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 you’re exploiting a new ecosystem.”