From trophic flows to causal effects: marine mammal–fish interactions in the Norwegian Sea and Barents Sea

Marine mammals, fish and fisheries exploit overlapping prey resources; yet the causal nature of their interactions remains unresolved due to limited data and complex ecosystem dynamics. In this study, we combine food web modelling with causal inference to quantify causal effects between marine mammals and commercially important fish species in the Norwegian Sea and Barents Sea. We first extract multiple food web dynamics from an existing mass-balanced linear inverse food web model and use these as inputs to causal modelling. A key methodological step is to translate the food web network into a temporally explicit causal diagram. By mapping biomass flows onto causal links, we quantify causal effects for both the short term (1 year lag) and the long term (30 year simulation). We find that the increase in marine mammal biomass has predominantly negative and potentially large-but highly uncertain-effects on fish biomass. In contrast, an increase in fish biomass produces smaller, positive effects on marine mammals. Long-term simulations reveal dampened impacts, indicating partial compensation over time. The results from this study, which combines food web dynamics modelling with causal inference, can support more precise, data-informed and ecologically grounded fisheries and marine mammal management in the North Atlantic.