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.