Underwater optics and behavioral responses of krill to presence of fish predators affect fish-plankton interactions

In clear oceanic waters off the Norwegian shelf and in outer shelf waters, mesopelagic fish (Maurolicus muelleri) were located at approximately 200 m by day. Across a front into water of lower light penetration, M. muelleri ascended about 100 m. Concurrently, a layer of krill (Thysanoessa inermis) appeared at between 150-200 m, i.e. below their potential predator M. muelleri and generally about 100 m above the bottom. The bottom associated fish Norway pout (Trisopterus esmarkii), occasionally ascended from the benthic boundary layer, foraging in the lower part of the krill layer. However, evidence was found that T. inermis responded to their presence by upward swimming. These results indicate that optical properties of water masses may be prominent in governing plankton and fish distributions and their predator-prey interactions. They furthermore suggest instantaneous behavioral responses in fish and krill to the presence of their respective prey and predator