Headstarts and setbacks: an evidence-based approach to conservation of an evolutionarily distinct and globally endangered crocodylian, the gharial
Keywords: conservation biology
phylogenetic diversity
Nepal
gharial
zoology
herpetology
crocodile
conservation
functional traits

Crocodylians are instantly recognisable, large aquatic predators with the potential to act as flagship species in their freshwater habitats. Over half of crocodylian species are threatened with extinction and where threats to freshwater systems are particularly acute, such as Asia, all species are threatened. In order to mitigate multiple threats, effective conservation actions require a species-specific focus and site-specific solutions. Such an approach, combined with a broader understanding of critical differences and similarities of threatened freshwater crocodylians sets the stage for appropriate conservation actions elsewhere. This thesis provides a comprehensive assessment of specific conservation actions in a single case study, the conservation of the gharial in Chitwan, Nepal. It also aims to improve an understanding of extinction risk across Crocodylians by consideration of species-specific functional traits.
In this thesis, I begin by investigating the imperilled functional and phylogenetic diversity of crocodylians, identifying traits that may increase extinction risk, and identifying functional groupings and distinctiveness measures for all crocodylians. Based on this research, I propose the gharial is the highest priority crocodylian for conservation action, due not only to its highly threatened status, but also its unique contribution to functional and phylogenetic diversity.
I then focus on the case study of the Chitwan gharial population, a recovering gharial population in Nepal. Firstly, I compare the use of VHF and GPS telemetry data to estimate home ranges in wild gharial. I find that with appropriate processing of data and selection of analysis methods, estimates from GPS and VHF data are very similar, despite the higher spatial precision and temporal resolution of GPS transmitters. I recommend different systems in which GPS or VHF might be more suitable for studying gharial, according to the differing biases of the two technologies.
Using three different population estimation methods, I then provide the first estimates of population size for gharial in Chitwan, and identify the environmental covariates that influence gharial detectability in daylight surveys. I show that gharial are most abundant in rivers with high dry-season water levels and low seasonal variability. Gharial appear to be more abundant further from a barrage, and in places with low human disturbance. Our results suggest there may be a threshold level of human disturbance, above which gharial do not occur. All methods found that the Chitwan population did not increase from 2018-2021, despite the release of hundreds of gharial from a headstarting (rear and release) programme, highlighting the need for an assessment of the programme.
I then conduct an analysis of the gharial headstarting programme in Chitwan. Using a range of data sources, I show that mortality remains high for headstarted gharial in Chitwan, primarily due to bycatch-related mortality, but also predation of naïve individuals. Retrospective analysis of the headstarting programme shows it was a failure prior to 2004, but has had some success since. This is most likely due to improved survival in captivity and the shift of the release site to an upstream tributary of the initial release site, reducing downstream deleterious dispersal through a dam on the India-Nepal border. Movement patterns and mortality of headstarted gharial suggest that gharial should be released at a smaller size and younger age, and even further upstream of the dam. Successful recruitment observed in-situ, and continued high mortality and biased sex ratios in captivity, suggest more nests should be left in-situ in Chitwan, to promote natural population recovery. The priority moving forward must be tackling in-situ threats to reduce mortality to immature size classes and increase nesting site options. These results can inform other headstarting programmes, and I make predictions about when and where headstarting programmes are likely to fail or succeed.
Taken together, these results show that headstarting alone is insufficient for population recovery in gharial, although it may be a useful tool under some circumstances and specific conditions. This is because gharial are sensitive to in-situ threats such as habitat fragmentation, modification and over extraction. Gharial are particularly sensitive among the crocodylians due to traits such as their habitat specificity and lack of behavioural or physiological adaptations to cope with environmental extremes. I use my findings to suggest key areas of conservation action and ecological research for the future, both for the gharial in Chitwan and across its range, as well as for crocodylian headstarting globally.