Formula for Oxygen Consumption for Atlantic Salmon (Mathematical) - insecurities regarding variable
Yeah, it's hours.
Further down in the paper, you can read,

The mass exponent was estimated to be −0.33 (b in Eq. (4)) for the specific metabolic rate \begin{equation} V_{O_{2}} (mg\space {O_{2}} \cdot kg^{-1} \cdot h^{-1}) \end{equation}

Rate by very definition implies an extent of time; since mg refers to oxygen and kg refers to water, the only variable left is h, which evidently refers to hours. Hours also jives well with the experimental design.

Source:

Grøttum JA, Sigholt T. 1998. A model for oxygen consumption of atlantic salmon (salmo salar) based on measurements of individual fish in a tunnel respirometer. Aquacultural Engineering 17(4): 241-251.