Individual variability in diel vertical migration of a marine copepod: Why some individuals remain at depth when others migrate

The diet vertical migration (DVM) of the cope-pod Metridia pacifica was examined in Dabob Bay (47 degrees 45.05'N, 122 degrees 49.71'W), a fjord in Washington state. Although the population showed deep daytime residence (75-175 m), a proportion of the population was found in the surface beaters at night. For individuals that migrated to the surface. the mean size of the oil sac, as much smaller than those that remained at depth (mean lengths of oil sac 0.25 mm for individuals, collected between 0 and 25 m at night, compared with 0.43 mm for individuals from between 125 and 175 m). Similarly. the C:N ratio was lower for animals collected from near the surface, indicative of their lower lipid reserves. These results suggest that individual variability in DVM as influenced hy body condition, with those animals vith larger lipid store, not needing to risk coming to the surface to feed at night.