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 survi ve periods with low food, and gain reproductive success the following season by transforming fat to eggs. Also, copepods face a trade-off be tween survival and growth, as surface waters generally are more risky and productive than the dark refuges at greater depths, both in the di el and annual temporal scale. Implicit in these trade-offs are the num bers of generations, population dynamics and productive potential of t he copepod. The authors develop a model which is used to investigate ( 1) how optimal diel and ontogenetic vertical migration might vary with season, individual state (size and lipid reserves) and growth conditi ons; (2) whether fat is mainly used for overwintering or for fueling r eproduction 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 ge nerations per year in its basic version. It shows that migratory strat egies 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 phe notypic plasticity in DVM patterns. The model is very sensitive to the rate of metabolism during dormancy.