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Sampling mobile oceanic fishes and sharks: Implications for fisheries and conservation planning

Tom Letessier, Phil Bouchet, Jessica Meeuwig | Dec 18, 2015

Tom Letessier, Phil Bouchet, Jessica Meeuwig

Dec 18, 2015

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Marine protected areas of the world currently cover 2.2% of the global ocean but are rapidly increasing in size and numbers.

Photo: IUCN and UNEP-WCMC (2013); The World Database on Protected Areas (WDPA) Official Map Series.

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Dr. Tom Letessier
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Letessier TB, Bouchet PJ, Meeuwig JJ. 2017. Sampling mobile oceanic fishes and sharks: Implications for fisheries and conservation planning. Biological Reviews, 92(2): 627–646.


  • Mobile sharks, tunas, and sailfish are amongst the most threatened and iconic predators on the planet.
  • Their numbers have however collapsed in the last 50 years due to overexploitation.
  • Marine reserves have sprouted over all of the world’s oceans in response to these declines, but their merits for protecting migratory species still remain uncertain.
  • One key challenge lies in that most of what we know about these animals comes from decades of commercial fishing records, which have historically been the only data available for stock management. Lethal sampling is often not permissible in protected spaces, so measuring how well ocean sanctuaries can conserve biodiversity will therefore require innovative solutions.
  • We describe a number of emerging technologies (e.g. underwater action cameras, satellite tags and acoustic echo-sounders) that may be of value for monitoring elusive marine vertebrates in offshore waters. We demonstrate their use in a large no-take area and discuss how they may generate the knowlegdge necessary to inform conservation on increasingly large scales.


Tuna, billfish, and oceanic sharks [hereafter referred to as ‘mobile oceanic fishes and sharks’ (MOFS)] are characterised by conservative life-history strategies and highly migratory behaviour across large, transnational ranges. Intense exploitation over the past 65 years by a rapidly expanding high-seas fishing fleet has left many populations depleted, with consequences at the ecosystem level due to top-down control and trophic cascades. Despite increases in both CITES and IUCN Red Listings, the demographic trajectories of oceanic sharks and billfish are poorly quantified and resolved at geographic and population levels. Amongst MOFS trajectories, those of tunas are generally considered better understood, yet several populations remain either overfished or of unknown status.

MOF population trends and declines therefore remain contentious, partly due to challenges in deriving accurate abundance and biomass indices. Two major management strategies are currently recognised to address conservation issues surrounding MOFS: (i) internationally ratified legal frameworks and their associated regional fisheries management organisations (RFMOs); and (ii) spatio-temporal fishery closures, including no-take marine protected areas (MPAs). In this context, we first review fishery-dependent studies relying on data derived from catch records and from material accessible through fishing extraction, under the umbrella of RFMO-administrated management. Challenges in interpreting catch statistics notwithstanding, we find that fishery-dependent studies have enhanced the accuracy of biomass indices and the management strategies they inform, by addressing biases in reporting and non-random effort, and predicting drivers of spatial variability across meso- and oceanic scales in order to inform stock assessments.

By contrast and motivated by the increase in global MPA coverage restricting extractive activities, we then detail ways in which fishery-independent methods are increasingly improving and steering management by exploring facets of MOFS ecology thus far poorly grasped. Advances in telemetry are increasingly used to explore ontogenic and seasonal movements, and provide means to consider MOFS migration corridors and residency patterns. The characterisation of trophic relationships and prey distribution through biochemical analysis and hydro-acoustics surveys has enabled the tracking of dietary shifts and mapping of high-quality foraging grounds.

We conclude that while a scientific framework is available to inform initial design and subsequent implementation of MPAs, there is a shortage in the capacity to answer basic but critical questions about MOFS ecology (who, when, where?) required to track populations non-extractively, thereby presenting a barrier to assessing empirically the performance of MPA-based management for MOFS. This sampling gap is exacerbated by the increased establishment of large (>10,000 sq. km) and very large MPAs (VLMPAs, >100,000 sq. km) – great expanses of ocean lacking effective monitoring strategies and survey regimes appropriate to those scales. To address this shortcoming, we demonstrate the use of a non-extractive protocol to measure MOFS population recovery and MPA efficiency. We further identify technological avenues for monitoring at the VLMPA scale, through the use of spotter planes, drones, satellite technology, and horizontal acoustics, and highlight their relevance to the ecosystem-based framework of MOFS management.



Declines in tuna and billfishes reported by catch per unit effort (CPUE) and stock assessment in the literature over the last 12 years globally, in order of declining medians (Letessier et al. 2016).


T.B.L. and P.J.B. were supported by the Marine Biodiversity Hub through the Australian Government’s National Environmental Research Program (NERP). T.B.L. and J.J.M. would like to acknowledge the support of the Bertarelli Foundation. Marine Biodiversity Hub partners include the Institute for Marine and Antarctic Studies, University of Tasmania; CSIRO, Geoscience Australia, Australian Institute of Marine Science, Museum Victoria, Charles Darwin University and the University of Western Australia. P.J.B. was the recipient of a scholarship for international research fees (SIRF) during the course of this work. We thank Heather Koldewey for ideas and concepts behind this review. We are grateful to Sara Maxwell and one anonymous reviewer for constructive comments.