Climatide

Humans are pummeling the ocean with a powerful one-two punch of fishing and climate change that could turn delicacies like tuna sashimi and baked cod into things of the past … as soon as 2050. That was the message from leading scientists at last weekend’s annual meeting of the American Association for the Advancement of Science.

Fishing down the food web

flickr/wanderlass

A new study predicts that large predatory fish could be gone by 2050, leaving a sea full of nothing but small fish, like herring and sardines.

Villy Christensen of the University of British Columbia announced that the biomass of large predatory fish in the ocean has dropped by two thirds in the past hundred years. Over half that change has taken place just in the last four decades. Meanwhile, small forage fish – herring, capelin, sardines, anchovies – have doubled, presumably because there have been fewer big predators to eat them. At this rate, Christensen says, there may be nothing but little fish left by 2050. The dramatic prediction is based on a computer model built using more than 68,000 estimates of fish biomass from around the globe, dating from 1880 to 2007.

Too hot to handle

If that’s not enough, what predatory fish remain in 2050 may be smaller and harder to catch than their modern counterparts. The ocean has absorbed the vast majority of the extra heat energy trapped by rising greenhouse gases. The resulting rise in water temperatures near the ocean’s surface is already driving fish toward the poles and into deeper offshore waters in search of the cooler waters they prefer. William Cheung of the University of East Anglia says the poleward movement could be as rapid as two and a half miles per year. Imagine George’s Bank without cod. It could be just decades away.

In addition, Cheung says the combination of warmer waters, shifting ocean chemistry, and anticipated drops in the microscopic plants at the base of the food chain are likely to cut the maximum size of future fish almost in half. All told, Cheung predicts a 30% drop in New England fish harvests.

Recipe for revival

The kinds of changes Christensen and Cheung are talking about could spell economic disaster for already struggling fishermen, not to mention major dietary changes for seafood lovers. But it’s not a fait accompli. Here’s what scientists say needs to happen in order to keep fish populations healthy enough to sustain fisheries and weather climate change:

1. Reduce greenhouse gas emissions immediately – The only way to limit changes in ocean temperature and chemistry is slow the meteoric rise in atmospheric levels greenhouse gases, notably carbon dioxide. There are two ways to do that: cut fossil fuel use and/or start capturing emissions to keep them out of the atmosphere.



flickr/yoshing_BT

2. Reduce fishing pressure immediately – In 2007, Congress gave federal fisheries regulators a deadline: end overfishing in U.S. waters by 2011. Strict catch limits put into place in the past year are expected to meet that mandate. But overfishing is a growing problem in other parts of the world, particularly Asia.
3. Diversify our diets – Christensen says we need to start eating through the food web, rather than fishing down it. That means putting fish like herring onto dinner plates, instead of turning them into bait for wild-caught predatory fish or fishmeal to feed farmed fish.