Marine mammal Bycatch (fishing)

Addressing the activities and threats


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 of marine mammals 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, 3]. The major fishery gears contributing to marine mammal bycatch include trawls, purse seines, longlines, gillnets and pot/traps [3].
Jeff Hester | Ocean Image Bank
Jeff Hester | Ocean Image Bank

Impacts of bycatch

It is estimated that more than 500,000 marine mammals are incidentally captured each year from a range of fisheries which is generally acknowledged as the principal threat to the persistence and recovery of many marine mammal populations [4]. Reports of bycatch from gillnets abound [5] however, threats from specific fishery gears range between species and cetacean groups with Baleen whales at high risk from gillnets and buoy lines [6] with baited longlines posing a significant threat to toothed cetaceans [7]. Ghost fishing, a threat from discarded, lost or abandoned gear, also causes high mortality amongst marine mammals [8].

Pinnipeds are also major victims of bycatch and pinniped mortality estimated in the hundreds of thousands with significant interactions from trawls [7].

Strategies to reduce marine mammal bycatch

MPA management actions 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 MPA.

The most complete form of protection would be a complete ban on fishing within the MPA, which is rarely implemented within large MPAs (Papahānaumokuākea Marine National Monument is a notable exception [9]). More common strategies include bans or restrictions of gear types within the MPA, restrictions within critical marine mammal areas inside an MPA, or seasonal restrictions.

The substitution of fishing methods may provide mitigation, while technological strategies such as the use of acoustic deterrent devices (pingers), exclusion devices for trawls, or various types of pot/trap guard designs may all be useful, depending on the species of marine mammal concerned [3].

Bycatch monitoring can involve compulsory reporting of marine mammal bycatch (aided by observers on commercial vessels), with data centrally maintained and used in research for the purpose of minimising bycatch. Further strategies include outreach and training programs for commercial fishers [10].

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.

Threat Management Plan for Hector’s and Māui dolphin 
Banks Peninsula Marine Mammal Sanctuary 

& references

Additional resources


[1] Web page: Marine Mammal Commission: Marine Mammal Bycatch
[2] Web page: International Whaling Commission: Bycatch
[3] PDF: 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] PDF: 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] PDF: 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] Paper: 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] Paper: 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

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