ONTOGENIC DIEL VERTICAL MIGRATION BEHAVIORS OF THE MARINE PLANKTONIC COPEPODS CALANUS-PACIFICUS AND METRIDIA-LUCENS

Daytime and nighttime vertically stratified zooplankton samples spanning the entire water column were obtained from Dabob Bay, Washington, USA, during several years. Vertical distributions of all the copepodid stages of Calanus pacificus and Metridia lucens were analyzed from 8 cruises representing the range of seasons, as well as several dates when inferred invertebrate or vertebrate predation pressures on zooplankton were strong or weak. Migration behaviors of similar-sized stages of C. pacificus and M. lucens differed. C. pacificus was closely associated with the surface waters; a large percentage of every stage was always in the surface 50 m at night except when the C5s were in diapause. During the day the different stages of C. pacificus showed varying degrees of avoidance of the surface layers, with the older, larger stages generally being deeper. The C4 and younger stages were particularly tied to the surface waters, with the majority of the population usually in the top 25 m during the day and night. M. lucens was less strongly associated with the surface layers. While the adult females, and usually the C5s, underwent a normal diel vetical migration (DVM), entering the surface 50 m at night, the vast majority of the adult males always stadyed below 75 m. The C4 and younger stages showed more varied behavior. On some dates they underwent a reverse DVM, avoiding the surface 25 m at night, at other times they avoided the surface 10 m particularly during the day, while on still other dates the C3 and younger stages avoided the surface 25 m day and night. On most dates there were significant portions of all the stages in the deepest layers sampled. Differences in the 2 species' migration behavior may be due to differences in their susceptibility to predation, or some additional aspect of their biologies.