Ontogenetic vertical distribution patterns in small copepods: field observations and model predictions

We investigated fine-scale (5m, 4h) speciesand stage-specific (N1 to C6) distributions of common copepods at an anchor station in a Swedish fjord during two 24 h periods in October 1997. Generally, both calanoid and cyclopoid nauplii were found near the surface, while copepodids stayed deeper in the water column. No diurnal migration was observed. To analyze the observed distributions, we combined mechanistic models of predation risk from fish and copepods, formulations of temperature-dependent growth and a habitat optimization model, maximizing expected lifetime reproductive output. Motility pattern has implications for encounter rates with copepod predators, and therefore affected optimal vertical positioning in the model. By applying speciesand stagespecific motility, and accounting for the ambient copepod predator field, we computed depth profiles of the mortality risk for the observed field situation. Predicted diel and ontogenetic vertical distribution patterns for various levels of fish concentrations were compared with observed distributions, and much of the patterns in the field were explained by the model. While the risk of fish predation governs the deeper habitat selection of the larger copepodids, the risk of copepod predation is probably more important for nauplii and small copepods. In addition, the vertically homogeneous growth profile and dense layers of copepod predators may wipe out potential benefits of diurnal migration.