A model of optimal life history and diel vertical migration in Calanus finmarchicus

A copepod in a seasonal environment continuously faces the trade-offs among allocating surplus growth to a storage compartment (lipids), to somatic growth or to reproduction. By building up lipids, it can survive periods with low food, and gain reproductive success the following season by transforming fat to eggs. Also, copepods face a trade-off between survival and growth, as surface waters generally are more risky and productive than the dark refuges at greater depths, both in the diel and annual temporal scale. Implicit in these trade-offs are the numbers of generations, population dynamics and productive potential of the copepod. The authors develop a model which is used to investigate (1) how optimal diel arid ontogenetic vertical migration might vary with season, individual state (size and lipid reserves) and growth conditions; (2) whether fat is mainly used for overwintering or for fueling reproduction in early spring; and (3) the adaptive value of behavioural flexibility in migration patterns. The model suggests an explanation for observations of absence of vertical migration during spring bloom and before descent to overwintering, and it suggests 1-2 successful generations per year in its basic version. It shows that migratory strategies are not similar between stages and strongly depend on the level of accumulated reserves, that the risk of predation can affect growth, distribution and number of generations and that one should expect phenotypic plasticity in DVM patterns. The model is very sensitive to the rate of metabolism during dormancy.