Atlantic cod—and the closely related haddock, pollock, whiting and hake—once supplied a seemingly endless bounty to fishing fleets off the coast of Canada and New England, where the cod grew as large as people. They were finally depleted by trawlers in the 20th century.

In 1497, Europeans reported that the coast of North America swarmed with so many cod that they could be scooped up in baskets. But by the late 20th century, cod had virtually disappeared from the North Atlantic, as huge fleets wiped out entire schools by dragging enormous nets along the bottom.

Why is the bottom so important for fish such as cod?
And what will it take for the fish to come back?

Bottom Feeders
Many species of fish and shellfish do most of their feeding near the bottom. The bottom also offers places for animals to hide from predators.

Dragging heavy nets stirs up the muddy sediment, removing vital nutrients that have settled on the bottom. Trawling also crushes the tunnels and tubes of small animals. With their food and homes destroyed, there are too few small animals left for fish such as cod to eat. Frequent trawling also levels the hiding places young fish need to survive.

The prime fishing grounds of Georges Bank once were home to a seemingly limitless abundance of cod, haddock, hake and scallops. But when collected with longlines or giant nets dragged across the bottom, their populations plummeted. Trawling removed so many fish and destroyed so much habitat that their populations have not been able to recover.

Fish Nurseries
The continental shelf is home to many sea creatures that feed near the shallow bottom. Young fish seek shelter among rocks and invertebrates to escape being eaten by larger fish.

Even after fishing boats trawled Georges Bank regularly, about a third of the bottom remained too rocky to drag nets over, so some young fish could always find a refuge.

But in the late 20th century, rubber wheels were added to trawling nets so they could be dragged over rocks. Using these "rockhoppers," almost all of Georges Bank could be trawled, contributing to the collapse of the fisheries there.

To rebuild their numbers, some adult fish must be left in the sea to breed. But limiting the total catch may not be enough for overfished species to recover. Some crucial bottom habitat may have to be set aside as marine reserves so that young fish can reach adulthood.