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Body size shifts and early warning signals precede the historic collapse of whale stocks


Predicting population declines is a key challenge in the face of global environmental change. Abundance-based early warning signals have been shown to precede population collapses; however, such signals are sensitive to the low reliability of abundance estimates. Here, using historical data on whales harvested during the 20th century, we demonstrate that early warning signals can be present not only in the abundance data, but also in the more reliable body size data of wild populations. We show that during the period of commercial whaling, the mean body size of caught whales declined dramatically (by up to 4 m over a 70-year period), leading to early warning signals being detectable up to 40 years before the global collapse of whale stocks. Combining abundance and body size data can reduce the length of the time series required to predict collapse, and decrease the chances of false positive early warning signals.

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Figure 1: Data from the IWC on the number and body size of whales caught from 1900 to 1985.
Figure 2: Metrics that include body-size-only (orange), catch-only (green) and size–catch (blue) data, which produced early warning signals in each population prior to the inflection point of the fishery.
Figure 3: Early warning signals were detectable with as little as 10 years of data prior to the inflection point.
Figure 4: Consistent early warning signals were present up to 40 years prior to inflection points.

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This work was made possible by an ERC starting grant (no. 337785) to A.O. and an SNF international short visit grant (no. IZK0Z3_166526) to C.F.C. Whale illustrations by K. Askaroff.

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This work was jointly conceived by all the authors. C.F.C. performed the analyses and wrote the first draft of the manuscript. All co-authors contributed substantially to revisions.

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Correspondence to Christopher F. Clements.

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The authors declare no competing financial interests.

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Supplementary Notes 1,2; Supplementary Figures 1–8; Supplementary Table 1; Supplementary References. (PDF 494 kb)

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Clements, C., Blanchard, J., Nash, K. et al. Body size shifts and early warning signals precede the historic collapse of whale stocks. Nat Ecol Evol 1, 0188 (2017).

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