The trophic fingerprint of marine fisheries

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Abstract

Biodiversity indicators provide a vital window on the state of the planet, guiding policy development and management1,2. The most widely adopted marine indicator is mean trophic level (MTL) from catches, intended to detect shifts from high-trophic-level predators to low-trophic-level invertebrates and plankton-feeders3,4,5. This indicator underpins reported trends in human impacts, declining when predators collapse (“fishing down marine food webs”)3 and when low-trophic-level fisheries expand (“fishing through marine food webs”)6. The assumption is that catch MTL measures changes in ecosystem MTL and biodiversity2,5. Here we combine model predictions with global assessments of MTL from catches, trawl surveys and fisheries stock assessments7 and find that catch MTL does not reliably predict changes in marine ecosystems. Instead, catch MTL trends often diverge from ecosystem MTL trends obtained from surveys and assessments. In contrast to previous findings of rapid declines in catch MTL3, we observe recent increases in catch, survey and assessment MTL. However, catches from most trophic levels are rising, which can intensify fishery collapses even when MTL trends are stable or increasing. To detect fishing impacts on marine biodiversity, we recommend greater efforts to measure true abundance trends for marine species, especially those most vulnerable to fishing.

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Figure 1: Changes in MTL relative to unfished ecosystem MTL.
Figure 2: Trends in MTL from global marine catches.
Figure 3: Measured MTL.
Figure 4: MTL for each Large Marine Ecosystem.

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Acknowledgements

This work arose out of a working group at the National Center for Ecological Analysis and Synthesis (NCEAS), funded by the University of California Santa Barbara, the US National Science Foundation (NSF), and the Moore Foundation. T.A.B. was additionally funded by the School of Aquatic and Fishery Sciences, University of Washington. The RAM Legacy stock assessment database was funded by the Canadian Natural Sciences and Engineering Research Council and the Canadian Foundation for Innovation; the Sea Around Us Project was funded by Pew Charitable Trusts. Additional collaborators and data contributors are acknowledged in the Supplementary Information.

Author information

T.A.B. designed the study, analysed the data, and wrote the paper; R.W. analysed the Sea Around Us Project catch data; E.A.F. designed and ran the ecosystem model analyses; S.J. analysed some trawl survey series; C.R.M. combined trawl survey data into a global time series; G.T.P. provided and calculated trophic level estimates; D.R. collated and analysed stock assessment data; and S.R.T. analysed the FAO and Sea Around Us Project catch data. All authors discussed the results and contributed to the manuscript.

Correspondence to Trevor A. Branch.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Methods, Supplementary acknowledgments and additional references, Supplementary Tables 1-6, and Supplementary Figures 1-18 with legends. (PDF 2086 kb)

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Branch, T., Watson, R., Fulton, E. et al. The trophic fingerprint of marine fisheries. Nature 468, 431–435 (2010) doi:10.1038/nature09528

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