Fishing for metrics

by , 20 November 2010

In 1998, Dan Pauly et al. of the University of British Columbia published a classic paper called “Fishing Down Marine Food Webs.” They reported that humans were steadily depleting oceans of the top predators in the food web and working our way down the food web as the fish ran out.  Pauly measured a value called “Mean Tropic Level,” (MTL), essentially the average place in the food web that our fish come from, finding that it had declined in fisheries globally since the 1970s.  (For a more elaborate but readable treatment of Pauly’s paper, see Corey Bradshaw.)

MTL has become a popular measure of health in fisheries, and was even used as a key metric for implementation of the Convention on Biological Diversity.  Now, though, a new study in Nature builds on Pauly’s findings and finds that things are considerably more complex than previously thought.  Trevor Branch of the University of Washington and his co-authors report that MTL has been driven down, in part, by spikes in the catch of small fish low on the food web like anchovies and sardines.  Also, when one zooms in to various fisheries, some of the changes in MTL appear to be due to artifacts in the way fisheries data are reported.  When MTL is measured by ocean surveys rather than by the reported tonnage of fish caught, trends are very different.  For instance, when Thai fisheries collapsed, MTL reported from fisheries catch actually went up, while MTL measured by surveys went down.  The reverse occurred when Atlantic Cod fisheries collapsed in the 1980s.

The picture that emerges from the study is one of complexity and ambiguity.  While there’s little doubt that global fisheries are in decline, they differ in how and how fast, and simple metrics like MTL can fail to capture these subtleties.

This paper is likely to show up in the media in the next few days, and I hope the story doesn’t get twisted into a caricature of Pauly vs. Branch, or even worse, a false debate over whether our fisheries are in danger.  Science already has a news commentary on the paper that plays up the conflict angle in some troubling ways.  Let’s see if other science journalists are able to resist. Pauly, D. (1998). Fishing Down Marine Food Webs Science, 279 (5352), 860-863 DOI: 10.1126/science.279.5352.860 Branch, T., Watson, R., Fulton, E., Jennings, S., McGilliard, C., Pablico, G., Ricard, D., & Tracey, S. (2010). The trophic fingerprint of marine fisheries Nature, 468 (7322), 431-435 DOI: 10.1038/nature09528