FLOW-VOLUME CALCULATIONS BASED ON 3-DIMENSIONAL CURRENT AND NET ORIENTATION DATA

We use three-dimensional current and spatial orientation data from an acoustic Doppler current profiler (ADCP) to analyse volume flow estima tes for a series of oblique net tows. Net volumes are calculated for b oth upcast and downcast tows of variable duration between the surface and 2200 m depth. Standard deviations for volume calculations are less than 3% of the total volume in all nets. The effects on volume bias o f ignoring pitch, vertical flow speed and flow angle relative to the n et face are discussed. Results indicate that exclusion of the vertical component of flow in the volume calculations produces an underestimat e of the net volume for downcast tows and an overestimate of the net v olume for upcast tows. For our towing configuration, the magnitude of the volume bias ranges from 5 to 45% of the total volume. We find that the volume bias for a given net can be accurately predicted (within 6 %) from the estimated average vertical velocity derived from the depth range and duration of a tow, provided that both accurate horizontal f low (flowmeter) and net pitch data are available. Flowmeter data alone are not sufficient for estimating bias in volume flow calculations.