Distribution And Habitat Selection Of Yangtze Finless Porpoise In Nanjing Section Of The Yangtze River

The Yangtze finless porpoise(Neophocaena asiaorientalis)is a small toothed whale,which is only distributed in the main stream of the middle and lower reaches of the Yangtze River,Dongting Lake,Poyang Lake and its tributaries.After the functional extinction of the Lipotes vexillifer in 2007,the Yangtze Finless porpoise became the only whale living in the Yangtze River.The Nanjing Yangtze River Porpoise Provincial Nature Reserve(hereinafter referred to as the"Nanjing Nature Reserve")is one of the important habitats for the Yangtze Finless porpoise in the lower reaches of the Yangtze River.In order to understand the distribution characteristics and habitat selection status of the natural population of the Yangtze Finless porpoise in the Nanjing Nature Reserve,an investigation and habitat survey of the Yangtze Finless porpoise were conducted in May,August,and November 2022.The correlation between the distribution characteristics of the Yangtze Finless porpoise and the fish resources and non biological factors in the Nanjing Nature Reserve was analyzed,And use the Maxent model to evaluate the habitat suitability of the Yangtze Finless porpoise in the Nanjing Nature Reserve,and explore the distribution characteristics and key influencing factors of the Yangtze Finless porpoise in the Nanjing Nature Reserve.The main research conclusions drawn from this study are as follows:1.In 2022,a total of 189 groups of Yangtze Finless porpoises were witnessed in the Nanjing Nature Reserve,with a total of 344 sightings,mainly distributed in the waters near Qianzhou,and scattered in other waters.In May,a total of 74 sightings were witnessed,totaling 130,mainly distributed in the waters of Qianzhou and Meizizhou;In August,there were a total of 32 sightings,totaling 53,mainly distributed in the waters of Meizizhou;In November,a total of 83 sightings,totaling 161,were observed,mainly distributed in the Qianzhou and adjacent waters.In May,1775 incidents of Yangtze Finless porpoise vocalization were monitored,with the highest number of vocalization events recorded in study area B4 being 608,followed by study area A4 with 520,and study area B1 with the lowest number of vocalization events with 3;In August,2608sound events of the Yangtze Finless porpoise were monitored,with the study area B2recording the highest number of sound events at 600,followed by study area A1recording 382,and study area B4 recording the lowest number of sound events at 49;In November,6425 vocal events of the Yangtze Finless porpoise were monitored,with the study area B3 recording the highest number of vocal events at 683,followed by the A4 area at 579,and the study area B1 recording the lowest number of vocal events at57.The distribution of Yangtze Finless porpoises varies at different times.2.(1)Water environment characteristics:In 2022,the temperature(T)of the Nanjing Nature Reserve varied from 16.48 to 32.17℃,the dissolved oxygen varied from 7.73 to 9.7 ml/L,and the turbidity was the lowest in August,at 28.44±9.05 NTU;The chlorophyll content was the highest in August,at 4.07±1.24 ug/L,and other physical and chemical indicators showed little change and no significant difference.The comprehensive trophic state index of the water area in the study area varies from29.57 to 33.99,with an average of 31.54±2.14.The trophic level of the water is medium trophic level,and the water quality is evaluated as good.(2)Hydrological characteristics:The results of two hydrological index surveys showed that the flow velocity and flow rate in non dense distribution areas were generally higher than those in dense distribution areas in August.The difference in flow velocity between non dense and dense areas in December was not significant,and flow velocity was not the main environmental variable affecting the distribution of Yangtze Finless porpoises;The correlation analysis results show a negative correlation between the vocal events of the Yangtze Finless porpoise and the average flow velocity in the study area,indicating that the Yangtze Finless porpoise tends to be distributed in the slow flow area of the Yangtze River main stream.(3)Shoreline characteristics:A1,A3,A4,and B2 regions have the highest proportion of natural shorelines at 100%;Secondly,the proportion of natural shorelines in Zone A2 is 27%,and the proportion of semi natural shorelines is 72%;The proportion of semi natural shorelines in B3 area is the highest,at 81%;The area with the highest proportion of solidified shoreline is B4,accounting for 100%,followed by B1,accounting for 68%.The number of sightings of Yangtze Finless porpoises between the Nanjing Yangtze River Third Bridge and the Nanjing Yangtze River Bridge is 252,accounting for 73.25%of the total sightings in the Nanjing Nature Reserve.Among them,the natural shoreline from the Nanjing Yangtze River Third Bridge to the Nanjing Yangtze River Bridge has 185 sightings of Yangtze Finless porpoises,accounting for73.41%of the total sightings;The number of sightings of Yangtze Finless porpoises on the semi natural shoreline is 43,accounting for 17.06%of the total number of sightings;The number of sightings of Yangtze Finless porpoises along the solidified coastline is24,accounting for 9.52%of the total sightings.The results indicate that the natural shoreline is the preferred habitat type for the Yangtze Finless porpoise.(4)Bottom features:The total measured area of the study area is approximately32.75 km~2.The water depth is shallower on the west side and deeper on the east side.The deepest depth is 50.11 meters,with an average depth of 14.24 meters.Among them,the water depth area less than 4 meters is about 4.15 km~2,and the area between 4-10meters is about 2.8 km~2.The water depth area between 10 m and 30 m is approximately23.76 km~2,and the water depth area below 30 m is approximately 2.04 km~2.The average water depth in densely distributed areas is smaller than that in non densely distributed areas,and the slowest slopes in the study area are A1 and A4,which are also the two regions with the highest frequency of occurrence of Yangtze Finless porpoises.Compared with previous survey results,the distribution characteristics of Yangtze Finless porpoises in the protected area have changed in 2022.Originally,A1 area had the highest value,but this survey found A4 area had the highest value.It is speculated that the corresponding changes in the distribution characteristics of Yangtze Finless porpoises may be due to changes in water level in 2022.3.A total of 9 orders,15 families,and 60 species of fish were collected.Based on the analysis of feeding types,the proportion of omnivorous fish and carnivore fish is dominant,48.35%and 31.38%respectively;Based on the analysis of habitat layers,the proportion of bottom fish and middle and bottom fish species is dominant,with 24.69%and 37.86%respectively;Based on the analysis of migratory habits,freshwater sedentary fish are the most abundant,accounting for 88.48%of the total number of species.Based on the statistical analysis of three survey results,Shannon diversity index(H’)is 3.08,Pielou evenness index(E)is 0.75,Simpson dominance index(C)is 0.075,and Margalef richness index(R)is 7.60.The survey results of fish resource density show that the distribution range of fish resource density in May is 0.35-1.00 ind./m~2,with an average of 0.76±0.22 ind./m~2;The distribution density of fish resources in August ranged from 0.006 to 0.16 ind./m~2,with an average of 0.064±0.05 ind./m~2;The distribution density of fish resources in November ranged from 0.68 to 1.57 ind./m~2,with an average of 1.03±0.25 ind./m~2.The statistical analysis results indicate that the distribution characteristics of Yangtze Finless porpoises are not significantly correlated with fish resource density,but Yangtze Finless porpoises tend to be distributed in areas with higher fish resource density.4.Using the Maxent model to analyze the habitat selection mechanism of Yangtze Finless porpoises in the Nanjing Nature Reserve,it was found that the main factors affecting the distribution of Yangtze Finless porpoises include distance from shore,water depth,and surface water temperature.Overall,the head and tail of Qianzhou,the southern branch of Qianzhou,and the head of Meizizhou are highly suitable areas.The high suitable area in May is mainly located in the southern branch of Qianzhou,while there are scattered high suitable areas at the head of Meizizhou;The high suitable areas for August are Qianzhoutou,Zhouwei,and Nancha,while there are scattered high suitable areas at Meizi Zhoutou;The high suitable area for November is mainly in the Qianzhou South Branch,and there are scattered high suitable areas near the Fifth Bridge of the Yangtze River.This study fully grasps the distribution characteristics and habitat status of the typical distribution waters of the Yangtze Finless porpoise in the Nanjing Nature Reserve,analyzes the correlation between the distribution and habitat factors of the Yangtze Finless porpoise,determines the suitable distribution area of the Yangtze Finless porpoise in the Nanjing Nature Reserve,explores the habitat selection mechanism of the Yangtze Finless porpoise,provides scientific basis for the on-site protection work planning of the Yangtze Finless porpoise in the Nanjing Nature Reserve,and provides reference for other protected areas to carry out related work.