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Marine Ecology Progress Series

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MEPS 654:53-66 (2020)  -  DOI:

Body length-dependent diel vertical migration of Antarctic krill in relation to food availability and predator avoidance in winter at South Georgia

Taro Ichii1,*, Yoshihisa Mori2, Kedarnath Mahapatra3, Philip N. Trathan4, Makoto Okazaki1, Tomonari Hayashi5, Takehiro Okuda1

1Fisheries Resources Institute, Japan Fisheries Research and Education Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan
2Department of Animal Sciences, Teikyo University of Science and Technology, 2525 Yatsusawa, Uenohara, Yamanashi 409-0193, Japan
3School of Marine Science and Technology, Tokai University, 3-20-1 Orido, Shimizu, Shizuoka, Shizuoka 424-8610, Japan
4British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK
5Japan Fisheries Resource Conservation Association (Scientific observer), Towa-Akashi Building, 1-1, Akashi, Chuo, Tokyo 104-0044, Japan
*Corresponding author:

ABSTRACT: We analyzed diel vertical migration (DVM) of overwintering Antarctic krill at South Georgia, a region that remains ice-free during the austral winter. We considered DVM in relation to krill body length, based on Japanese krill fishery data (1990-2012), and examined DVM in relation to food availability and predator (Antarctic fur seal) avoidance. We report that diel changes in median trawling depth (a proxy for krill vertical distribution) showed significant interannual variation; the overall trend was such that during both daytime and nighttime, the larger the average size of krill, the deeper their median depth. Consistent with the literature, this size-dependent DVM relates to food availability and size-dependent diet; that is, with increasing body length, krill tend to rely less on phytoplankton (which are available in surface layers) as a winter food source. Concerning predator avoidance, and based on analyses using an optimal foraging dive model for fur seals, DVM showed close agreement with size-dependent predation risk; that is, larger krill remained deeper, thereby reducing mortality from fur seals. Therefore, DVM of overwintering krill appears to reflect a compromise between adequate feeding conditions and minimizing predation risk. There was, however, an exception that krill occurred at a shallow depth in winter 2006 when phytoplankton abundance was particularly low and krill density was very high. This supports the hypothesis that physiological demands (i.e. hunger) may become a more important factor affecting DVM than predator avoidance under conditions of insufficient food availability.

KEY WORDS: Euphausia superba · Overwintering · DVM · Predator avoidance · Trade-off · Antarctic fur seal · Optimal foraging dive · South Georgia

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Cite this article as: Ichii T, Mori Y, Mahapatra K, Trathan PN, Okazaki M, Hayashi T, Okuda T (2020) Body length-dependent diel vertical migration of Antarctic krill in relation to food availability and predator avoidance in winter at South Georgia. Mar Ecol Prog Ser 654:53-66.

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