Yahoo News 17 Nov 10;
PARIS (AFP) – A yardstick for estimating ocean fish stocks, many of which are under intensifying pressure from industrial trawling, is badly flawed, a study released Wednesday said.
As a result, global stocks of some commercially valuable top predators -- including certain species of tuna, sharks and halibut -- may be closer to collapse than thought, it warned.
Since the late 1990s, scientists and regional management organisations have used catch data to measure changes in the balance of species across so-called "trophic levels."
The trophic level is the species' rank in the food chain. Microscopic sea algae have a trophic level of one, while large predators such as sharks or tuna are at the highest level, four.
Proportional changes within this ranking have been used as the indicator of how well a particular species is faring.
If, for instance, a species of "Trophic Four" fish was in disproportionate decline compared with "Trophic Three" fish on which they feed, this would likely indicate overfishing.
The method presumes that humans "fish down the food web" by over-harvesting fish at the highest levels and then sequentially going after fish further down the chain.
But the new study says this technique is not smart enough.
"Applied to individual ecosystems, it's like flipping a coin -- half the time you get the right answer and half the time you get the wrong answer," said Trevor Branch, a University of Washington professor.
"This is important, because that measure is the most widely adopted indicator by which to determine the health of marine ecosystems."
The method's shortcomings are illustrated by the case of the Gulf of Thailand, according to the paper, which appears in the journal Nature.
The average trophic level of what is being caught is rising -- and this in principle should indicate improving ecosystem health.
But it turns out that fish at all levels have declined by about tenfold since the 1950s because of overharvesting.
This disastrous drop is masked because the "trophic level" system is based on looking at the top predators first, say the authors.
But in the Gulf of Thailand, industry first targeted mussels and shrimps near the bottom of the food web before shifting to predators higher up, says the study.
When the researchers compared the catch-based method with a more accurate one, based on trawling over a long period of study, the results differed sharply in 13 out of the 29 ecosystems they evaluated.
Applying both methods to worldwide data, the scientists say industrial fishing over the past decades has not simply worked its way downwards from the top of the food chain -- it has gone upwards, too.
"Globally we're catching more of just about everything," Branch said.
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."
Scientists not involved in the study said the findings could revolutionise the way fish populations are measured.
"This study makes clear that the most common indicator, average catch trophic level, is a woefully inadequate measure of the status of marine fisheries," said Henry Gholz, an environmental biologist at the US National Science Foundation.
Scientists Question Widely Adopted Indicator of Fisheries Health and Evidence for 'Fishing Down Marine Food Webs'
ScienceDaily 17 Nov 10;
The most widely adopted measure for assessing the state of the world's oceans and fisheries led to inaccurate conclusions in nearly half the ecosystems where it was applied according to new analysis by an international team led by a University of Washington fisheries scientist.
"Applied to individual ecosystems it's like flipping a coin, half the time you get the right answer and half the time you get the wrong answer," said Trevor Branch, a UW assistant professor of aquatic and fishery sciences.
In 1998, the journal Science published a groundbreaking paper that was the first to use trends in the trophic levels of fish that were caught to measure the health of world fisheries. The trophic level of an organism shows where it fits in food webs, with microscopic algae at a trophic level of one and large predators such as sharks, halibut and tuna at a trophic level of around four.
The 1998 paper relied on four decades of catch data and averaged the trophic levels of what was caught. The authors determined those averages were declining over time and warned we were "fishing down the food web" by overharvesting fish at the highest trophic levels and then sequentially going after fish farther down the food web.
Twelve years later, newly compiled data has emerged that considers such things as the numbers and types of fish that actually live in these ecosystems, as well as catch data. An analysis in the Nov. 18 issue of Nature reveals weaknesses in assessing ecosystem health from changes in the trophic levels of what is being caught.
"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 new analysis in Nature. Those involved with the U.N.'s Convention on Biological Diversity, for instance, chose to use the average trophic level of fish being 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 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 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.
"The 1998 paper was tremendously influential in gathering together global data on catches and trophic levels and it warned about fishing impacts on ecosystems," Branch says. "Our new data from trawl surveys and fisheries assessments now tell us that catches weren't enough. In the future we will need to focus our limited resources on tracking trends in species that are especially vulnerable to fishing and developing indicators that reflect fish abundance, biodiversity and marine ecosystem health. Only through such efforts can we reliably assess human impacts on marine ecosystems."
"In this paper we conducted the first large-scale test of whether changes in the average trophic levels of what is caught are a good indicator of ecosystem status," says Beth Fulton, a co-author and ecosystem modeler with the Commonwealth Scientific and Industrial Research Organisation, Australia. "Catch data might be easiest to get, but that doesn't help if what it tells us is wrong. Instead we really need to look directly at what the ecosystems are doing."
Other co-authors are Reg Watson and Grace Pablico, University of British Columbia; Simon Jennings, Centre for Environment, Fisheries and Aquaculture Science and University of East Anglia, England; Carey McGilliard, University of Washington; Daniel Ricard, Dalhousie University in Halifax, Nova Scotia; and Sean Tracey, University of Tasmania, Australia.
The work was supported by the National Science Foundation, Gordon and Betty Moore Foundation and the UW School of Aquatic and Fishery Sciences. It used data from the National Center for Ecological Analysis and Synthesis working group, used the stock assessment database funded by the Canadian Natural Sciences and Engineering Research Council and the Canadian Foundation for Innovation and used data from the Sea Around Us project funded by Pew Charitable Trust.
Journal References:
1. Trevor A. Branch, Reg Watson, Elizabeth A. Fulton, Simon Jennings, Carey R. McGilliard, Grace T. Pablico, Daniel Ricard, Sean R. Tracey. The trophic fingerprint of marine fisheries. Nature, 2010; 468 (7322): 431 DOI: 10.1038/nature09528
2. D. Pauly. Fishing Down Marine Food Webs. Science, 1998; 279 (5352): 860 DOI: 10.1126/science.279.5352.860
