"The measure only declines if fisheries aimed for top predators first, but for the Gulf of Thailand the measure fails because fisheries first targeted mussels and shrimps near the bottom of the food web, before shifting to predators higher up in the food web," Branch said.
Including the Gulf of Thailand, Branch found that changes in the average trophic levels of what was being caught and what was found when fish populations were surveyed differed in 13 of the 29 trawl surveys from 14 ecosystems. Trawl surveys, generally done from research vessels, count the kinds and abundance of fish and are repeated over time to reveal trends.
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 tu
|Contact: Sandra Hines|
University of Washington