FishDet-XR and FishBoT-SLR-TDR: A YOLO11s-Based Detection and Tracker-Side Recovery-Reranking Framework for Underwater Fish Tracking

Underwater fish detection and multi-object tracking support marine ecological monitoring, underwater robot inspection, and fish behavior analysis. In a tracking-by-detection framework, this study targets three specific problems: unstable detection inputs for small- and medium-scale elongated fish, short-term trajectory breaks when low-confidence true detections are discarded, and identity switches caused by local candidate-edge competition when fish cross or move close together. We address these problems with a joint framework that combines the FishDet-XR detector and the FishBoT-SLR-TDR tracker. FishDet-XR stabilizes detector inputs through the proposed SFP-AFPN-XR feature-fusion neck, strip-shaped directional feature modeling, the Simple Parameter-Free Attention Module (SimAM), and positive-prior sampling. FishBoT-SLR-TDR improves tracker-side association through Spatially-Gated Low-Score Recovery (SLR) and Trajectory-Direction Reranking (TDR). Experiments on BrackishMOT-onlyfish show that the framework improves detection localization and tracking continuity while maintaining real-time inference. At the detection stage, FishDet-XR improves mean average precision at an Intersection over Union threshold of 0.50 (mAP50) from 74.48% to 76.19%, mean average precision averaged over Intersection over Union thresholds from 0.50 to 0.95 (mAP50–95) from 40.72% to 43.55%, and Precision from 86.75% to 88.00% compared with YOLO11s-640, while maintaining 54.76 frames per second (FPS). Compared with the YOLO11s-640 + BoT-SORT baseline, the final detection-tracking chain increases Higher Order Tracking Accuracy (HOTA) from 40.090 to 42.497 and identity F1 score (IDF1) from 51.372 to 54.926, while reducing identity switches (IDSW) from 184 to 159 and trajectory fragmentations (Frag) from 270 to 253.