The Fingerprint of Fishing

ResearchBlogging.orgHow is fishing changing the ocean? This simple question has motivated a slew of fantastic research. One of the most pervasive ideas has been that of Fishing Down Marine Food Webs. Popularized by Daniel Pauly and colleagues in their 1998 paper, the idea simply states that when humans began fishing, we hit the top predators first. Gradually, as we depleted those stocks, human fishing moved down to the next trophic level. And the next. And the next.

Fishing down marine food webs. A conceptual diagram (left) and data from Pauly et al. 1998 (right). At right, the figure shows change in mean trophic level - the position in the food web - of fisheries from 1950 to 1994 in open ocean (top) and inland (bottom) areas.

This idea has a great deal of intuitive appeal and has guided quite a bit of research. Heck, its one of the primary reasons I focused my dissertation on the consequences of losses of predator diversity in the oceans. But, the idea has been challenged. Essington and colleagues formalized an alternative with Fishing Through Food Webs. The idea here was not that we were serially depleting different parts of the web, but, rather, that as demand for more seafood grew, we added more and more lower trophic levels. In many ways, this idea was slightly more terrifying. If true, we are essentially asking different levels of ocean food webs to simultaneously maximize production for our needs.

So, fishing down? Fishing through? Or, are neither of these right – is fishing purely based on what’s available? Or are we just fishing the heck out of everything?

A new paper by Branch and colleagues in Nature elegantly demonstrates that, in most cases, we’re just fishing everything. They begin by looking at multiple global fisheries and laying out concrete predictions for the fingerprint of fishing as proposed by each of the above scenarios. They then go on to say, ok, what do we see? How has the average trophic level of fisheries changed over time?

Predictions for the fingerprint of fishing under different scenarios. Predictions cover average trophic level of catch (top row), the whole ecosystem (middle row), and the porportion of fisheries collapsed (bottom row).

They find that, overall, we’re fishing everything. Sure, there are exceptions. In the Northern Atlantic in the US and Canada, we’ve fished down the web. And in a few places, it’s all about what species are easiest to catch. But on the whole, exploitation targets the whole food web.

Catch per year of species in different trophic levels. Fishing pressure on all trophic levels has increased since 1950.

These results conflict a bit with the earlier work of Pauly and colleagues. However, the explanation is fairly simple – the way we calculate the position of a fish in a food web has changed. As an aside as a food web ecologist, I’d say, let the buyer beware in terms of what your trophic level calculation is telling you. They different techniques all have various assumptions. If you are going to draw a strong conclusion, make sure it is conditioned upon your assumptions or conduct a sensitivity analysis.

Not only that, but fishing data is not always a reliable indicator of the status of whole marine food webs. There is greater complexity out there, and we need to be careful when using fishing records as proxies for the health of our oceans.

The results of Branch and colleague’s work have critical implications for assessing the health of our oceans. First off, the situation is potentially even more precipitous than what would be possible if we were simply fishing through food webs. Patterns of exploitation are more complex, and in overfished areas, the consequences may be far more unpredictable. Second, just assessing the average position of fished species in a food webs is not sufficient to determine ecosystem health. Rather, we need to look at ecosystem diversity, stability, and the vulnerability of the resources provided by the ocean. We need to not only think about individual species, but how ocean ecosystems work as a whole.

It’s a daunting challenge, and a grim picture. But the analysis here provides some helpful guiding lights through the murk of assessing the health of our oceans. And the future looks hopeful with some major initiatives tackling the problem.

Refs:
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

Essington, T. (2006). Fishing through marine food webs Proceedings of the National Academy of Sciences, 103 (9), 3171-3175 DOI: 10.1073/pnas.0510964103

Pauly, D. (1998). Fishing Down Marine Food Webs Science, 279 (5352), 860-863 DOI: 10.1126/science.279.5352.860