A powerful method for measuring fish size of small-scale fishery catches using ImageJ

Measuring fish length is useful for exploring size structure of catches and fishing effects on marine resources. However manually measuring large numbers of individuals requires time-consuming manipulations, which are hardly operational in the context of tropical small-scale fisheries. To overcome this limitation, we developed a method to automatically measure total fish length using digital images and an analytical procedure using the ImageJ software. For testing this procedure, a total of 180 fishes (3 to 50 cm TL) from 19 families, presenting two color patterns and two caudal fin types, were obtained from small-scale fishers in southwestern Madagascar in 2018. We took standardized images of the fishes using a locally made, cost-effective camera-equipped device. The accuracy of fish size estimates and the time taken for measurements were compared across the ImageJ automatic procedure, manual measurements made on-screen using ImageJ, and measurements using an ichthyometer, the latter providing the reference fish length. The ImageJ automatic measurement procedure reached 98.4% accuracy while the measurement time was reduced by 57% and 40% compared to the measures using an ichthyometer and ImageJ on-screen tools, respectively. We conclude that the automatic measurement procedure provides very effective results for fish < 50 cm TL. This method is relevant for governmental, non-governmental, and research organizations that investigate size-related fishing and management impacts on fish community structure and dynamics.