Research press release


Nature Ecology & Evolution

Shrinking whales signal population collapse



1900年以降、世界的な捕鯨モラトリアムが初めて実施された1985年までの期間について、商業捕鯨船で捕獲されたクジラ4種の個体数および個体サイズに関する歴史的データが収集されており、Christopher Clementsたちは、そこに切迫した崩壊のシグナルが表れているかどうかを調べた。その結果、シロナガスクジラ、ナガスクジラ、イワシクジラ、およびマッコウクジラの平均個体サイズは、いずれも20世紀の後半に急速に小型化しており、最も小型化が急激であったマッコウクジラでは、1905年と比較して1980年代の平均サイズが4メートル小さかった。この個体サイズの小型化に基づく初期的な警告シグナルは、クジラ資源の世界的急減の40年も前に検知することができた。


The global collapse of whale stocks during the twentieth century - caused/driven by commercial whaling - could have been predicted by tracking changes in their body size, finds a study published in Nature Ecology & Evolution. Although this retrospective analysis comes too late for global whale stocks, the use of body-size data as a reliable early warning signal of population collapse could be usefully applied to assess the health of today's fish stocks.

Commercial whaling was extensive in the eighteenth and nineteenth centuries, with whale fisheries exploiting more species as stocks became depleted through overfishing. Although the whaling fleets are now a fraction of the size they once were, the ability to reliably predict when population collapses will occur is still crucial to ensure the sustainable management of commercially exploited wild fish stocks.

Christopher Clements and colleagues explored whether signals of impending collapse were apparent in historic data collected on the abundance and body size of four species of whales caught by commercial whaling vessels between 1900 and 1985, when the global moratorium on whaling first took effect. They show that the average body size of blue whales, fin whales, sei whales and sperm whales all declined rapidly during the second half of the twentieth century, with the most rapid decline seen in sperm whales, whose average size was four metres shorter in the 1980s than in 1905. Early warning signals based on this decline in body size were detectable up to 40 years before global whale stocks collapsed.

Using a previously developed method for detecting when populations are likely to collapse, the authors show that the variation in whale body size decreased in all four species as stocks became less commercially viable and global fishing pressure was reduced.

doi: 10.1038/s41559-017-0188

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