Diel vertical migration behavior of Calanus finmarchicus and its influence on right and sei whale occurrence

Diel vertical migration (DVM) by herbivorous copepods likely has a profound effect on the behavior and ecology of copepod predators. We characterized the DVM behavior of late-stage Calanus finmarchicus in the southwestern Gulf of Maine during the spring seasons of 2005 to 2007, and investigated the influence of this behavior on the occurrence of zooplanktivorous baleen whales. On 5 occasions, we occupied an oceanographic station for 1 to 2 d and conducted (1) a half-hourly census of whales and (2) a half-hourly cast with an instrument package measuring temperature, salinity, chlorophyll fluorescence, and copepod abundance. We observed significant variability in DVM behavior both within and among years that was unrelated to stratification or chlorophyll concentration. Instead, DVM appeared to be influenced by the vertical distribution of phytoplankton, the presence of visual predators (sand lance Ammodytes spp.), copepod developmental stage, and the feeding history of individual copepods. Migrating copepods had oil sacs that were 44% larger than non-migrating copepods at the surface after accounting for developmental stage, which suggests that well-fed copepods are more likely to vertically migrate. While the occurrence of North Atlantic right whales Eubalaena glacialis was unrelated to variability in the migration behavior of C. finmarchicus, sei whales Balaenoptera borealis were significantly less abundant during times of strong DVM behavior. We speculate that right whales do not compete directly with sand lance and herring for C. finmarchicus, but by inducing DVM behavior, these fish are likely influencing the distribution and abundance of sei whales in the southwestern Gulf of Maine.