.org/articles/20051112/fob4.asp .

You can learn more about the sea lamprey at www.seagrant.wisc.edu/greatlakesfish/sealamprey.html (Wisconsin Sea Grant) and www.science.mcmaster.ca/Biology/Harbour/
SPECIES/SEALAMP/TITLE.HTM (McMaster University).

Whether or not the story is true, the chances of catching “the big one” seem to be getting worse and worse. In response to fishing pressures, fish are becoming smaller, growing more slowly, and having a harder time reproducing, says David O. Conover of the State University of New York at Stony Brook.

In response to fishing pressures, fish are becoming smaller, growing more slowly, and having a harder time reproducing.

The average size of fish of many species has declined in recent years. That’s not surprising. Smaller fish are more likely to squirm out of nets than larger ones. And laws often require that fish smaller than a certain size be returned to the sea. People tend to catch and keep the bigger, meatier ones.

Atlantic silversides.

Courtesy of David Conover

Fishing can also induce changes in fish that are passed on from generation to generation. To check this idea out, Conover grew several generations of Atlantic silversides in six aquarium tanks. In two of the tanks, he kept removing the biggest fish, to copy what commercial fishermen do in the ocean. He found that this caused each generation of fish to grow more slowly than the one that came before it.

When Conover repeatedly removed the smallest fish from two other tanks, each generation grew more quickly than the previous one. When he removed fish at random, there was no change in how fast the fish of each generation grew.

After four generations in which large fish are harvested, the descendents tend to be smaller than they were originally (bottom). Harvesting small fish leads to fish that are larger in later generations (top).

Courtesy of David Conover

Taking only big fish penalizes fish that grow quickly. Removing these animals from the gene pool leads to breeds that grow more slowly and don’t grow to be as large.

To see if the trend is reversible, Conover then stopped fishing from his tanks. So far, it seems to take longer than expected for the fish to recover their earlier growth patterns.

It’s not practical to stop fishing altogether. Instead, Conover proposes that fishing boats be required to throw back the biggest fish along with the smallest ones. This would help preserve fast-growing fish. And your chances of catching “the big one” might actually improve.—E. Sohn

Going Deeper:

Harder, Ben. 2005. Shrinking at sea: Harvesting drives evolution toward smaller fishes. Science News 167(Feb. 26):132-133. Available at http://www.sciencenews.org/articles/20050226/fob4.asp .

You can learn more about the impact of fishing on people and fish populations at www.ourplanet.com/aaas/pages/issues05.html (American Association for the Advancement of Science).

Chartering a boat or taking your own vessel out to sea to go fishing for fun is a large industry. Yet, for a long time, people thought that all the fish caught in this way represent just a tiny fraction of the number of fish caught by commercial fishing boats and sold in markets or brought to factories.

Once targeted by both recreational anglers and commercial fishing boats, the lingcod is one of many saltwater species being overfished.

OAR, NURP, Alaska Dept. Fish and Game

The impact of sportfishing is much bigger than previously suspected, say researchers from Florida State University in Tallahassee.

When the researchers took a close look at U.S. fisheries data, collected over more than 22 years, they found that recreational fishing accounts for 4 percent of fish caught. Two types of fish, menhaden and pollack, make up the bulk of the commercial catch, however. When these two types of fish are left out of the analysis, the fraction of all other fish caught and killed in recreational fishing jumps to 10 percent.

Finally, the researchers looked just at species that have been classified as “overfished.” That’s where the numbers are more troubling. Recreational anglers account for 59 percent of red snapper landings in the Gulf of Mexico, for example, and 93 percent of red drum landings in the southern U.S. Atlantic.

In some places and for some types of fish, it might be time for stricter regulations, the researchers say. There are already limits on the number of fish that someone can catch in many locations. But regulators rarely limit the number of people allowed to go fishing.

People aren’t going to stop fishing. If they just threw back what they caught, though, the fish now at risk might be better off.—E. Sohn

Going Deeper:

Milius, Susan. 2004. There’s a catch: Recreation takes toll on marine fish. Science News 166(Aug. 28):133-134. Available at http://www.sciencenews.org/articles/20040828/fob6.asp .

You can learn more about U.S. marine recreational fisheries at www.st.nmfs.gov/st1/recreational/index.html (National Oceanic and Atmospheric Administration).

Comments:

I now know that fishing for fun can really kill. And the orcas really need help. I hate what the world is doing to all the animals!—Megan, 13