A game model of coexistence between longline fishers and large marine predators

Abstract Fishers often report the presence of large marine predators, such as sharks and marine mammals, that hinder fishing operations, and even feed on their fishing lines. This latter behaviour, known as “depredation,” has been reported in many fisheries worldwide. Both marine predators and fishers develop interplaying strategies to minimize their energy costs or maximize daily catch. Fishers may choose to avoid depredation by moving away from depredators, thus incurring extra costs and forgone profits. In that regard, game theory can assist fishers and managers reduce negative interactions between humans and other predators. In this research, we developed a game theory framework in which both human and non-human species prey on the same fish stock, while depending on the sustainable dynamics of this shared resource. Analytical solutions with various game models representing non-cooperative scenarios show that the prey stock grows with higher depredation rates because fishing effort is reduced to meet the sustainability constraint. The outcome of an avoidance strategy is far more complex to analyse in terms of parametric conditions. The sensitivity of theoretical results to parameters in the depredation function is therefore illustrated by a numerical application to the French demersal longline fishery targeting Patagonian toothfish (Dissostichus eleginoides). The shape of the depredation function is also tested empirically with data collected in the same fishery to highlight the killer whales’ behaviour associated with fishing vessels. The results show how coexistence with killer whales could be more profitable for fishers than avoidance despite depredation losses.