Vertical distribution and mortality of overwintering Calanus

Overwintering Calanus spp. were studied in four Norwegian fjords with diffe rent predator regimes and ranging in depth from 380 to 1300 m. Three fjords held both the planktivorous mesopelagic fish Maurolicus muelleri and Benth osema glaciale and invertebrate predators, whereas one lacked mesopelagic f ish but had especially high abundance of several invertebrate predators. Co -occurrence of C. finmarchicus, C helgolandicus, and C. glacialis rendered distinction between effects of environmental conditions and inherent specie s properties in choice of depth difficult. The highest daily per capita mor tality rate for Calanus was estimated at 0.024-0.027 d(-1) (95% Cl) in a fj ord with high fish abundance and with the clearest water. Predation by M. m uelleri and B. glaciate alone could explain the estimated winter mortality. The fjord devoid of mesopelagic fish but particularly rich in invertebrate predators gave the lowest estimated mortality rate; 0.008-0.009 d(-1) (95% Cl). Our results indicate that mesopelagic fish pose a stronger predatory threat than invertebrates to overwintering. Calanus. This concurs with Cala nus selection of oceanic winter habitats below depths where planktivorous f ish can forage efficiently by sight.