Vertical distribution and mortality of overwintering Calanus
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 f...
| Main Authors: | , , , |
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.503.1307 http://www.aslo.org/lo/toc/vol_46/issue_6/1494.pdf |
| Summary: | 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 abun-dance of several invertebrate predators. Co-occurrence of C. finmarchicus, C. helgolandicus, and C. glacialis ren-dered 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 d21 (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 d21 (95 % CI). Our results indicate that mesopelagic fish pose a stronger predatory threat than invertebrates to overwintering Calanus. This concurs with Calanus se-lection of oceanic winter habitats below depths where planktivorous fish can forage efficiently by sight. The life cycles of Calanus spp. in the North Atlantic in-clude periods of dormancy in deep waters. Preadult stages descend for ‘‘overwintering’ ’ in summer or autumn and spend winter in an inactive, nonfeeding mode (Gran 1902; Sømme 1934; Marshall and Orr 1955; Østvedt 1955). The particular depth stratum occupied varies strongly geograph-ically (Kaartvedt 1996), spanning from 600 to 2,000 m in |
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