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Marine mammal population decline linked to obscured by-catch

  1. Martin Krko?ekc
  1. aDepartment of Zoology, University of Otago, Dunedin 9054, New Zealand;
  2. bWildbase, Institute of Veterinary, Animal and Biomedical Science, Massey University, Palmerston North 4442, New Zealand;
  3. cDepartment of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada M5S 3B2
  1. Edited by Alan Hastings, University of California, Davis, CA, and approved September 7, 2017 (received for review February 23, 2017)


Declines of marine megafauna such as turtles, pinnipeds, and whales are often related to mortality caused by capture or entanglement in fisheries gear. To help recovery of these species, trawl fisheries have implemented exclusion devices that release nontarget species. Despite decades of use, there has been no empirical evaluation of whether or not exclusion devices aid recovery of affected species. Long-term data on the endangered New Zealand sea lion and a trawl fishery in the Southern Ocean indicate that exclusion devices have paradoxically contributed to ongoing decline rather than recovery. Exclusion devices obscure the postrelease impact of elevated mortality or reproductive failure; meanwhile, reduced levels of reported by-catch may mislead management that continued decline is not associated with fisheries.


Declines of marine megafauna due to fisheries by-catch are thought to be mitigated by exclusion devices that release nontarget species. However, exclusion devices may instead conceal negative effects associated with by-catch caused by fisheries (i.e., unobserved or discarded by-catch with low postrelease survival or reproduction). We show that the decline of the endangered New Zealand (NZ) sea lion (Phocarctos hookeri) is linked to latent levels of by-catch occurring in sub-Antarctic trawl fisheries. Exclusion devices have been used since 2001 but have not slowed or reversed population decline. However, 35% of the variability in NZ sea lion pup production is explained by latent by-catch, and the population would increase without this factor. Our results indicate that exclusion devices can obscure rather than alleviate fishery impacts on marine megafauna.


  • ?1To whom correspondence should be addressed. Email: stefanmeyer621{at}gmail.com.

Published under the PNAS license.

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