Climatide

flickr/Pieter Pieterse

Selecting the overfishing-related discovery of the year was a no-brainer. When this story hit, I overheard people talking about it in the local coffee shop. Granted, that’s in Woods Hole, where the vast majority of people in the coffee shop are likely to be ocean scientists. Still, this was big news. Here’s a lightly edited version of my original post:

“Trophic level” is a fancy term for whether you’re more likely to eat or be eaten. In the ocean, microscopic algae sit at the bottom of the food chain – a trophic level of one – while large predators such as sharks and tuna are on the top of the world, around trophic level four or so. Currently, many fisheries managers use the average trophic level of fish harvests as a leading indicator of ecological health. The system is based on a 1998 study of over four decades of data that indicated the average trophic level of catches was declining as we “fished down the food web” by overharvesting the highest trophic levels, then moving on to fish lower and lower on the chain. It’s a familiar storyline here on Cape Cod – when the cod dried up, fishermen moved on to dogfish and herring. But a high-profile study published in the journal Nature upended this theory and left scientists and managers fishing for a new measure of ecosystem health (sorry, subscriptions needed for both articles).

Trevor Branch/U of Washington

Contrary to the theory that humans are fishing down the marine food chain, a new study shows that catches of top predators have generally increased in recent decades. Named next to each trophic level is the species most often caught, for example bigeye tuna is the most often caught fish at level 4.5. The wider the line, the more fish are being caught at that trophic level.

WHAT WE KNOW

If we were fishing our way down the marine food chain, catches of high-trophic-level fish should be dropping (the lines on the graph to the right should be slanting down to the right). Instead, as you can plainly see, most of the lines are upward sloping, indicating that harvests are increasing. After a brief drop in the average trophic levels of fish being in the 1970′s, the new study finds that catches of fish at all trophic levels have generally gone up.

HOW WE KNOW IT

The current analysis is based on a comprehensive set of data – including worldwide catch data, stock assessments, scientific trawl surveys, small-scale fishery data and modeling results – compiled by the National Center for Ecological Analysis & Synthesis. Rather than calculating the average trophic level of all fish caught in a fishery or region (the basis of the 1998 study that sparked the “fishing down the food chain” idea), this new study tracked catches of individual species at different trophic levels.

WHAT IT MEANS

This study calls into question the most widely accepted indicator of fishery health, the one chosen by the U.N.’s Convention on Biological Diversity as the primary measure of global marine diversity. Trevor Branch, the lead author of the study and a University of Washington fisheries scientist, says that using average trophic level “like flipping a coin, half the time you get the right answer and half the time you get the wrong answer.” If fishermen targeted all levels of the food chain equally, entire ecosystems could be fished to collapse while the average trophic level of the catch remained steady – an indicator of fishery health under the current system. If fishermen targeted low-level shellfish before moving on to top predators, things could even look like they’re improving while they’re actually crashing. That’s exactly what has happened in the Gulf of Thailand, where fish at all levels have declined tenfold since the 1950s while the average trophic level indicator has been rising.

WHAT WE DON’T KNOW

Throwing out the old system leaves two enormous questions: What is the actual status of the world’s fisheries? And how do we monitor them now? There aren’t easy answers for either question.

Henry Gholz, program director for the National Science Foundation’s Division of Environmental Biology, points out the difficulty of the situation: “Monitoring all the fish in the sea would be an enormous, and impossible, task. But this study makes clear that the most common indicator, average catch trophic level, is a woefully inadequate measure of the status of marine fisheries.”