Diel vertical migration and feeding in adult female Calanus pacificus, Metridia lucens and Pseudocalanus newmani during a spring bloom in Dabob Bay, a fjord in Washington USA

Diel vertical migration and feeding on phytoplankton by adult female Calanus pacificus, Metridia lucens and Pseudocalanus newmani were simultaneously measured near the end of a phytoplankton bloom. Almost the entire Calanus population migrated out of the deep layer (108–50 m) at night but only about 30% came to the surface (25–0 m). Feeding occurred only at night and was equally high in the surface and intermediate layers, in spite of much higher food concentrations in the surface. Like Calanus, the entire Metridia population was found in the deep layer during the day but unlike Calanus, 20–50% remained in the deep layer at night and most migratory Metridia were collected from the surface layer. Metridia feeding at night was highest in the surface layer but significant feeding also occurred in both the intermediate and deep layers. Migratory behavior of Pseudocalanus was weak, with the proportion of the population in the surface layer increasing from slightly <10% during the day to approx 30% at night. Feeding occurred in both surface and intermediate layers throughout the 24 h but was greater in both layers at night. The different migratory patterns are discussed in the context of our current understanding of the contributions of predator avoidance and feeding to diel vertical migration.