Relationship of oxygen consumption to swimming speed in Euphausia pacifica

Oxygen consumption rate was measured as a function of swimming velocity for the vertically migrating euphausiid Euphausia pacifica at two temperatures (8° and 12°C) and pressures (1 and 40 atm) typical of its bathymetric distribution. Increased swimming speed (x; mh^-1) required increased oxygen consumption (y; μl O₂ mg dry weight^-1 h^-1), described by the equation y = 0.012x + 0.64 at 8°C, and by y = 0.020x + 0.85 at 12° C. The current concept of low swimming costs of zooplankton, based on determinations of dead drag in copepods, is contradicted by our measurements. Temperature had a more profound effect on metabolism at higher swimming speeds (112 m h^-1; Q₁₀=2.8) than on standard metabolism (O m h^-1; Q₁₀=2.0), indicating that activity is more costly at higher temperatures. Pressure caused a small but significant (P>0.05) rise in the relationship of respiratory rate to swimming speed at both temperatures. The energy cost of vertical migration for E. pacifica was estimated by applying our data on oxygen consumption vs swimming speed to published observations on sonic scattering layer movement and the day-night distribution pattern of this species. Results indicate that the cost of a diel migration of 254 m, through a temperature change of 4 °C (8° to 12° C), would cancel any energetic benefit gained by time spent at the lower temperature typical of daytime depth. The act of vertical migration is energetically expensive; its cost should be thoroughly considered in attempts to describe the energetics of vertically migrating species.