Underwater Computer Vision for Ecologists: A Framework for Curating Image Training Datasets for Object Detection

As marine ecosystems experience accelerating change, there is an urgent need for efficient and scalable biodiversity monitoring tools. We present a 10-step framework for integrating computer vision (CV) tools into long-term underwater biodiversity monitoring, using a case study from coastal British Columbia. Over 9000 h of unbaited remote underwater video footage were collected from two kelp farms and reference sites between March 2022 and June 2023. The framework includes steps for creating an annotated training dataset using unsupervised and supervised CV tools, culminating in the training and validation of a YOLOv8 object detection model. This process produced over 241,000 annotations across 54 pseudo-taxonomic categories (representing both taxa and visually similar groups of fauna), with a focus on fish and gelatinous zooplankton groups. The final model achieved an overall F1 (2 × precision × recall/(precision + recall)) of 0.74 and a mean average precision at 0.5 intersection over union (mAP50) of 0.78. The model had the highest performance on fine-resolution taxa such as Phanerodon vacca (F1 = 0.88) and Aurelia labiata (F1 = 0.90), and lowest performance on pseudo-taxa with limited visual distinctiveness such as Actinopterygii (F1 = 0.60) and Cnidaria (F1 = 0.60). A re-training experiment using annotation thresholds between 25 training images to full dataset availability (~200-24,000 images per group) found that model performance was positively correlated with annotation effort, with F1 averaging 0.84 and mAP50 averaging 0.91 at the maximum training dataset size. Our results suggest that the model is most effective for abundant and visually distinctive taxa, while performance declines for groups with coarse taxonomic resolution. We recommend optimizing annotation effort by targeting genus- or species-level taxa, having at least one broad-level group to capture order-level abundances, and supplementing annotations of rare groups with common but morphologically similar groups, which may further improve model performance.