Small fish populations threatened by overfishing

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The plight of the Bluefin tuna as an overfished species is well documented, but research to be published in a forthcoming issue of the Proceedings of the National Academy of Sciences provides evidence that smaller fishes are just as threatened, if not more so, by overfishing.

Malin Pinsky and coauthors analysed 223 scientific assessments of fisheries for 120 fish species conducted worldwide and examined global landings reported by the Food and Agriculture Organisation (FAO) for 1950 to 2006, treating each FAO statistical area as a stock, for a total of 891 stocks across 458 species.  

After their analysis, they came to the conclusion that populations of small fish such as anchovies and smelt were at least as likely to have collapsed at some point in the last 50 years as stocks of large fish.

The authors tested the hypothesis that large, high trophic-level species would show a higher incidence of collapse than small, low trophic-level species and found that 16% of the stocks of large fish species had collapsed, while almost twice the percentage (29%) of the stocks of small fish species suffered a collapse.  

The findings run contrary to what is known for terrestrial ecosystems, where large, slow-growing species (such as top predators) are more vulnerable to population collapse due to human interference.  

One reason that the smaller, faster-growing fish species suffer more population collapses due to overfishing is because they are fished more intensively (current fisheries management practices often recommend higher exploitation rates for species that grow faster and have shorter generation times and greater productivity), according to the authors.  

Another reason is that the short generation times of the smaller species makes them vulnerable to overfishing, especially if fishing pressures are not reduced when these populations undergo large-scale decreases due to other environmental (e.g. climatic) conditions.

The high incidence of population collapses seen in small, short-lived fish species occupying the lower trophic levels has important implications for management of ecosystems.

Populations of larger-bodied fishes, seabirds and marine mammals may be threatened by these collapses, especially if they rely heavily on the smaller fishes for food.

For more information, see the paper: Pinsky ML, OP Jensen, D Ricard and SR Palumbi (2011) Unexpected patterns of fisheries collapse in the world's oceans. Proceedings of the National Academy of Sciences of the USA doi: 10.1073/pnas.1015313108