Copepod foraging and predation risk within the surface layer during night-time feeding forays

Vertical distribution data seem to indicate that certain species of diel vertical migrating copepods avoid the surface high chlorophyll (Chl) region within coastal and estuarine environments, even during the night. Copepods may avoid this layer to reduce predation mortality, avoid advective loss or to avoid consuming too much toxic algae. We hypothesize that copepods make several intermittent feeding 'forays' into shallow surface layers during the night, returning to intermediate depths between forays. Using an individual-based model (IBM) of Calanus pacificus, we examined the implications of this behavior on feeding success and mortality risk, and tested whether a practical field-sampling scheme would be able to detect foray-like behavior. In some cases, mortality of the foray-foraging copepods was up to 50% less than that of randomly behaving controls, for a given amount of food ingested. The trapping scheme devised should be able to detect the occurrence of foray behavior (FB) in the field and should show differences in the gut contents of copepods entering and leaving the uppermost food-rich layer. The presence or absence of foray-like behavior significantly altered the relative concentration of copepods within various surface strata and thus could influence the temporal availability of copepods as prey for the larvae and juveniles of several important managed fish species.