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, Gavialis gangeticus）是一种极危鳄类物种，受生存威胁影响，尼泊尔河流中的该物种种群丰度偏低。本研究针对奇旺国家公园（Chitwan National Park, CNP）内种群规模最大的主要河流之一——拉普蒂河（Rapti River），开展了恒河鳄种群丰度估算工作。研究将横贯奇旺国家公园的拉普蒂河划分为18个单段长约4千米的河段，通过三次重复实地直接计数法完成恒河鳄个体统计。本研究采用N混合模型（N-mixture model，负二项分布，negative binomial）对恒河鳄计数数据进行分析，并利用单季占用模型（single season occupancy model）估算恒河鳄整体占用率，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，拉普蒂河现存恒河鳄种群数量为150只（置信区间119-181），各河段的平均种群丰度为8.3只（标准差SD=3.45）。人类活动存在情况与拉普蒂河水深的平方值为对恒河鳄种群丰度具有显著影响的协变量，二者分别对种群丰度产生负向与正向作用。此外，沙洲存在数量会影响恒河鳄的检测概率。

本研究表明，借助N混合模型可优化恒河鳄种群数量的估算流程。基于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、河床资源开采等行为）。若该河流内的恒河鳄种群出现衰退，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河段划定为“无开采区”。