Can predator avoidance explain varying overwintering depth of Calanus in different oceanic water masses?

Distributions of overwintering Calanus spp. in the upper 1000 m in the Norw egian and Greenland Seas were studied in relation to hydrography, invertebr ate predators and acoustic scattering layers (38 kHz). C, finmarchicus was concentrated deepest in the Atlantic domain, inhabiting the Arctic Intermed iate Water (AIW) below the Atlantic Water (AW). It was less deeply distribu ted in the Arctic domain, where AMI reached the surface. The temperature of the overwintering habitats was mainly between -0.5 and +0.5 degrees C, irr espective of region and overwintering depth. The distributions of invertebr ate predators such as chaetognaths and the carnivorous copepods Euchaeta sp p. and Chiridius sp. provided no explanation for the varying vertical distr ibutions of C. finmarchicus. A conspicuous coupling between the level of ac oustic backscattering (38 kHz) and the type of water mass (i.e. AW or AIW) was found. High acoustic backscattering, which we ascribe to planktivorous mesopelagic fishes, was confined to the relatively warm AW. The colder AIW was almost devoid of backscattering. Thus C, finmarchicus appear to overwin ter in waters with a low abundance of planktivorous fish in all parts of th e ocean. Since temperature and predator distribution as revealed by acousti cs were closely coupled, predator avoidance is an alternative explanation t o metabolically motivated temperature preference in Calanus' choice of over wintering depth.