VERTICAL-DISTRIBUTION OF FISH AND KRILL BENEATH WATER OF VARYING OPTICAL-PROPERTIES

The distribution of acoustical scattering layers of fish and krill cha nged markedly in concert with fluctuating fluorescence (chlorophyll a) in upper waters, possibly due to a varying 'shadow effect'. Beneath c lear waters on the outer Norwegian shelf (about 300 m depth), mesopela gic fish (Maurolicus muelleri) were located at approximately 150 to 20 0 m by day. Krill (mainly Thysanoessa inermis) was primarily found bel ow the mesopelagic fish and above planktivorous demersal fish (Norway pout Trisopterus esmarkii). The vertical distributions changed abruptl y across a front into water with less Light penetration associated wit h increased fluorescence (chlorophyll a). Mesopelagic fish ascended by about 100 m accompanied by a rise of krill. Demersal fish left the be nthic boundary zone, with ascending Norway pout foraging in the lower part of the krill layer. We suggest that the intermediate light condit ions inside the front provided an 'antipredation window' (sensu Clark & Levy 1988: Am Nat 131:271-290) and thereby favorable feeding conditi ons for the planktivore. These results indicate that properties of upp er layers may impact plankton and fish distributions and their predato r-prey interactions throughout the water column on continental shelves .