A tenet of modern fisheries science may be unfounded, suggests a study of how catches are affecting marine ecosystems. The finding has sparked a heated debate about how best to measure humanity's impact on the ocean.
A landmark study in 1998 found that we are 'fishing down the food chain' worldwide -- in other words, exhausting stocks of top predators such as cod before switching attention to smaller marine animals. This has since become accepted wisdom. But a study published in Nature today suggests that the indicator on which this claim is based -- 'mean trophic level' or MTL -- is severely flawed.
The authors of the 1998 paper have hit back, with one of them branding the latest research "completely invalid". But Trevor Branch, lead author of the Nature study and a fisheries scientist at the University of Washington, Seattle, stands by the work. "At a global level we are not fishing down," he says. "The results are quite clear on that."
Branch says that fishing down may have occurred in some local areas, for example as happened with cod in the Atlantic. But in other places -- such as the Gulf of Thailand -- fisheries first targeted creatures low in the food chain, such as mussels or prawns, and are now 'fishing up'.
Individuals wanting the bloody details can read the article and/or consult the original article and comment in Nature, The trophic fingerprint of marine fisheries. As for the policy implications of the work:
Timothy Essington, a fisheries researcher at the University of Washington, Seattle, whose work was cited by Branch, argues that for a detailed picture of what is happening in ecosystems, MTL must now be used in combination with other measures. Pauly's original paper took a "20,000-feet view", he says. "What we're seeing now is the view from that altitude isn't very clear."
Essington compares looking at MTL to a physician taking a patient's temperature. If the temperature changes dramatically it is probably an indication that something is wrong, he says. But if the doctor decided on a treatment based on temperature alone, "you would never go back to that doctor again".