Distribution patterns of Yangtze finless porpoises in the Yangtze River: implications for reserve management

Passive eDNA sampling facilitates biodiversity monitoring and rare species detection

Environmental DNA (eDNA) technology has revolutionized biomonitoring, but challenges remain regarding water sample processing. The passive eDNA sampler (PEDS) represents a viable alternative to active, water filtration-based eDNA enrichment methods, but the effectiveness of PEDS for surveying biodiverse and complex natural water bodies is unknown. Here, we collected eDNA using filtration and glass fiber filter-based PEDS (submerged in water for 1 d) from 27 sites along the final reach of the Yangtze River and the coast of the Yellow Sea, followed by eDNA metabarcoding analysis of fish biodiversity and quantitative PCR (qPCR) for a critically endangered aquatic mammal, the Yangtze finless porpoise. We ultimately detected 98 fish species via eDNA metabarcoding. Both eDNA sampling methods captured comparable local species richness and revealed largely similar spatial variation in fish assemblages and community partitions between the river and sea sites. Notably, the Yangtze finless porpoise was detected only in the metabarcoding of eDNA collected by PEDS at five sites. Also, species-specific qPCR revealed that the PEDS captured porpoise eDNA at more sites (7 vs. 2), in greater quantities, and with a higher detection probability (0.803 vs. 0.407) than did filtration. Our results demonstrate the capacity of PEDS for surveying fish biodiversity, and support that continuous eDNA collection by PEDS can be more effective than instantaneous water sampling at capturing low abundance and ephemeral species in natural waters. Thus, the PEDS approach can facilitate more efficient and convenient eDNA-based biodiversity surveillance and rare species detection.

Spatiotemporal relationships of threatened cetaceans and anthropogenic threats in the lower Yangtze system

The impacts of fisheries interactions on cetaceans can be challenging to determine, often requiring multiple complementary investigative approaches. The Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis), one of the few Critically Endangered cetaceans, is endemic to the middle-lower Yangtze drainage, a system impacted by multiple anthropogenic pressures. Bycatch mortality is implicated in regional porpoise decline, but the significance and dynamics of porpoise interactions with fishing activities and other threats remain poorly understood. We conducted boat-based surveys to map seasonal distributions and spatial congruence of porpoises and two potential threats (fishing and sand-mining), and an interview survey of fishing communities to understand temporal patterns and drivers of regional fishing activity, across Poyang Lake and the adjoining Yangtze mainstem. Variation in harmful and non-harmful gear use (non-fixed nets versus static pots and traps) between these landscapes might be an important factor affecting local porpoise status. Within Poyang Lake, spatial correlations between porpoises and threats were relatively weak, seasonal porpoise and threat hotspots were located in different regions, and two protected areas had higher porpoise encounter rates and densities than some unprotected sections. However, porpoise hotspots were mostly in unprotected areas, threats were widely observed across reserves, and more fishing and sand-mining was seasonally observed within reserves than within unprotected areas. Compared to null distributions, porpoises were detected significantly closer to fishing activities in summer and further from sand-mining in winter, indicating possible spatial risks of gear entanglement and disturbance. Reported porpoise bycatch deaths are associated with fixed and non-fixed nets, hook-based gears, and electrofishing. Longitudinal patterns in reported gear use indicate that hook-based fishing has decreased substantially and is generally conducted by older fishers, and significantly fewer respondents now practice fishing as their sole source of income, but electrofishing has increased. This combined research approach indicates a continued potential risk to porpoises from changing fisheries interactions and other threats, highlighting the importance of fishing restrictions and appropriate support for fishing communities impacted by this legislation. A potential “win–win” for both biodiversity and local livelihoods could be achieved through wider use of static pots and traps, which are not associated with bycatch deaths.

Seasonal Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) movements in the Poyang Lake, China: Implications on flexible management for aquatic animals in fluctuating freshwater ecosystems

Identifying seasonal high-use habitats and migration corridors is the basis for migratory species conservation. Previous studies have focused on Marine Protected Areas, while freshwater ecosystems, often accompanied by severe hydrological fluctuations, suggest new perspectives for flexible management. Poyang Lake is an essential habitat for the critically endangered Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis, YFP), supporting almost half of their natural population. However, studies on the movement patterns and habitat preferences of YFPs still lack there, preventing potential protected areas' effectiveness. In this study, we conducted 10 surveys for the YFP population in the main body and tributaries of Poyang Lake from October 2018 to August 2020. We used habitat modeling to analyze their seasonal habitat use and the environmental predictors. YFPs showed a seasonal movement pattern consistent with water level fluctuations. They entered the tributaries from the main lake body in low and medium water levels and returned to the main lake during high water level periods. The water depth was the most important environmental variable in each hydrological season that affects YFPs' habitat selection. The suitable water depth was about 4-8 m during the low water level, 6-12 m during the medium water level, and 7-20 m during the high water level. YFPs' 50% core habitats distribution showed noticeable seasonal changes. The proportion of their suitable habitats in the whole lake was relatively low, which was highest during high water level (16.89%), and the weakest during low water level (12.11%). Considering the seasonal movements of the YFPs in the lake, we recommend flexible management measures for their core habitats to alleviate human interference and restore their movement rhythm between the river and the lake, which shed light on protected area management for aquatic animals in such seasonal fluctuating habitats.

Thresholds of population persistence for the Yangtze finless porpoise: implications for conservation managements

The Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis), a critically endangered species, is the only cetacean species in the Yangtze River following the functional extinction of baiji (Lipotes vexillifer). To inform conservation actions, two important questions need to be addressed: what is the threshold value of survival rate, and what is the threshold value of population size? We calculate the instantaneous rate of population increase ( r ¯ ) for the Yangtze finless porpoise for various combinations of the calf and the non-calf survival rates. We also test the probability of extinction for different minimum carrying capacities for 100 and 500 years using a stable population model. The threshold value of the non-calf survival rate is never lower than 0.869, but current estimates from field data have been far below this threshold. Our model based on extinction probability and carrying capacity suggests that the threshold for the population size to persist 100 years required 113 animals, and 472 animals are required to persist 500 years. Therefore, we recommend establishing an ex situ reserve network to guarantee the minimum 100-year carrying capacity. To ensure the long-term population viability, we suggest establishing two in situ reserve zones in two lakes and their surrounding reserves to meet a minimum 500-year carrying capacity. In addition, measures to avoid further habitat fragmentation should be priority.

Physiological consequences of biologic state and habitat dynamics on the critically endangered Yangtze finless porpoises (Neophocaena asiaeorientalis ssp. asiaeorientalis) dwelling in the wild and semi-natural environment

The objectives of this study were to investigate the effects of habitat and biological state on the physiology of critically endangered wild and semi-natural Yangtze Finless Porpoises (YFPs; Neophocaena asiaeorientalis ssp. asiaeorientalis) by measuring and comparing serum biochemical parameters. A total of 168 YFPs were sampled, 68 living in the semi-natural (Tian-E-Zhou Oxbow) and 98 living in the wild (Poyang Lake, PL) environment. The YFPs in the Tian-E-Zhou Oxbow were sampled from 2002 to 2015 and in the PL from 2009 to 2017. Each population was divided into Juvenile Male, Juvenile Female, Adult Male, Pregnant and Lactating Female life history categories. Overall, with location, 19/33 of the analytes and with season 18/33 of the analytes were significantly different. Similarly, within each location, 15/33 of the analytes changed with time in PL while only 8/33 changed with time in Tian-E-Zhou Oxbow, respectively. Finally, 15/33 of the analytes demonstrated significant differences between the different age and sex groups of animals. In our study, a significant variation, as well as an increasing and decreasing pattern of several parameters in both populations, suggest a worsening ecological environment of both habitats. This study will help in health assessment, improving conservation and management practices, a crucial requisite for biodiversity conservation.

How to detect an elusive aquatic mammal in complex environments? A study of the Endangered Antillean manateeTrichechus manatus manatusin French Guiana

The Antillean manateeTrichechus manatus manatusis a cryptic mammal that inhabits, among other areas, murky rivers and estuaries of Central and South America. The difficulty in detecting and counting manatees is an obstacle to monitoring their population status, as traditional count sampling is unsuitable. We conducted a quantitative analysis of the distribution and abundance of the manatee across its known range in the rivers and estuaries of French Guiana, based on data from a range-wide line transect visual survey combined with a side-scan sonar survey, identification of feeding sites, and silent observations at fixed points. A total effective effort of 248 hours and 1,129 km of boat survey was completed. We used two relative abundance indices: the combined encounter rate, which combines encounters per km from either visual or side-scan sonar surveys, and the global detection index, which is defined as the sum of all evidence per unit time. Manatees were detected in all nine study units. Niche modelling was used to analyse the space selection by manatees, and helped to detect differential use of habitats according to the season. The model predicted that coastal areas are used more extensively during the dry season. In the absence of better techniques to detect wild manatees in complex habitats this monitoring protocol may be relevant and replicable in hydrological systems where manatee detectability is constrained by biogeographical characteristics.

Effects of fish community on occurrences of Yangtze finless porpoise in confluence of the Yangtze and Wanhe Rivers

The Yangtze finless porpoise is a subspecies of narrow-ridged finless porpoise endemic to the middle and lower reaches of the Yangtze River and the adjoining Poyang and Dongting Lakes. With the depletion of fish stocks in the Yangtze River in recent decades, food availability has become the most important factor affecting the survival of this subspecies. Despite this, the relationships between fish community and occurrences of porpoise are far from being fully understood. Therefore, during September 2013 to August 2014, the occurrences of porpoise were investigated in confluence of the Yangtze and Wanhe Rivers; fish community was also surveyed synchronously in confluence and two adjacent transects. The results showed that (1) the confluence had maximum fish species richness, and the main dominant species was upper fish, while the other two transects were mainly dominated by demersal fish. ANOVA analyses showed that individual number and yield of upper fish which can be eaten by porpoise (upper edible fish) in the confluence were significantly higher than other two transects. (2) Average group size of the porpoise was 3.7 ± 1.8 individuals. The occurrences of porpoise in different seasons had great differences, and the porpoise was more likely to be detected in autumn and winter. (3) Fish community had significant effects on occurrences of porpoise, and the main influencing factors were fish species richness, individual number, and yield of edible fish, especially the upper edible fish. The results of this study will have important implications for the conservation of porpoise.