A BLAZING row has broken out between fisheries scientists over
whether fishing fleets are depleting the world's oceans of their
large species. Fishing boats are thought to target smaller
species only once the big, predatory ones are fished out. In the
1990s, Daniel Pauly of the University of British Columbia in
Vancouver, Canada, calculated how high up the food chain the
boats' catches were on average, and found that this mean trophic
level (MTL) was falling. The MTL has since been adopted by the
UN Convention on Biological Diversity as its main measure of
ocean biodiversity. Now Trevor Branch of the University of
Washington in Seattle is contesting Pauly's conclusions. When he
compiled the MTL for catches between 1950 and 2005, he found
that it fell until the late 1980s then rose again (Nature, vol
468, p 431). "I'm saying we're depleting everything equally," he
says. Pauly says the interpretation is flawed, as fishing fleets
moved into new areas during the 80s and 90s. "Expansion resets
the clock, because y