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Reply to ‘Whaling catch data are not reliable for analyses of body size shifts’

Nature Ecology & Evolution volume 2, pages 757–758 (2018)Cite this article

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Finally, the authors suggest a potential problem regarding the blue whale analysis2. In the 1960s, blue whale landings shifted from harvesting ‘true’ blue whales to the smaller pygmy blue whale, which may well account for the observed sharp decline in the 95% mean body size after 1960. We eliminate all cases where a catch is recorded as a pygmy blue whale, but cannot account for those falsely recorded as ‘true’ blue whales. Moreover, we restricted our analysis of early warning signals to data from the years 1900 to 1935 (when the number of blue whales harvested declined significantly and before the shift to the pygmy blue whale). Indeed, we do not specifically discuss the final declines in mean body size of harvested blue whales in our original publication (noting that body size of caught individuals increased initially, and then declined steadily for the rest of the period of commercial whaling). However, we recognized that the declines seen after 1960 were not attributed to a potential shift to the smaller subspecies in that text, an oversight on our part that should have been included in the original submission.

In light of the concerns highlighted by Clapham and Ivashchenko2 we would like to expand briefly upon our original piece, and clarify where we see these analyses fitting into conservation biology more broadly. Our original analysis was intended to be a general, and consequently coarse, first step analysis of the overall trends in the body size distributions of whale catches. We ascribed no mechanisms to the observed trends, but their presence fits in to what is known of the effects of commercial harvesting on marine systems4. It should be noted that the IWC data that we analyse are not direct measures of whale population abundance or size distributions, but are measures of the landed individuals. Fisheries catch data such as these also reflect changes in the fishery and other associated drivers, including overexploitation, technological change, markets, price, targeting different sexes, ages and sizes, or changes in environmental conditions that together can act to influence the collapse in the catches as well as the observed changes in body size. The IWC dataset provides perhaps a unique opportunity to test these methods for predicting declines on real-world populations where, critically, the populations are unequivocally known to have collapsed. As with any fisheries data there are inconsistencies and misreporting, although such issues seem to be particularly prevalent in the IWC data2. Given the evidence presented above, we do not believe such vagaries invalidate our approach or the overall findings, given that the observed collapses are still predicted even when the data are less than perfect.

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Fig. 1: Global aggregated data from the IWC on the body size of sperm whales caught from 1900 to 1985 with data from the USSR and Japanese whaling fleets included and omitted.
Fig. 2: Trends in body size of harvested sperm whales across different whaling areas.

References

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  2. Clapham, P. & Ivashchenko, Y. Nat. Ecol. Evol. https://doi.org/10.1038/s41559-018-0534-2 (2018).

  3. Clements, C. F., Blanchard, J. L., Nash, K., Hindell, M. & Ozgul, A. Nat. Ecol. Evol. 1, 0188 (2017).

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  4. Cheung, W. W. L. et al. Nat. Clim. Change 3, 254–258 (2013).

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Acknowledgements

The authors would like to thank P. Clapham and T. Branch for bringing these issues to light and discussing them at length with the authors.

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Authors and Affiliations

  1. School of BioSciences, University of Melbourne, Parkville, Victoria, Australia

    Christopher F. Clements

  2. Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia

    Julia L. Blanchard, Kirsty L. Nash & Mark A. Hindell

  3. Centre for Marine Socioecology, Hobart, Tasmania, Australia

    Julia L. Blanchard & Kirsty L. Nash

  4. Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania, Australia

    Mark A. Hindell

  5. Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland

    Arpat Ozgul

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  1. Christopher F. Clements
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Correspondence to Christopher F. Clements.

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Clements, C.F., Blanchard, J.L., Nash, K.L. et al. Reply to ‘Whaling catch data are not reliable for analyses of body size shifts’. Nat Ecol Evol 2, 757–758 (2018). https://doi.org/10.1038/s41559-018-0537-z

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