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

The diet vertical migration (DVM) of the cope-pod Metridia pacifica was exa mined in Dabob Bay (47 degrees 45.05'N, 122 degrees 49.71'W), a fjord in Wa shington state. Although the population showed deep daytime residence (75-1 75 m), a proportion of the population was found in the surface beaters at n ight. For individuals that migrated to the surface. the mean size of the oi l sac, as much smaller than those that remained at depth (mean lengths of o il sac 0.25 mm for individuals, collected between 0 and 25 m at night, comp ared with 0.43 mm for individuals from between 125 and 175 m). Similarly. t he C:N ratio was lower for animals collected from near the surface, indicat ive of their lower lipid reserves. These results suggest that individual va riability in DVM as influenced hy body condition, with those animals vith l arger lipid store, not needing to risk coming to the surface to feed at nig ht.