Hiding in plain sight: predator avoidance behaviour of mesopelagic Maurolicus muelleri during foraging

Mesopelagic fishes are ubiquitous, ecologically important and a potential protein resource. However, how mesopelagic fish use their 3D environment and facilitate encounters is fairly unknown. We tracked the swimming trajectories of juvenile Maurolicus muelleri and adult M. muelleri and Benthosema glaciale acoustically within 2 distinct vertical layers, measured swimming speed, and used a self-overlap model to analyse the geometry of the trajectories. Our aim was to investigate if and how the fishes were optimizing their swimming behaviour, maximizing prey encounters while minimizing predator encounters simultaneously, and how different ontogenetic stages move differently in the 2 layers. We found that some of the mesopelagic fishes were moving actively, displaying a range between ballistic and convoluted movements. Some of the fishes, especially the adults, had higher degrees of self-overlap, reducing predator encounters. Juveniles took higher risks, increasing the potential for prey encounters. Individual behaviour was highly variable and may be in response to differing ontogenetic stages, potential predator and prey surroundings, and abiotic factors such as light, water turbulence and currents. The presence of active individual spatial behaviour challenges the assumption of mass action in modelling species interactions and underlines the importance of accounting for environmental changes modifying individual-level predator-prey encounter conditions.