In the Mean Trophic Level (MTL) system, fish species are assigned a value according to where they fit in in the food chain. A predatory fish like tuna that eats shrimp, mackerel and other smaller fish, fits at the top of the scale, while an oyster, whose dietary staple comprises of tiny plants, is at the bottom.
The concept assumes that humans are working their way down the food chain by consuming the larger fish more rapidly than the smaller fish. This idea, called “fishing down the food web”, was introduced in 1998.
It came about when a group of Canadian researchers put forth the suggestion that, according to data from the United Nations Food and Agriculture Organization, the MTL of landings, or fish sold, had declined (meaning relatively fewer big fish) since 1950, particularly in the northern hemisphere. The idea has served as a widely accepted paradigm assessing the state of the world oceans and fisheries.
In a more recent study, an American team introduced a different technique. They compared the MTL of catches with more directly observable data from trawl surveys and stock assessments for fish populations. Both of these methods measure the abundance of fish in the ocean, rather than what is coming up as catches in fishermen’s nets.
In contrast to the MTL measure of ocean health, the new study found that, on a global scale, predatory fish are not being replaced in nets by smaller prey fish. To the contrary, catches of many big predator fish, like selected varieties of tuna, have increased, as have those of many smaller species such as anchovy down to the filter-feeding oyster.
When comparing the methods, the team found the survey and assessment data conflicted with the catch data in about half of the cases for which the comparison was possible. Consequently, the American researchers suggested that the MTL of fish catches is not reliable at showing what is happening in undersea ecosystems.
“It’s not clear what this means for marine diversity on a global scale,” says Trevor Branch, lead researcher, University of Washington. “On one hand, it could be that, on a large scale, we’re not overfishing as previously thought. Or, on the other hand, it could mean that we’re fishing too hard in isolated locations.”