Marine mammal interactions with fisheries are a growing major world-wide issue with both substantial ecological and economic consequences. Most longline fisheries operating around the world, including many toothfish fisheries in the Southern Ocean, are seriously affected by these interactions i.e. marine mammals eating fish directly from fishing gear, known as “depredation”, causing losses of tens of millions dollars to the fishing industry annually (Tixier et al. 2010). s.

Depredation can also have negative impacts on the conservation of recovering marine mammal populations as a result of incidental bycatch, lethal responses from illegal fishers, and increased dependence through artificial food provisioning. Finally, depredation can affect the management of fisheries and the fish stock assessment if depredated fish are disregarded when defining quotas. Stocks that have also been under pressure from illegal fishing activity are even more so sensitive to the effects of depredation. Most depredation activity in the Southern Ocean is due to interactions with odontocetes (toothed whales), specifically sperm whales and killer whales.

To begin addressing these issues globally, the first international workshop dedicated to odontocete depredation on demersal longlines operating in high latitudes was organised by the Coalition of Legal Toothfish Operators (COLTO) in Punta Arenas, March 2016. This workshop emphasised the urgent need to find global solutions to this issue. The Patagonian toothfish (Dissostichus eleginoides) fisheries, which are operated in Australian, French, SG&SSI, Argentinian, Chilean, South African and international waters of the Southern Ocean, experience either well-established or increasing depredation by killer whales (Orcinus orca) and/or sperm whales (Physeter macrocephalus). However, the state of local knowledge about, and efforts and means to reduce depredation varies substantially between these fisheries. As was discussed at the workshop, in several fisheries the potential or suspected increase of depredation paired with a lack of research effort could lead towards a threat to the fisheries’ economic viability, and further have affect the targeted fish stocks and depredating marine mammal populations if mitigation solutions are not found in the near future.

The workshop concluded that routine data collection framework across fisheries confronted with interactions with odontocetes is important for a better understanding of depredation and for solutions to be developed, both locally and globally.

Using the COLTO workshop as an excellent example of international science, industry, government and NGO collaboration, this resulting document aims to provide basic guidelines for observer programmes that are relatively new to dealing with marine mammal interactions or are interested in expanding their observation efforts. It is based on ten years of experience around the islands of Kerguelen and Crozet (Southern Indian Ocean), mainly with sperm whales and killer whales interacting with the longline fishery targeting Patagonian toothfish in this region. In this fishery seven vessels operate all year round using only automatic (weight integrated) bottom longlines. The fishery has had 100% observer coverage for the last 15 years. Data on interactions with marine mammals has been collected for 60,000 fishing events and more than 100,000 pictures of odontocetes have been taken for photoidentification purposes.