Seasonal and inter-annual dynamics in the energy seascapes of a marine top predator.

Ecosystems are undergoing global changes that disrupt energy transfer across trophic levels, reducing energy availability and destabilizing predator-prey interactions. In marine systems, small pelagic fish (SPF) are key energy vectors to higher trophic levels. In the Bay of Biscay, common dolphins rely heavily on SPF, which have declined in size and condition in recent decades, while dolphin by-catch has increased. To explore whether prey field changes could explain shifts in dolphin distribution, we mapped prey energy by integrating biomass, size and energy content for four SPF species (sardine, anchovy, sprat and small horse mackerel), representing 80% of the dolphin diet. We analysed acoustic survey data from PELGAS (2000-2023) and JUVENA (2009-2023) to assess temporal and spatial changes in prey energy. Energy per prey has declined, mainly due to decreasing fish length and energy density. Total energy available for common dolphin remained stable, through increase of lower-energy anchovies and decrease of larger, higher-energy sardines. Spatial redistributions were also evident, with sardine energy becoming more coastal and anchovy energy expanding northward. These energy seascapes dynamics, probably climate-driven, may have led dolphins to forage closer to shore, in areas of higher fishing pressure, thereby contributing to the recent rise in by-catch.