Successful adaptation to the environment by zooplankton is constrained by the agents of mortality (starvation, predation) and losses due to advection. A fitness measure which explicitly includes risk of advective loss is presented. The authors show that as horizontal current speeds and vertical shear increase, the behavioral strategies that maximize fitness of zooplankton possessing different life history strategies are affected. For a Calanus finmarchicus-type life history, fitness is maximized by did vertical migrations when advection risk in surface waters is low and by spending less time in the surface layers as flow rate increases. For a Paracalanus parvus-type life history, vertical migration is postulated to occur as advective loss increases. The geographic length scale of the habitat of residence also affects the optimal mode of habitat selection. In the Calanus case an abrupt change in the optimal vertical migration pattern is postulated as a function of length scale of habitat and rate of advection.

We have applied the population surface method to estimate the mortality rates of eggs, nauplii, and copepodids of Calanus spp. in two Norwegian fjords with contrasting predator guilds. Efforts were concentrated in Lurefjorden on the western coast (sampled weekly from late February to mid-June), an unusual fjord dominated by invertebrate (tactile) planktivores. In addition, monthly sampling in Sørfjorden was carried out for comparisons. The latter fjord resembles most Western Norwegian fjords in that it contains populations of vertebrate (visual) planktivores but lower abundance of the invertebrate planktivores. In Lurefjorden, mortality of Calanus spp. was high early in life (∼0.35 d-1), but significantly lower (∼0.00 d-1) in the early copepodid stages. In Sørfjorden, mortality was rather uniform throughout ontogeny (∼0.08 d-1). The noted differences are consistent with different patterns of selective predation between the two fjords. Visual predation in Sørfjorden gave rise to persistent high mortality over the life cycle. Tactile predation in Lurefjorden gave rise to a concave mortality curve for the later developmental stages. Our results indicate that mortality patterns suffered by natural zooplankton populations may differ greatly over short geographical distances, and we suggest that the predator community and mortality patterns must be considered when addressing the spatial and temporal dynamics of zooplankton populations.