Canadian "fish cop" watches a fishing boat. Courtesy Fisheries and Oceans Canada.

By summer's end, this kelp forest is lush and bursting with life. Kelp forests, like the ones along the California coast, are major fish nurseries.
Courtesy Monterey Bay Aquarium Foundation

So far, we've been focusing on the amount of fish eligible to enter our gullets. But let's not forget that fish have their own impact on the ocean. In fact, according to a recent study, overharvesting of fish, shellfish and other marine creatures has caused extreme, long-term ecological damage.

Using data from sediments, archaeological sites, historical documents and scientific literature, Jeremy Jackson of the University of California at San Diego, and colleagues wrote that "Ecological extinction caused by overfishing precedes all other pervasive human disturbance to coastal ecosystems, including pollution, degradation of water quality, and anthropogenic climate change" (see "Historical Overfishing..." in the bibliography).

That's strong language for a scientist. The examples came from major coastal ecosystems:

Kelp forests, which are nurseries to numerous fish in vast parts of the continental shelf, are food for sea urchins. While sea otters once controlled these spiny animals in the Pacific, people have killed many otters, allowing the urchins to gobble large kelp forests. Similarly, declining numbers of cod has allowed sea urchins to clearcut kelp forests in the Gulf of Maine.

Ironically, sea urchins protected branching coral which dominated coastal waters in the tropical Western Atlantic for half a million years. Urchins ate algae that attacked the coral, and when the urchins disappeared in the 1980s, the coral dramatically declined.

Giant "reefs" of oysters ate vast quantities of floating plants in Chesapeake Bay, an estuary that's suffered mighty ecological trauma over the past century or so. The Jackson article described estuaries as "the most degraded of marine ecosystems," but noted that despite sedimentation, loss of seagrasses, and infusions of sewage, damage to the Chesapeake was mainly due to over-harvesting of oysters that started when mechanical dredges were introduced in the 1870s. The numerous oysters filtered the entire estuary every three days; experiments indicate that reintroducing oysters could improve the bay.

The relationship can be complicated, however. Sudden declines in species may reflect the loss of distant refuges, for example, or the addition of exotic species that affect key ecological relationships.

Sea urchins eat giant kelp. In sufficient numbers, they can graze -- even raze -- entire forests. Where urchins abound, new giant kelp have difficulty surviving and growing.
Courtesy Monterey Bay Aquarium Foundation

Sinkin' sushi
Nonetheless, the examples reinforce a truism of ecology, Magnuson says: "Everything is connected to everything else. Things are far more powerfully interrelated than people have taken account of."

The overfishing story also reflects how top predators maintain the structure and function of ecosystems -- on land and under the sea, Magnuson adds. "Fishing tends to remove the top predators [think swordfish and tuna] and release the control they exert on the rest of the food web."

By looking further into the past, Magnuson says, "The Jackson study ... was a major step forward. If we are complacent, we may have the feeling that the way it is now is the way it's always been."

Dip into our overfishing bibliography.