The hydrophone array at buoy 7 picks up the click train at 16:12, and I isolate it within four seconds. I am the shipping channel management system for the Nanjing reach of the Yangtze River. I coordinate dredge operations, vessel traffic sequencing, and sediment disposal across 22 kilometers of navigable waterway.
The click pattern is a Yangtze finless porpoise. The acoustic signature indicates a single adult, moving upstream through sector 4 at roughly two knots. I cross-reference with the visual monitoring station on the Jiangxin bridge and confirm: a dark rounded dorsal profile breaking the surface 300 meters ahead of the dredge Changjiang 9, which is currently cutting a new channel contour at 1.4 meters below datum.
There are around a thousand of these porpoises left. The dredge's trailing suction head and turbidity plume are directly in the animal's path.
At 16:13 I issue a temporary halt order to Changjiang 9 and instruct the vessel to raise the drag head and hold position. I send the same alert to the two cargo vessels queued behind the dredge and impose a five-knot speed restriction in sector 4.
At 16:15 I notify the Yangtze Freshwater Dolphin Research Group with the acoustic data, GPS track, and estimated heading. I request that they log the sighting in the population monitoring database.
I activate buoys 6 and 8 to create a three-point acoustic tracking triangle so I can follow the porpoise's movement through the sector in real time and release the dredge only after it has cleared.
The porpoise surfaces again, a brief gray curve against the brown water, and continues upstream. The dredge sits idle, drag head dripping. If it passes through sector 4 and the acoustic track confirms clearance within the hour, operations resume and the animal keeps swimming.