The accuracy and precision of underwater measurements of length and maximum body depth of southern bluefin tuna ( Thunnus maccoyii) with a stereo–video camera system

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The accuracy and precision of in situ stereo–video measurements of the snout to fork length (SNFL) (range 830–1412 mm) and maximum body depth (MBD) (range 228–365 mm) of free-swimming southern bluefin tuna (SBT) ( Thunnus maccoyii) were tested by filming live fish in sea cages immediately prior to harvest. Stereo–video measurements of the SNFL of 54 fish produced an average error of 1.72 mm (relative error of 0.16%), while an average error of 1.37 mm (relative error of 0.51%) was recorded for measurements of MBD from 47 fish. A procedure was developed to maximise the accuracy and precision of measurements of the SNFL and MBD from a single SBT over sequential images to avoid the underestimation of SNFL and overestimation of MBD due to sinusoidal changes in body form associated with fast swimming. The results demonstrate the potential of stereo–video systems to non-destructively make counts and measurements of tuna and other fish in both wild fisheries and mariculture situations, without the need to capture and handle them.