Shrinking fish

You’ve probably heard the story about “the big one that got away.” Someone goes out fishing and claims to have caught a monster fish. There’s no proof, though, because the fish managed to free itself before it could be landed.

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.

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).