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Oversimplified Doomsday Prophecy

16.11.2006
The recent article in the journal Science: “Impacts of Biodiversity Loss on Ocean Ecosystem Services” by Worm et al.* has attracted a great deal of media attention. The article extrapolates the negative trend in world fisheries to predict a total collapse by 2048. However, there are good reasons for claiming that the prediction is based on an oversimplified and partly meaningless presentation of the current situation.

Using data from the FAO’s fisheries statistics, the article presents a time-based overview of world fisheries, which shows that 29% of the species that are currently being fished (in 2003) are classified as having collapsed. In the article, “collapse” is defined as when less than 10% of the maximum catch of the species remains – a wrong definition, according to Ole Arve Misund, Research Director at the Institute of Marine Research, who also believes that it is wrong to say that it is species that collapse; rather, it is the fisheries on specific stocks that collapse. There are many examples of species that are still surviving perfectly well in the sea, even though the fisheries that exploit them have collapsed.

The total amount of any given fish species can also vary considerably due to natural causes, as in the case of capelin in the Barents Sea. Fishing of this species has been stopped on three occasions during the past 20 years, because the stock has collapsed due to natural causes.

Natural oscillations not taken into account

The article goes further, with a time-based overview of “cumulative collapses”, which demonstrates that since the 1950s, 65% of the species that are fished have collapsed. On the basis of these figures, the authors extrapolate to a collapse of 100% of all fished species by 2048. This is an oversimplified Doomsday prophecy, first and foremost because it does not take into account natural oscillations which alone may be sufficient to produce reductions of more than 10% in catches from one year to another. “However, we do need to take seriously the possibility that natural oscillations may have been amplified during the past few decades by over-harvesting and the poorer age-group composition of many stocks”, says Head of Research Group Kjell Nedreaas.

In fact, the definition of “cumulative collapses” also includes species and stocks which have been regenerated. For example, the stock of Norwegian spring-spawning herring, which collapsed during the 1970s, has been built up again thanks to a successful management policy, so that within the coming years, the stock will be capable of supporting a sustainable fishery of around 1.3 million tonnes a year!

Developments in world fisheries

The authors’ overview and their use of the concept of “cumulative collapses” thus leaves us with a false impression of how the world’s fisheries are evolving. It is quite true that the global catch of fish has fallen somewhat in recent years, and many stocks have been over-fished, resulting in partial collapse of the fisheries concerned. But many stocks have regenerated and are again providing the basis for large catches. This is a result of ever more comprehensive management measures, such as setting total allowable catches, technical regulations, restrictions on fishing effort, and temporal and spatial restrictions. It is therefore meaningless for the authors to include regenerated stocks as still being in a state of collapse.

For several decades, fisheries management has been based on the precautionary principle, and it is now evolving in the direction of a more highly integrated ecosystem-based process.

The aim is fisheries based on a limited number of target species, and a minimal impact of fisheries on other parts of the ecosystem. This aspect is also emphasised by the authors of the Science article, who find that conservation efforts and improvements in biological diversity should be capable of raising productivity, thus reversing a negative trend, in favour of larger, more stable catches.

Yvonne Robberstad | alfa
Further information:
http://www.imr.no/english/news/news_2006/oversimplified

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