Rebuilding the critically endangered Yangtze finless porpoise population: Successful release from an ex situ conservation programme

Ex situ conservation and population reinforcements or reintroductions are vital strategies for protecting endangered species, yet efforts for cetaceans have been notably limited. Through post-release monitoring based on wearable radio tag and passive acoustic methods, we report the first successful release of a critically endangered small-toothed whale, the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis), into the wild. Our monitoring results indicate that, by the third day after release, the two porpoises from the ex-situ population had joined a local individual and gradually integrated into the core distribution area of the local population. Continuous monitoring by the Yangtze Cetacean Protection Network revealed no recorded deaths among the released porpoises. This successful release demonstrates the potential of this approach as a supplementary measure for the restoration of the Yangtze finless porpoise wild population.

Methods
Animals and the release site

Two male YFPs (T21M42: 135.0 cm, 42.4 kg; T21M02: 127.0 cm, 33.2 kg), both from the ex-situ population in Tian-e-Zhou, were captured on April 28, 2021. Following health examinations and the implantation of identification chips, they were transferred to the Laowan branch for a two-year acclimatization period. On April 25, 2023, at 10:30 AM (UTC + 8), both porpoises were released into the upstream waters of the Middle branch, near the Laowan branch (Fig. 1). This release site was chosen due to its relatively low levels of human activity and the presence of abundant fish resources. It is not part of the main navigation channel, and during the dry season, it hosts a stable group of over ten local YFPs.

Tracking with wearable radio tags

The released porpoises were tracked using a wearable radio tag system. This system comprises a flexible vest-style miniature radio tag that transmits signals collected by base stations positioned around the release area (Fig. 2). The vest is designed to fit the porpoise's body, securing the tag on the dorsal side to ensure proper attachment for an extended duration. The vest automatically detaches after the tracking period to minimize any long-term impact on the animal. When the porpoises surface to breathe, the tag exposed to the air emits electromagnetic waves, which are collected by the base stations, allowing us to obtain spatiotemporal information on the tagged porpoises. During tracking, the radio tag was programmed to emit signals every 2 seconds for 2 minutes at 5-minute intervals.

Stationary passive acoustic monitoring (PAM) and vessel-based visual surveys

Long-term monitoring was conducted from April 25 to June 2, 2023, using stationary real-time porpoise click detectors (RPCD-A4, Wuhan Pindu Technology Co., China) and two Soundtraps (Ocean Instruments, New Zealand) near the release site (Fig. 1). Visual surveys based on boats were also carried out from April 26 to 30 and May 7 to 8, 2023 (see Supplementary Methods).

Yangtze Cetacean Protection Network collection of deceased porpoises

The Yangtze Cetacean Protection Network (YCPN), managed by the Institute of Hydrobiology, Chinese Academy of Sciences (IHB, CAS), operates with the support of local governments along the Yangtze River. Residents report any sightings of deceased YFPs to the network. The carcasses are collected and scanned to verify whether the identification chips match the released individuals.