Depredation Dynamics in Longline Fisheries

Depredation in longline fisheries occurs when marine predators remove or damage target catch from baited hooks, undermining stock assessments, reducing catch rates and inflating operational costs. This phenomenon is most commonly associated with species such as killer whales, sperm whales and false killer whales, but can also involve seals and pelagic sharks. Depredation dynamics are shaped by predator foraging behaviour—including deep and rapid diving—and by human activities such as the spatial concentration of fishing effort, vessel density and gear deployment depth. As predators learn to associate vessels with easy prey, social transmission can spread depredation techniques through populations. The global scale of longline operations means that small changes in fishing strategy can have large effects on both economic outcomes for fisheries and conservation status of marine mammals. Mitigation approaches range from altering hauling speed and gear configuration to coordinated relocation of effort and multi-stakeholder management frameworks.

Research from Nature Portfolio

Analyses of several toothfish longline fisheries reveal that the probability of odontocete interactions is strongly influenced by how, where and when fishing effort is deployed. Variations in seasonal effort concentration, size and mobility of fishing areas, vessel density and operating depth explain between-fishery differences in interaction levels, which may exceed 50 % of fishing days in some regions. This work provides quantitative models linking fleet behaviour to depredation risk and demonstrates that by adjusting the timing, spatial footprint and intensity of operations, fishers can meaningfully reduce opportunities for whales to exploit longline gear. Such insights stress the value of integrating predator distribution data and dynamic fishing strategies in management plans.

Depredation Dynamics in Longline Fisheries publication trend

The graph below shows the total number of articles in depredation dynamics in longline fisheries across all publications each year (not limited to Nature Index journals).

Technical terms

Depredation: Removal or damage of target catch from fishing gear by predators.

Catch per unit effort (CPUE): Standardised metric of catch relative to fishing effort.

Spatio-temporal avoidance: Strategy of altering fishing location and timing to minimise predator overlap.

Bycatch: Unintentional capture of non-target species in fishing operations.

Social transmission: Spread of learned foraging behaviours within a predator population.

References

1. Movements and dive behaviour of a toothfish-depredating killer and sperm whale. ICES Journal of Marine Science (2018).
2. Mitigating killer whale depredation on demersal longline fisheries by changing fishing practices. ICES Journal of Marine Science (2014).
3. Commercial fishing patterns influence odontocete whale-longline interactions in the Southern Ocean. Scientific Reports (2019).
4. The Challenges of Managing Depredation and Bycatch of Toothed Whales in Pelagic Longline Fisheries: Two U.S. Case Studies. Frontiers in Marine Science (2021).
5. Patterns of depredation in the Hawai‘i deep‐set longline fishery informed by fishery and false killer whale behavior. Ecosphere (2021).
6. Using Models of Social Transmission to Examine the Spread of Longline Depredation Behavior among Sperm Whales in the Gulf of Alaska. PLOS ONE (2014).
7. Southern elephant seals (Mirounga leonina Linn.) depredate toothfish longlines in the midnight zone. PLOS ONE (2017).
8. Toothed whale and shark depredation indicators: A case study from the Reunion Island and Seychelles pelagic longline fisheries. PLOS ONE (2018).