Swarms, schools, and scattering layers : interactions between pelagic predators and aquatic prey aggregations

Pelagic predators rely on prey which are patchily distributed over a range of scales; studying these interactions is challenging due to the difficulty of making direct observations. Scientific echosounding partially overcomes this challenge. This thesis explores interactions between predators and prey aggregations observed by echosounding, spanning diverse biomes, spatial and temporal scales, foraging modalities, and aggregation structures. Associations between brown booby, Sula leucogaster, foraging and shallower, higher biomass prey layers suggest this species can successfully locate suitable foraging habitat and highlights the importance of vertical accessibility to plunge diving predators. In contrast, Chaoborus edulis larvae effectively exploit environmental refuge by performing diel vertical migrations into the anoxic zone of Lake Malawi to evade fish predation. Where refuge is unavailable, species adopt collective anti-predation strategies such as swarming; however, this may create a valuable prey resource for predators with high energetic needs. Higher whale density was found near large, dense Antarctic krill, Euphausia superba, swarms than other swarm types, showing many predators rely on the aggregative behaviour of prey, and there are trade-offs associated with anti predation strategies. Copepod density at a bowhead whale, Balaena mysticetus, foraging site was driven by advection and entrapment by tides and bathymetry. Lower prey densities at this site than required for adult female bowhead whales gives an insight into large-scale sexual segregation in this population. Oceanography and limnology were found to shape prey distribution and patch characteristics, while depth, density, speed, and size of aggregations were important for determining predator behaviour. Across diverse systems, prey distributions emerge from both environmental forcing and behaviour. Predators respond not only to abundance, but to aggregation structure and accessibility as constrained by their foraging modes. Models that rely solely on environmental proxies risk misrepresenting predator–prey linkages, which may explain many historically weak or inconsistent predator–prey relationships in pelagic ecosystems.