Capture efficiency and its uncertainties from depletion experiments with bottom trawls: A case study of benthic organisms sampled using toothed dredge

Capture efficiency of bottom-towed fishing gears is a fundamental parameter in fisheries science. In this study, we used a dataset of benthic organisms collected during a field depletion experiment consisting of 45 transects conducted in two consecutive days with toothed dredges to estimate capture efficiency and abundance using the patch model, which enabled simultaneous evaluation of multiple benthic taxa. Catch data indicated that tooth-bars retained a substantial number of organisms, suggesting that they contribute meaningfully to total catches and should be considered when evaluating capture processes. Capture efficiency varied among species and between gear parts (cod-end and tooth-bars), indicating that gear structure and species-specific characteristics can influence depletion-based abundance estimates. Further analysis suggested that at least 20 transects are recommended to obtain reliable estimates of capture efficiency in depletion experiments under the conditions of the current experiment. Additionally, a two-phase pattern in catches between Day 1 and Day 2 was also observed, consistent with diel redistribution of whiskered velvet shrimp ( Metapenaeopsis barbata ) and dorippid crab ( Dorippe sinica ). Overall, the results demonstrate the value of large-scale depletion datasets for evaluating capture efficiency and improving understanding of sampling processes in benthic surveys. These findings provide practical guidance for benthic survey design, particularly in determining appropriate sampling effort and improving the reliability of abundance estimates derived from depletion experiments.