Predator-borne acoustic transceivers and GPS tracking reveal spatiotemporal patterns of encounters with acoustically tagged fish in the open ocean

Pinnipeds are abundant upper trophic level predators in many marine ecosystems. Top-down effects of their consumption can play important roles in ecosystem structure and functioning. However, interactions between pinnipeds and their prey remain poorly understood due to their inaccessibility while foraging at sea. This uncertainty has fueled debate on the impact of seal predation on fish stocks of commercial or conservation interest. Here, we show that a novel combination of acoustic (Vemco Mobile Transceiver, VMT) and GPS technology can be used to determine the spatial and temporal pattern of interactions between grey seals Halichoerus grypus and fish species in 2 large marine ecosystems, the Eastern Scotian Shelf and the southern Gulf of St. Lawrence, Canada. During 4 yr of study, the VMT on 9 of 64 adult grey seals recorded detections from 3 species of fish, namely 17 adult Atlantic cod Gadus morhua, 7 Atlantic salmon Salmo salar and 1 American eel Anguilla rostrata implanted with coded acoustic tags. An examination of the temporal and spatial pattern of these seal-fish interactions suggested that 1 salmon and 2 cod might have been predated on. However, to have confidence in the occurrence of a predation event, more conclusive evidence is required, which may be gathered through validation experiments. These preliminary results provide proof-of-concept that large upper trophic level marine predators fitted with VMT and GPS tags can provide information on species locations in areas where fixed receiver arrays are not present and allow new insights into the nature of interspecies interactions in otherwise inaccessible environments.