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.

中文翻译：

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南乔治亚州冬季取决于南极磷虾体长的diel垂直迁移与食物供应和避免捕食者的关系

摘要：我们分析了南乔治亚州越冬的南极磷虾的diel垂直迁移（DVM），该地区在南半球冬季保持无冰。我们根据日本磷虾渔业数据（1990年至2012年）考虑了与磷虾体长有关的DVM，并研究了与食物可获得性和避免捕食者（南极海狗）有关的DVM。我们报告中位数拖网深度（磷虾垂直分布的代理人）的狄尔变化显示出明显的年际变化；总体趋势是，无论在白天还是晚上，磷虾的平均尺寸越大，其中值深度越深。与文献一致，这种依赖于大小的DVM与食物供应和依赖大小的饮食有关。也就是说，随着身体长度的增加，磷虾往往较少依赖浮游植物（可从表层获得）作为冬季食物来源。关于避免捕食者，并基于使用针对海豹的最佳觅食潜水模型进行的分析，DVM显示出与依赖大小的捕食风险密切相关；也就是说，更大的磷虾仍然更深，从而降低了海狗的死亡率。因此，越冬磷虾的DVM似乎反映了适当的喂养条件和最大程度降低捕食风险之间的折衷。然而，有一个例外是磷虾在2006年冬季发生在一个较浅的深度，当时浮游植物的丰度特别低，而磷虾的密度却很高。这支持了这样的假说：在食物供应不足的情况下，生理需求（即饥饿）可能比避免捕食者成为影响DVM的更重要因素。