Foray foraging behavior: seasonally variable, food-driven migratory behavior in two calanoid copepod species

Abstract

We tested a hypothesis we termed 'foray foraging', which states that zooplankton make repeated short-term and short-distance migrations between food-rich surface layers and deeper layers throughout the night. Ultimately, the reason for the behavior is to balance the neces- sity of feeding with the predation risk in surface waters. We tested the hypothesis on 2 species of marine copepods, Calanus pacificus and Metridia pacifica, in Dabob Bay, Washington, USA. Zoo- plankton nets and traps were used to collect females of both species from specific layers and while migrating up into and down out of the surface mixed layer to determine turnover rates in the sur- face layer. Gut contents of individuals were measured to determine if feeding history varied between individuals caught migrating upward and downward or in different layers. Turnover rates of the surface layer were highly variable, ranging from near zero to over 1000%, and were higher in summer and autumn than in spring for both species. Gut contents were consistently higher in animals migrating downward than in those migrating upward, but overall gut contents were higher in spring than in summer or autumn. These results suggest that foray behavior varies in magnitude seasonally and occurs throughout the year for both species but is most pronounced for M. pacifica. These findings suggest that vertical migration behaviors occurring in periods shorter than diel scales may affect zooplankton population dynamics through feeding and preda- tion and likely impact the flux of energy and material into and out of the surface mixed layer.