Shorter lives and faster growth in overfished communities


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Overfishing has caused modern marine fish communities to grow faster and live shorter lives, according to a study by researchers from the Wildlife Conservation Society to be published in a forthcoming issue of the journal Conservation Biology.

Timothy McClanahan and Johnstone Omukoto compared species composition of fishes off the coast of Kenya using data from historical bone middens (domestic refuse heaps that indicate sites of historical human settlements) dating back to AD 750, modern fisheries, and areas closed to fishing.

They did this to test the hypothesis that species in assemblages of fish differ in relative abundance as a function of their size, growth rates, vagility (capacity to move or disperse), trophic level, and diet, and that this difference is caused by overfishing.

After analysing a total of 15,977 fish from middens, closures, and fisheries, the authors found that the fishes in the historical and modern assemblages were noticeably different, both in terms of species composition and population characteristics.

The fishes from about AD 750 to 950 had longer lifespans, older age at maturity, and longer generation times than those after AD 950.

At the same time, fishes in the modern assemblages had smaller mean body sizes, higher growth and mortality rates, and higher rates of food consumption; the proportion of herbivores, omnivores and species that fed on microinvertebrates was also higher than in the historical assemblages (which had a higher proportion of species feeding on fishes and large invertebrates).

These differences are consistent with an unsustainable fishing hypothesis in which an increase in fishing effort and time is predicted to reduce abundance of slow-growing apex predators and mean trophic level and to increase abundance of opportunistic species (those with broad diets, high feeding rates, and high growth and mortality rates).  

This was supported by the finding that the modern fish communities in areas closed to fishing had life histories more similar to those of the historical assemblages than to modern fisheries.

For more information, see the paper: McClanahan, TR and JO Omukoto  (2011) Comparison of modern and historical fish catches (AD 750–1400) to inform goals for marine protected areas and sustainable fisheries. Conservation Biology, DOI: 10.1111/j.1523-1739.2011.01694.x

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