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Marine mammal Bycatch (fishing)

Addressing activities and threats

Co-authored by Bianca Cisternino, Bycatch Coordinator, Whale and Dolphin Conservation (WDC)


Marine mammal bycatch is the term given to any marine mammal adversely affected as a result of being unintentionally captured in fishing gear such as nets, lines, traps, or hooks, or otherwise impacted by fishing gear [1, 2]. Bycatch has long been recognised as the main anthropogenic threat to marine mammal species globally, hindering the persistence and recovery of many marine mammal populations [2, 3]. It is estimated that more than 500,000 marine mammals are incidentally captured each year on a global scale [3].
Jeff Hester | Ocean Image Bank
Jeff Hester | Ocean Image Bank

Impacts of bycatch

Marine mammal bycatch occurs in all kinds of fishing operations, from large industrial to localised artisanal fisheries, and can be a result of mobile or fixed fishing gears [2, 5]. The major fishery gears contributing to marine mammal bycatch include gillnets, trawls, purse seines, longlines, and pot/traps [5]. Gillnets are identified as the highest risk fishing gear for marine mammal bycatch [6] however, threats from specific fishery gears range between species and cetacean groups with Baleen whales at high risk from buoy lines [7] with baited longlines posing a significant threat to toothed cetaceans [8]. Ghost fishing, a threat from discarded, lost, or abandoned gear, also causes high mortality amongst marine mammals [9]. Pinnipeds are also major victims of bycatch and pinniped mortality estimated in the hundreds of thousands with significant interactions from trawls [8].

Strategies to reduce marine mammal bycatch

Due to the dynamic nature of fisheries and different types of interactions with marine mammals, each fishery should be handled on a case-by-case basis to determine the appropriate strategies to reduce bycatch.

  1. Area Closures / Fishery Closures: Marine spatial management actions, including Marine Protected Areas (MPA), and fishing closures to reduce marine mammal bycatch will vary with the particular marine mammal species at risk, and the type and extent of existing or potential fisheries in and around the area. A total ban on fishing within an MPA, for example, is appropriate and essential for removing fishing gear from the habitat of critically endangered species. However, due to the highly mobile nature of most marine mammal species, adaptive management with spatial and temporal closures that change based on marine mammal population movements will provide the most complete form of protection [11]. More common strategies include bans or restrictions of gear types within a particular area, restrictions within critical marine mammal areas inside a protected area, or seasonal restrictions.
  2. Changes to Fishing Gears and Practices: The use of alternative gears for high-risk fishing gear for bycatch can reduce potential entanglements such as the use of rope-less gear or sinking lines on pots/traps, whale-safe hooks for longline fishing, reduced gillnet length, thinning twine diameter, or switching gears completely (for example, longlines to pole and line fishing, gillnets to pots/traps) [12].

    Technological and sensory strategies can be used for bycatch mitigation such as the use of acoustic deterrent devices (pingers), light emitting diodes (LEDs), acoustic reflective nets, alternative bait types, changes to gear colours, exclusion devices for trawls, or various types of pot/trap guard designs may all be useful, depending on the species of marine mammal concerned and the interactions observed [5].

    Fishing practices can be altered to reduce the risk of bycatch and must be done in close collaboration with fishers. Reducing fishing effort and soak times, altering the time of day when fishing operations occur, and changing the depth to which gear is set, can all limit the amount of fishing gear interactions with marine mammals [13].
  3. Monitoring: For robust bycatch monitoring, the use of remote electronic monitoring (REM) is the optimal choice to obtain verifiable and accurate data on entanglements of marine mammals [14]. Bycatch monitoring should involve compulsory reporting of marine mammal bycatch using REM or at-sea observers, with data centrally maintained and used in research for the purpose of minimising bycatch.

Case study

Banks Peninsula Marine Mammal Sanctuary (New Zealand)

Hector’s dolphin (Cephalorhynchus hectori), found only within New Zealand waters, is currently listed as endangered. In response to threats of entanglement and by-catch from fishing, the Banks Peninsula Marine Mammal Sanctuary was gazetted in 1988. Covering 413,000 ha, the sanctuary extends 12 nmi out to sea.

Within the sanctuary, there is a year-round ban on amateur set-net fishing, and seasonal set-netting restrictions are in place. While the survival rate within the sanctuary, this was considered insufficient to allow population recovery. Therefore, broader management actions were introduced across the majority of the species’ range, including a ban on gill-netting within 4nmi of the coast in some areas and 2 nmi of the coast in others. Other strategies have involved fishing awareness campaigns, changing fishing methods, and substantial research using compulsory reporting of entanglement and bycatch and encouragement to report sightings. Early indications of population recovery illustrate the frequent need to extend management beyond small and medium-sized MPAs, even for relatively sedentary coastal species.


Image credit; Commercial and recreational set net prohibition areas off the west coast of the North Island – Hector’s and Māui Dolphin Threat Management Plan 2020

& references


[1] Web-page: Marine Mammal Commission: Marine Mammal Bycatch 

[2] Web-page: International Whaling Commission: Bycatch 

[3] Article: Hamilton, S., & Baker, G. B. (2019). Technical mitigation to reduce marine mammal bycatch and entanglement in commercial fishing gear: lessons learnt and future directions. Reviews in Fish Biology and Fisheries, 1-25. 

[4] Article: FAO (2021). Fishing operations. Guidelines to prevent and reduce bycatch of marine mammals in capture fisheries. FAO Technical Guidelines for Responsible Fisheries No.1, Suppl. 4. Rome. 

[5] Article: Peltier, H., Authier, M., Dabin, W., Dars, C., Demaret, F., Doremus, G., Van Canneyt, O., Laran, S., Mendez-Fernandez, P., Spitz, J., Daniel, P., & Ridoux, V. (2020). Can modelling the drift of bycaught dolphin stranded carcasses help identify involved fisheries? An Exploratory Study. Global Ecology and Conservation, 21: e00843. 

[6] Article: Van der Hoop, J. M., Moore, M. J., Barco, S. G., Cole, T. V. N., Daoust, P. Y., Henry, A. G. & Solow, A. R. (2012). Assessment of Management to Mitigate Anthropogenic Effects of Large Whales. Conservation Biology, 27(1): 121 – 133. 

[7] Article: Werner, T. B., Northridge, S., Press, K. M. & Young, N. (2015). Mitigating Bycatch and Depredation of Marine Mammals in Longline Fisheries. ICES Journal of Marine Science, 72(5): 1576 – 1586. 

[8] .pdf: FAO. 2019. Voluntary Guidelines on the Marking of Fishing Gear. Directives volontaires sur le marquage des engins de pêche. Directrices voluntarias sobre el marcado de las artes de pesca. Rome/Roma. 88 pp. Licence/Licencia: CC BY-NC-SA 3.0 IGO.

[9] Web-page: Papahānaumokuākea Marine National Monument.

[10] Web-page: Understanding Bycatch, NOAA

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