Climatide

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How many and what types of fish are in the sea is a perennial – and difficult to answer – question at the heart of an ongoing debate about the state of the world’s fisheries.

University of Washington fisheries scientist Ray Hilborn has a thought-provoking, upbeat take on news that large predatory fish, like salmon, tuna, and cod, have declined by two thirds in the past hundred years. I first encountered it in a succinct letter to the editor in the Washington Post:

The Feb. 21 news article “Experts sound alarm over decline of predator fish” reported on an unpublished, not peer-reviewed presentation at the American Association for the Advancement of Science’s annual meeting stating that the number of large predator fish in the oceans had declined by two-thirds in the past 100 years and that their prey, the forage fish, had doubled in abundance. If true, this is good, not bad news.

Harvesting the large fish of the ocean down to one-third of their original abundance is exactly how one maximizes the long-term sustainable yield of fish stocks. One-third of original abundance is not overfished; it is the objective of U.S. and international fisheries management.

In a longer (and rather caustic) column in Seafood News, Hilborn lays out three major points of disagreement or criticism of Villy Christensen’s recent announcement:

1. As above, he says that “the 2/3 decline is exactly what is supposed to happen in a well managed fishery in order to produce maximum sustained yield.” In other words, that’s the balance point where we’re taking as many fish as possible without triggering a population crash. He says that’s welcome news after Boris Worm’s high-profile 2003 study declared only a tenth of predatory fish remained. In fact, he says the news is even better than he and Worm (along with several colleagues) predicted back in 2009, when they decided to collaborate rather than fight. At that point, they concluded that many fisheries were headed in the right direction but that almost two thirds of fish stocks worldwide still needed reduced fishing limits to allow rebuilding and avoid collapse.
2. The increase in small forage fish, like herring and sardines, is even more encouraging, especially given high rates of fishing targeting those species in recent years. Hilborn gets enthusiastic, saying that “… despite the best efforts of world fishing fleets, these forage fish that provide perhaps 25% of the worlds catch, have doubled in abundance. Wow! That is certainly good news!”
3. Hilborn expresses concern about Christensen’s reliance on so-called ecosystem models rather than “actual trends.” While the point is well taken that computer models are not exact reproductions of the real world, they are a widely used and accepted technique in modern science, one that – used appropriately – can allow scientists to move beyond some limitations of the data at hand. The 2009 study was limited to ten well-studied ecosystems. Christensen’s model is based on a significant body of data and enables a global assessment.

Ultimately (as with most scientific debates), time and more research will be the tests of the competing assertions. But Hilborn and Christensen agree on one point – many of the world’s fisheries are in danger of collapse if fishing pressure is not curtailed soon.