Vertical distribution of fish and krill beneath water of varying optical properties

The distribution of acoustical scattering layers of fish and krill changed markedly in concert with fluctuating fluorescence (chlorophyll a) in upper waters, possibly due to a varying 'shadow effect'. Beneath clear waters on the outer Norwegian shelf (about 300 m depth), mesopelagic fish (Maurolicus muelleri) were located at approximately 150 to 200 m by day. Krill (mainly Thysanoessa inermis) was primarily found below the mesopelagic fish and above planktivorous demersal fish (Norway pout Trisopterus esmarkii). The vertical distributions changed abruptly across a front into water with less light penetration associated with increased fluorescence (chlorophyll a). Mesopelagic fish ascended by about 100 m accompanied by a rise of krill. Demersal fish left the benthic boundary zone, with ascending Norway pout foraging in the lower part of the krill layer. We suggest that the intermediate light conditions inside the front provided an 'antipredation window' (sensu Clark and Levy 1988: Am Nat 131:271-290) and thereby favorable feeding conditions for the planktivore. These results indicate that properties of upper layers may impact plankton and fish distributions and their predator-prey interactions throughout the water column on continental shelves.