Gharial abundance in Rapti river, Chitwan, Nepal

Gharial (Gavialis gangeticus) is a Critically Endangered crocodilian species whose abundance in Nepalese rivers are low due to the threat they face. We estimated gharial abundance in the Rapti River, one of the major rivers in Chitwan National Park (CNP) holding the largest numbers of gharials in Nepal. The Rapti River, running across the CNP, was divided into 18 segments, each measuring ~4 kilometers, and gharials were counted directly with three replicates. Gharial count data was analysed using an N-mixture model (negative binomial) and the overall occupancy of gharials was estimated using a single season occupancy model. Covariate effects were also investigated on gharial abundance. Our findings revealed that the Rapti River is home to 150 gharials (119 - 181), with a mean abundance of 8.3 (SD= 3.45) across each segment. The presence of humans and square of Rapti River depth were the significant covariates that had a negative and positive impact on gharial abundance, respectively. Similarly, the number of sandbank presence influenced the detection probability of gharials. Our study shows that gharial population estimation can be improved using the N-Mixture model. The overall gharial occupancy estimated using single season occupancy model was 0.84 (SD= 0.08), with a detection probability of 0.37 (SD =0.02). The management authority should concentrate on segments to minimize human disturbance (e.g., fishing, washing clothes, extraction of riverbed materials). If the gharial population in this river declines, their population in central Nepal will be threatened. Hence, we suggest designating the Rapti River section that passes across the CNP as a 'no extraction zone'.

恒河鳄（Gharial，学名Gavialis gangeticus）是一种极危（Critically Endangered）的鳄类物种，受各类生存威胁影响，尼泊尔境内河流中的恒河鳄种群数量偏低。本研究针对奇旺国家公园（Chitwan National Park, CNP）的主要河流之一拉普蒂河（Rapti River）开展恒河鳄种群丰度估算——该流域拥有尼泊尔境内规模最大的恒河鳄种群。研究团队将横穿奇旺国家公园的拉普蒂河划分为18个河段，每个河段长度约4千米，并通过三次重复直接计数法统计恒河鳄的数量。本研究采用N-混合模型（负二项分布，N-mixture model (negative binomial)）对恒河鳄计数数据进行分析，并通过单季占用模型（single season occupancy model）估算恒河鳄的整体占用率，同时探究了协变量对恒河鳄种群丰度的影响效应。研究结果显示，拉普蒂河境内现存恒河鳄150只（置信区间119~181），各河段平均种群丰度为8.3（SD=3.45）。人类活动存在及拉普蒂河水深的平方项是两项显著协变量，分别对恒河鳄种群丰度产生负向与正向影响。此外，沙洲数量会影响恒河鳄的检测概率。本研究表明，采用N-混合模型可优化恒河鳄种群数量的估算精度。通过单季占用模型估算得到的整体恒河鳄占用率为0.84（SD=0.08），检测概率为0.37（SD=0.02）。相关管理部门应重点管控各河段，以降低人类活动干扰（例如捕鱼、洗衣、河床物料开采等）。若该流域的恒河鳄种群数量下降，尼泊尔中部地区的恒河鳄种群将面临生存威胁。因此，本研究建议将横穿奇旺国家公园的拉普蒂河段划定为“无开采区”。