Depth distribution during the life history of Neocalanus plumchrus in the Strait of Georgia

Neocalanus plumchrus is the most common large copepod in the Northeast Pacific, and plays an important role in the ecosystems of that area; it is particularly common in the Strait of Georgia. In this study, seasonal changes in the vertical distribution of late-stage N. plumchrus at a single site in the Strait of Georgia were measured with an optical plankton counter (OPC) and depth-stratified net samples. Particles measured in situ by the OPC were compared to an empirically measured size distribution for live N. plumchrus. Below 200 m (primarily overwintering individuals), OPC estimated abundance usually corresponded to abundance estimates from net samples. Abundance in the top 100 m was consistently overestimated by the OPC, relative to results from net samples. During overwintering, the vertical distribution shifted downward over time, coinciding with deepwater renewal events in the Strait, and was followed by a partial upward spawning migration by newly moulted adults. Spatial structure at several scales was inconsistent, but significantly different from random. A simple simulation model of depth as a function of time showed that both hydrographic changes and endogenous changes within the animals may influence depth distributions, and suggests that maintenance of position in the water column during diapause is not a passive process.