Depredation mitigation project

This project drew to a close in late June 2014. During its four year period, some very interesting outcomes have bene generated, which have considerable promise and applicability in situations where operational interactions between odontocetes (toothed whales) and pelagic longline fisheries occur. These outcomes demonstrate that physical and psychological deterrence strategies have the potential to mitigate the negative economic impact of depredation on the fisheries involved and the negative conservation impact of by-catch on the odontocete populations involved. Details about the project are provided below and have been communicated more widely in journal publications and Government reports, the references for which can be found by scrolling to the bottom of this page.

Background and context

Since the late 1980s, many major commercial fisheries have experienced declining catches due to overfishing, driven by the unabated and increasing demand for fish for human consumption. Fewer fish, increasing management restrictions designed to conserve the remaining viable fish stocks, and increased operational costs associated with fuel prices and crew wages, have all impacted negatively on the economic viability of affected commercial fisheries. Over the same period, many pelagic longline fisheries have found themselves in competition with odontocetes (toothed whales). In some cases this may be because local populations have finally recovered from harvesting during the global whaling period that ceased during the mid 1900s, which is the case for sperm whales in particular. However, most cases are probably the result of both major predators – humans and whales – competing for the same finite and dwindling resource.

Odontocetes are intelligent, thus quickly learn to opportunistically forage or ‘depredate’ fish caught in or on fishing gear. Depredation is the act of odontocetes (i) deterring fish from approaching fishing gear, (ii) completely removing caught fish, or (iii) damaging caught fish that are landed on the vessel. Some fish may feature in the natural diet of odontocetes, while others may be too large, too quick, or reside in waters that are too deep for them to catch when foraging naturally. In some cases, odontocetes may switch to targeting fish caught on fishing gear, because their natural prey has been overfished, either previously by the same fishery, or by another fishery. There are a growing number of reports about the economic impact of depredation on fisheries, with reports of resulting lost revenue in some affected fisheries being in the millions annually, with the global cost across lognline fisheries being in the 10-100s of millions.

While this phenomenon further reduces the profitability of commercial fishing, it also places odontocete populations at risk of decline if they become by-caught when depredating. Depredating individuals may misjudge their movements around the fishing gear, or may ingest a hook. These scenarios may lead to immediate drowning death if the animal is unable to reach the surface to breathe, or to prolonged death from injury and infection. There are reports that by-catch mortalities on pelagic longlines in Hawaiian waters may be responsible for the decline of a resident false killer whale population there. Aside from the obvious threats to the conservation of many of the species involved, the welfare of the individuals that are drowned, or that sustain life threatening injuries also needs to be considered.

Finding a solution

Given the impact that depredation has on the longline fisheries involved and that by-catch has on the odontocetes involved, there is a need to search for effective ways to mitigate the interaction. A project was initiated in early 2010 to investigate physical deterrence as a possible strategy, headed up by Dr. Derek Hamer, a former biologist at the Australian Antarctic Division. Derek has extensive experience in assisting fisheries to mitigate operational interactions with marine mammals and worked closely with fishing gear manufacturers to develop devices and with affected pelagic longline fisheries in Australia and Fiji to assess their efficacy under operational conditions. The devices were developed in consultation with fishers, with several reporting that depredating odontocetes avoided taking caught fish form hooks near gear tangles. In response, two devices were developed to simulate gear tangles.

Deterrence was thought to be either physical (by providing a barrier between the caught fish and the depredating whale), or psychological (assuming that depredating whales may have had a previous negative experience with a gear tangle, perhaps becoming temporarily entangled). Product development resulted in prototyping and manufacture of two devices that became known as the ‘cage device’ and the ‘chain device’. Extensive trials were undertaken to quantify their effect on (i) target fish catch rate, and odontocete (ii) depredation and (iii) by-catch.

After observing 100,000 hook hauls, the data obtained produced promising results. There was negligible impact on soak depth, although sink rate was slowed, which may be problematic in areas of overlap with seabirds. Additional weight may remedy this. Surprisingly, target fish catch rates increased in the presence of the devices, perhaps because more fish were attracted to their vicinity, or more depredators were deterred. Size and survival of caught fish also appeared unaffected. Hauling was slowed by the use of the devices, with the need for an extra crewmember during setting and hauling likely cost prohibitive in some regions where crew wages are high. Importantly for affected odontocetes and despite the small sample size, by-catch only occurred on unprotected fishing gear and all individuals were released alive. However, their fate remained uncertain and by-catch levels on unprotected gear is likely higher than reported given its cryptic nature.

Device details

Following are three videos showing how the cage and chain devices are deployed on and retrieved from pelagic longlines. For further information, please contact Mike Double at the AAD, or search the net for Derek Hamer’s contact details.



Hamer, D.J., Childerhouse, S.J., McKinlay, J.P., Double, M.C., Gales, N.J., in press. Two devices for mitigating odontocete by-catch and depredation in pelagic longline fisheries: physical and psychological deterrence at the hook. ICES Journal of Marine Science.

Hamer, D.J., Childerhouse, S.J., 2013. Mitigating odontocete by-catch and depredation in longline fisheries: non-lethal physical and psychological deterrence at the hook. Final report to the Pacific Islands Forum Fisheries Agency (FFA), World Wildlife Fund (WWF) South Pacific, and Pacific Islands Tuna Industry Association (PITIA). Australian Marine Mammal Centre (AMMC), Department of Sustainability, Environment, Water, Population and Communities (DSEWPaC). 14 pp.

Hamer, D.J., Childerhouse, S.J., 2012. Physical and psychological deterrence strategies to mitigate odontocete by-catch and depredation in pelagic longline fisheries: progress report. Progress report to Pacific Islands Forum Fisheries Agency (FFA), World Wildlife Fund (WWF) South Pacific, and Pacific Islands Tuna Industry Association (PITIA). Australian Marine Mammal Centre (AMMC), Department of Sustainability, Environment, Water, Population and Communities (DSEWPaC). 47pp.

Hamer, D.J., Childerhouse, S.J., Gales, N.J., 2010. Mitigating operational interactions between odontocetes and the longline fishing industry: a preliminary review of the problem and of potential solutions. Report to the International Whaling Commission (IWC) Scientific Committee, Report no. SC/62/BC3. Australian Marine Mammal Centre (AMMC), Department of Sustainability, Environment, Water, Population and Communities (DSEWPaC). 30pp.

Invited presentations

  • at three of the five multinational tuna Regional Fishery Management Organisations (RFMOs), particularly through the Western Central Pacific Fishery Commission (WCPFC) and the Indian Ocean Tuna Commission (IOTC)
  • the multinational Consortium for Wildlife Bycatch Reduction
  • the intergovernmental Forum Fisheries Agency (FFA)
  • Fiji Ministry of Fisheries and Forests
  • Australian Government Environment Department Sustainable Fisheries Section
  • Australian Government Department of Agriculture, Fisheries and Forestry (DAFF) and the Australian Fisheries Management Authority (AFMA)


  • extensive relationship building between governments, researchers and the fishing industry
  • dissemination of information about depredation and by-catch mitigation using novel physical and psychological deterrence strategies
  • production of related high quality reports and journal articles
  • development of products to springboard further development in this space

Project supporters

Government and inter-governmental agencies

  • Australian Fisheries Management Authority (AFMA)
  • Department of Environment
  • Fisheries Forum Agency (FFA)
  • Samoa Ministry of Agriculture and Fisheries
  • Fiji Department of Fisheries and Forests
  • Indian Ocean Tuna Commission (IOTC)

Fishery associations

  • Pacific Islands Tuna Industry Association (PITIA)
  • Fiji Tuna Boat Owners Association (FTBOA)

Conservation agencies

  • Worldwide Fund for Nature (WWF)
  • Conservation International (CI)
  • South Pacific Regional Environment Program (SPREP)
  • Humane Society International (HSI)
This page was last modified on August 31, 2015.