|Other Abstract||Line transect sampling method is widely employed in the study of marine mammals, but rarely used in the study of freshwater cetaceans living in the relatively narrow rivers. Some special environmental conditions of riverine systems violate some key assumptions of Line transect sampling method, which highly limit the application of this method in rivers. Limitations exist in survey design, field survey and data analysis stages.
By exemplifying the survey on Yangtze finless porpoise (Neophocaena phocaenoide asiaeorientalis) in the Yangtze River, we explored a promising systematic abundance survey methodology suitable for aquatic mammals in narrow river system by coping with the pointed limitations occurring in all stages of the survey and based on the conventional line transect sampling methodology. Thereafter, we tested the application of this survey method in different platforms with different environmental conditions and got reliable results. Based on the results derived from the case studies, we revaluated the status of the freshwater porpoise subspecies, predicted its population trends, and made assessment on its IUCN status according to the updated criteria of IUCN. In addition, we figured out three types of distribution patterns of the Yangtze finless porpoises (longitudinal, cross river and bank side preferences), which we think may greatly facilitate the survey design and conservation practice in the future. Main results we obtained and conclusions we drawn based on these results are as follows:
We proposed and made a detailed description of the survey methodology suitable for riverine systems including survey design, field method and analytical method based on the understanding on both conventional line transect method and the features of the riverine systems.
In 2006, we surveyed the entire current range of the porpoise population by using two boats and improved line transect sampling method, except for the two lakes (Poyang and Dongting). Sighting results were similar for both boats, so we pooled all data and analyzed them using two line transect models and a strip transect model. All models produced similar estimates of abundance (1111, 1225 and 1000). We then added independent estimates of the number of porpoises from the two lakes for a total estimate of approximately 1800 porpoises. Our findings indicate that the population continues to decline and that its distribution is becoming more fragmented. Our current estimate in the main river is slightly less than half the estimate from surveys between 1984 and 1991 (which was probably an underestimate). We also found an apparent gap in the distribution of porpoises between Yueyang and Shishou (~150km), where sightings had previously been common.
Between 2005 and 2008, we employed small fishing boat and line transect methods to survey the Balijiang section (~40 km) of the Yangtze River. We made correction on the density of middle river area using correction factor. The mean estimate of abundance of total 10 separate surveys is ~90 porpoises ranging from 44 to 121 among various surveys. The abundance variation indicates that specific seasonal migration may exist between this section and other water areas. Opposed to the upper area (i.e. Hukou - Jiujiang), animals showed obvious habitat preference in the lower sections (i.e. Hukou - Island #3). Small boat line transect survey was proved to be a good alternative method to monitor the abundance and even trends of porpoises when large boat survey is economically unavailable.
Between 2005 and 2007, we used small fishing boat and line transect methods to survey the Poyang Lake in different seasons. The abundance estimates of 5 independent surveys conducted in different seasons were 489 (95%CI: 256~932), 341 (95%CI: 215~539), 484 (95%CI: 248~657), 657 (95%CI: 396~1091) and 316 (95%CI: 226~441), respectively. This implies that the population size in Poyang Lake accounts for one quarter, or even one third of the total abundance of this subspecies. We speculated this population was presumably stable in the last two decades. The dramatic variation between seasonal abundance estimates may suggest the seasonal migration between the lake and other water areas. Apart from the possible migration between the lake and the main stem of Yangtze River, larger scale migration could happen between the lake and its 5 tributaries. Since the abundance in flood seasons is higher than in dry seasons, we speculated that the porpoises may swim into the lake during flood seasons, and leave the lake in dry seasons.
By analyzing the data achieved in present study and the data collected from previous surveys over the past three decades, we examined the abundance trends of the Yangtze finless porpoises. Our analysis indicates that the population in upper section (Yichang - Ezhou) showed the greatest rate of decrease (-7.7% yr-1), while those populations in middle and lower sections (Ezhou - Huayang and Huayang - Shanghai) shared lower decrease rates (-6.2% yr-1) between 1990 and 2007. The average rate of decline for the whole river was 6.4% yr-1, which means that the population will lose approximately half of the population in each decade. We hindcasted the abundance in the river in 1990 to be 3623 (95%CI: 2734~4446) which was much higher than earlier estimate of 2550. At the current rate of decline, we project that the population will decline to a total population size of 200 by 2035. We conclude that this subspecies is “Critically Endangered” in accordance with the criteria of IUCN.
We calculated the encounter rate in all small sections between Yichang and Shanghai. The longitudinal distribution map shows that the encounter rate varied between Yichang and Shanghai, including two high density areas (Wuxue – Island #3, and Nanjing - Zhenjiang) and one distribution gap area (Shishou – Yueyang, ~150 km). The cumulative density function of porpoises from bank line to middle river shows that 80% of the animals occurred within a narrow strip (~351 m wide) along the bank line. Moreover, the porpoises showed significant preferences on specific side of the river bank in some areas, although more ecological data need to be collected to clarify this phenomenon.
In summary, we are confident that the survey design, field method and analytical method developed in this study could provide reliable abundance estimate of the freshwater subspecies of the finless porpoises, and have great potential to apply in other riverine systems. In addition, the analysis on distribution patterns may help to improve the future survey design. Moreover, the current study contributed much to conservation of this animal by facilitating estimation of the abundance, prediction of its trends and evaluation of its endangered status.|