Depredation by toothed whales of Patagonian toothfish on demersal longline and tuna and swordfish on pelagic longlines is a growing problem internationally and the main problem exposed the French longline fisheries operated from Reunion Island. Pelagic and demersal longline fishery operated from Réunion Island is the first French fishery in terms of economic value and the second merchant economic sector of Réunion Island (100 M d'€/yr). These fisheries are highly affected by this depredation with an estimated financial loss of € 65 million over the 2003-2013 period. The observed depredation levels raise both economic and conservation issues. Indeed the artificial supply of food helps in creating an imbalance between these populations of cetaceans and their natural resources. Longlining is one of the fishing methods with the lowest environmental effect. The objectives of OrcaDepred are firstly to better understand the depredation behaviour of and ecology of cetacean species involved to offer fishing companies operational and technological solutions to depredation. Technological approaches tested to this day, namely the use of pots, acoustic repellents are ineffective. Under the OrcaDepred four Work Packages (WP) will be implemented to study and solve the depredation issue. - WP1 aims at better understanding the natural feeding and interaction behaviours with the fishery cetaceans interacting with the lines, and in the case of pelagic longlines identify the cetacean species involves. For this two dimensions tracking movements (tracks and dives) of these cetaceans will be studied using i) a new generation of satellite tags processing on-board the pressure and acceleration data and ii) by passive acoustic monitoring using hydrophones deployed along pelagic longline or a dedicated acoustic vertical array to the demersal fishery. The interactions of cetaceans with the lines will be studied using an experimental line on which the hooks are equipped with accelerometers to assess when fish are caught and depredated. - WP2 is devoted to assessments of the bio-economic consequences of depredation through an ecological economic for sustainable management of these fisheries taking into-account depredation. Finally, ecosystem modelling based on trophic links between species will be carried out to assess the ecological consequences of fishing-depredation at the ecosystem level. - WP3 will consider whether changes in levels of interaction between cetaceans and ships are related to fishing practice differences between captains and/or vessel characteristics, with a special focus on acoustic noise generated. These analyses are essential to guide the fishing companies in their strategic choices: training their fishing captains and/or conducting technical changes on their vessels. - WP4 will implement a technological approach to remove depredation. In partnership with industry, new prototypes of fish protection devices on the lines and not harmful to cetaceans and possibly limiting the levels of accessory catch such as skate on demersal long line will be tested and operational systems will be patented. OrcaDepred federates the complete French scientific community currently involved in addressing the longline depredation issue. Other fisheries at both national and international levels should benefit directly from the OrcaDepred as this problem is expending worldwide. Orcadepred outcomes will lead to mitigation solutions and should generate a strong national and international audience.