Climatide

flickr/Pieter Pieterse

It's a fish-eat-fish ocean out there. Whether humans are catching too many fish at the top of the food chain is the subject of a new study that calls into question over a decade of fisheries assessments.

“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 familiar storyline here on Cape Cod – when the cod dried up, fishermen moved on to dogfish and herring. But a new study published last week in the journal Nature has upended this theory and left scientists and managers fishing for a new measure of ecosystem heath (subscriptions needed for these articles).

Trevor Branch/U of Washington

If humans were fishing down the marine food web then catches of top predators, at trophic levels 3.5 and above, would be decreasing. But catches at those levels have generally increased according to newly compiled information. 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. Instead, the new study indicates that the average trophic levels of fish being caught declined in the 1970′s, but that 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.

HOW WE KNOW

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.

WHAT WE DON’T KNOW

This work brings into question the true status of many of the world’s fisheries – their health and sustainability (or lack thereof). Trevor Branch, a University of Washington assistant professor of aquatic and fishery sciences and the lead author of the study says the system currently in place is “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 their 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 IT MEANS

“This is important because that measure is the most widely adopted indicator by which to determine the overall health of marine ecosystems,” said Branch. For example, the U.N.’s Convention on Biological Diversity chose to use the average trophic level of fish being caught as the main measure of global marine diversity. This study suggests that we need a new metric of fishery and ecosystem health.

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.”