Vertical distribution and mortality of overwintering Calanus

Abstract

Overwintering Calanus spp. were studied in four Norwegian fjords with different predator regimes and ranging in depth from 380 to 1300 m. Three fjords held both the planktivorous mesopelagic fish Maurolicus muelleri and Benthosema glaciale and invertebrate predators, whereas one lacked mesopelagic fish 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 species properties in choice of depth difficult. The highest daily per capita mortality rate for Calanus was estimated at 0.024–0.027 d‐1 (95% CI) in a fjord with high fish abundance and with the clearest water. Predation by M. muelleri and B. glaciale 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% CI). Our results indicate that mesopelagic fish pose a stronger predatory threat than invertebrates to overwintering Calanus. This concurs with Calanus selection of oceanic winter habitats below depths where planktivorous fish can forage efficiently by sight.