"This is important because that measure is the most widely adopted indicator by which to determine the overall health of marine ecosystems," said Branch, lead author of the Nature paper.
Those involved with the U.N.'s Convention on Biodiversity, for instance, chose to use the average trophic level of fish caught as the main measure of global marine diversity.
An example of the problem with the measure is in the Gulf of Thailand where the average trophic level of what is being caught is rising, which should indicate improving ecosystem health according to proponents of that measure.
Instead, it turns out fish at all levels have declined tenfold since the 1950s because of overharvesting.
"The measure only declines if fisheries aimed for top predators first, but for the Gulf of Thailand the measure fails because fisheries first target mussels and shrimp near the bottom of the food web, before shifting to fish higher up," 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 co-authors are the first to combine 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, who gathered at the NSF-supported National Center for Ecological Analysis and Synthesis (NCEAS) in Santa Barbara, Calif.
They brought together world-wide catch data, stock assessments, scientific trawl surveys, small-scale fishery data and modeling results.
What emerged is the m
|Contact: Cheryl Dybas|
National Science Foundation