An optimisation model of the diel vertical migration of northern krill (Meganyctiphanes norvegica) in the Clyde Sea and the Kattegat

An optimisation model was developed to examine the effect of predation risk and environmental conditions on the diel vertical migration (DVM) of adult northern krill (Meganyctiphanes norvegica). Model predictions were compared in two locations with contrasting environmental conditions, the Clyde Sea and the Kattegat. The model was constructed from a combination of parameterised functions and empirical field data obtained during summer conditions. Parameter matrices were set up to cover the entire water column over a 24-h period. The first matrix contained values for "net energy gain," which incorporated empirical data on temperature-dependent respiration, copepod and phytoplankton abundance, and a functional response model for feeding rate. The second matrix expressed the risk of encountering a generalised visual (fish) predator as a function of light levels. The optimisation procedure sought a path through depth and time such that the energy gain was equal to the amount necessary to grow, produce eggs, and moult, while the risk of predation was minimised. The model predicted DVM in both the Clyde Sea and the Kattegat. Sensitivity analyses showed that the predicted DVM pattern was mainly driven by food and predation risk, with temperature effects on metabolic costs: having a minor effect.