Fish stocks: Unlimited by-catch limits recovery

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We argue that government decisions to increase the social and economic benefits of fisheries will be ineffective without improvements in data-reporting practices and in regulations for targeted by-catches (see J. Casey et al. Nature 530, 160; 2016).

Take the swordfish (Xiphias gladius), a target species in the Atlantic longline fishery with a strictly regulated low annual total allowable catch. The unregulated 'by-catch' consists mainly of shortfin mako (Isurus oxyrinchus) and blue sharks (Prionace glauca), whose fins and meat are commercially valuable. Similarities between the gutted carcasses of swordfish and mako, without heads or fins, mean that there is potential for illegal overfishing of a regulated species that can then be logged as an unregulated species on landing.

Catches of shortfin mako typically comprise 3–13% of blue-shark catches in the same longline or gill-net fishery (J. D. Stevens in Sharks of the Open Ocean Ch. 7, 90; Blackwell, 2008). Yet the 2008 mako landings of a European fleet were, on average, six times those of blue shark (unpublished data; available from D.W.S. and N.Q.). This implies that the excess 'mako' could have been a regulated species such as swordfish.

The scale of the problem may already have affected stock rebuilding. More stringent surveillance of by-catch species by national regulatory authorities is essential for spotting such irregularities.

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  1. Marine Biological Association, Plymouth, UK.

    • David W. Sims
  2. CIBIO/InBIO — University of Porto, Portugal.

    • Nuno Queiroz

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