Survival strategies and growth of krill: avoiding predators in space and time

Although Antarctic krill live in a relatively harsh and variable environment, they are extremely abundant and occur across a wide geographic range. Krill are also the dominant prey item for a variety of predators ranging from fish to whales. Thus, krill life histories and survival strategies represent an interesting and biologically relevant theoretical question: How do krill survive and what makes them so successful? The factors that influence krill size and spatial distributions and lifehistory patterns are not generally understood. We present a conceptual framework for studying krill life histories and patterns of distribution. This approach uses what is known about krill physiology and environmental conditions within an evolutionary framework to increase our understanding of krill distributions and abundance. Using a dynamic state-variable model, we determined the factors predicted to affect the distribution of krill among habitats and feeding behavior of krill. Habitats vary in their associated survival, predation risk, food availability, and metabolic costs. Existing data on variation in temperature, phytoplankton abundance, metabolic costs, growth rates and predator behavior are used to parameterize the model. The model predicts that krill will shrink when experiencing extreme temperatures or food deprivation (under negative energy budgets), but we also predict that krill may shrink due to predation risk (under positive energy budgets). Furthermore, predation patterns are predicted to strongly influence krill size and spatial distributions. If predation risk is size-dependent, krill are predicted to shrink between reproductive events which reduces predation. Differences between habitats in predation risk may cause krill to shift away from risky habitats even if this slows growth. In the presence of predation risk when feeding, krill are predicted to experience a tradeoff between growth and survival and shift their feeding behavior and habitat distribution accordingly. Differences between habitats in water temperature and travel costs are also predicted to cause size-dependent shifts in habitat use and migration behavior. Our results reinforce the importance of predator behavior on krill life histories, growth, and distributions. Patterns of predation risk may be the key to understanding krill distribution in space, time and size.