Branch and his co-authors are the first to combine so many trawl surveys for analysis – no one had combined more than a handful before. The trawl survey data came from efforts started three years ago by fisheries scientists and ecologists gathered at the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif. They brought together worldwide catch data, stock assessments, scientific trawl surveys, small-scale fishery data and modeling results. What emerged is the most comprehensive set of data yet for fisheries researchers and managers.
It paints a different picture from previous catch data and has revealed another major new finding: On a global scale humans don’t appear to be fishing down the food web, Branch said.
The new catch data reveal that, following declines during the 1970s in the average trophic levels of fish being caught, catches of fish at all trophic levels have generally gone up since the mid-80s. Included are high-trophic predators such as bigeye tuna, skipjack tuna and blue whiting.
“Globally we’re catching more of just about everything,” Branch said. “Therefore relying on changes in the average trophic level of fish being caught won’t tell us when fishing is sustainable or if it is leading to collapse.” That’s because when harvests of everything increase about equally, the average trophic level of what is caught remains steady. The same is true if everything is overfished to collapse. Both scenarios were modeled as part of the Nature analysis.
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