Stage-specific mortality of Calanus spp. under different predation regimes

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 Sorfjorden 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 (similar to0.35 d(-1)), but significantly lower (similar to0.00 d(-1)) in the early copepodid stages. In Sorfjorden, mortality was rather uniform throughout ontogeny (similar to0.08 d(-1)). The noted differences are consistent with different patterns of selective predation between the two fjords. Visual predation in Sorfjorden 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.