Vertical migration’s why and when revisited with new technology: Tracking individual timing to understand the adaptive significance of vertical migration

Each day, animals in high biomass aggregations referred to as ‘deep scattering layers’ migrate vertically through the mesopelagic. This migration, comprising the largest net animal movement on earth, is thought to be a result of the competing need to feed in energy rich surface waters and avoid predators in the dark refuge of the twilight zone. Previous work using a newly adapted autonomous vehicle to measure individual characteristics in scattering layers provided the first measures of the internal layer structure, demonstrating that deep scattering layers can be made up of many topologically-scaled, mono-specific aggregations, or ‘schools’ rather than a haphazard mix of species. Follow up measurements show these schools of mesopelagic animals remain coherent during dusk migration, allowing us to examine the timing and ordering of vertical migration. We found that groups of smaller animals began migrating sooner each night than larger individuals, consistent with their relatively lower detectability by visual predators. However, when individual size was accounted for, we also found a correlation with swimming capabilities with squid migrating first, followed by fish, and finally crustaceans, suggesting that the ability to avoid predators once they were encountered also plays in a role in the patterns of migration observed. The presence of squid predators, Risso’s dolphins (Grampus griseus), however, altered this pattern, making the largest squid remain at depth for approximately 45 minutes longer. The risk of predation by this air-breathing predator decreases with depth. Squid apparently assessed and appropriately responded to this non-visual predation risk, affecting their migration patterns. Understanding the timing of individual migration patterns provides insights into the bioenergetic and predator-prey processes in the mesopelagic that are critical for understanding their ecological and biogeochemical impacts of these high biomass layers in the ocean.