Old widely-adopted method of measuring health of fisheries shown inaccurate

Example of a trophic pyramid for marine organisms. (Source: ScienceLearn.org, <http://www.sciencelearn.org.nz/Contexts/Life-in-the-Sea/Science-Ideas-and-Concepts/Marine-food-webs>)

-       According to a team of scientists led by Trevor Branch of the University of Washington, the most used method of measuring the health of ocean ecosystems and fisheries yields accurate results only half the time. 
-       Science originally published the measurement method in 1998, which is based on the trends of fish at various trophic levels. The paper that was published said that based on catch data over 40 years, trophic level averages were falling. This led the researchers to believe that humans were taking a top-bottom approach to fishing; in other words, they thought humans were overfishing species at higher trophic levels first and then moving to lower trophic levels as the stocks became depleted. 
-       Now, new analyses have been developed that integrate and assess other factors besides average trophic levels of catches; these other factors include populations, catch data, and which species of fish live in the particular ecosystem being studied. 
-       With the new data, catches of species at higher trophic levels have actually increased rather than decreased (a decrease what would have been expected from fishing down the food web). 
-       The older measure of the health of fisheries ecosystems only works if there is top-down fishing rather than bottom-up fishing. For example, the assessment fails in Thailand because the fisheries in that area went after low-trophic level organisms like shrimp first and then moved to catching higher-level species later. The measure thus concluded that Thailand fisheries were healthy when in actuality the ecosystems were being overfished. 
-       Because we are catching more of everything in general, average trophic levels are not a good sole indicator of the health of an ecosystem, which is why the 1998 method of measurement fails half of the time.

Source:
"Scientists question indicator of fisheries health, evidence for 'fishing down food webs'". 17 Nov 2010. PhysOrg.com. <http://www.physorg.com/news/2010-11-scientists-indicator-fisheries-health-evidence.html>.

 -- Erika Najarro