Understanding the 3D environment of pelagic predators from multidisciplinary oceanographic surveys to advance ecosystem-based monitoring

Abstract

Marine predators move through the seascape searching for foraging resources. Prey configuration and oceanographic processes could therefore shape their 3-dimensional (3D) oceanographic habitats. Taking advantage of multidisciplinary oceanographic JUVENA surveys targeting biomass estimation of pelagic fishes, observations of 2 highly migratory pelagic seabirds were collected during line-transects: sooty shearwaters (SOSHs) Ardenna grisea and great shearwater (GRSHs) A. gravis. Every autumn these species visit the pelagic ecosystem of the Bay of Biscay (BoB). We developed generalised additive models to disentangle the effects of the 3D ocean environment and preyscapes at different depth ranges, in addition to static variables, on driving the spatial abundance of these predators. The species differed in their vertical habitat use, with SOSHs and GRSHs influenced by habitat conditions above the depth of the maximum temperature gradient and at the surface, respectively. SOSHs were more abundant in deeper shelf areas with localised hotspots associated with upwelling and river discharges. In contrast, GRSHs were more abundant in shallow slope areas in the outer BoB sectors, followed by less dense areas with intermediate levels of juvenile anchovy biomass. Therefore, both species integrate marine resources at different vertical and spatial dimensions, influenced by topographic features, oceanographic conditions and preyscapes. Relative abundance estimations provided mean values of 3203 SOSHs (95 \% CI: 1753-5748) and 12 380 GRSHs (95\% CI: 5797-28152) in the BoB during their annual migration; these numbers varied slightly inter-annually. This study provides an example of the combination of multiple pelagic components as a means to provide an integral assessment to advance ecosystem-based monitoring.