Life cycle strategies of Northern krill (Meganyctiphanes norvegica) for regulating growth, moult, and reproductive activity in various environments: the case of fjordic populations

Adaptive strategies of two fjord populations of Northern krill (Meganyctiphanes norvegica Sars) were studied and compared with other populations from different climatic and oceanic conditions. The Gullmarsfjord (West Sweden) and the Clyde Sea (West Scotland) resident populations followed the same basic pattern of development as the Kattegat (between Denmark and Sweden) and the Ligurian Sea (Northwest Mediterranean) populations, but the fjord krill reached a comparatively larger body size during their second year of life. The positive relationship between body size and fecundity means that fjord populations are potentially more productive than those of the open sea. High rates of moulting and spawning activity were limited to the spring and summer despite the fact that trophic conditions still seemed favourable in early autumn. We show here that, in autumn, the adult moult cycle became longer and growth stopped, large 2-year-old krill disappeared from the population and ovarian development was arrested. Resources already accumulated ill the oocytes (glycoproteic and lipid yolk) were recovered by oosorption and ovaries regressed for a winter rest. Autumn trophic conditions were still favourable in both sites, especially in terms of copepod abundance. However, the phytoplankton community changed from a dominance of diatoms in early season to dinoflagellates in late summer. We suggest that these changes triggered the autumn arrest of krill production.