US Long-Term Ecological Research Network
Calibrated mask-bar for underwater measurement of fish
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This paper describes a method for measuring fish under water by using a diving mask equipped with a calibrated mask-bar. The calibrated mask-bar consists of a plexiglass strip with calibration marks spaced 3.2 mm (1/8 in) apart, mounted to a diver’s mask by aluminum straps. Relationships between calibration mark counts made by underwater observers and the length of objects were described by regression analysis. Measurements of stationary objects were accurate to within ±5\%. Estimates of fish length under field conditions were generally accurate to within ±10\%. Mean lengths calculated from multiple mask-bar measurements were consistent with those calculated from measurement of captured fish.