The gharial is a river dwelling crocodilian (Whitaker and Basu 1983) with adult gharial showing widespread use of river systems with seasonal migratory behaviours and social hierarchies (Lang and Kumar 2013, 2016). Gharials congregate for mating and nesting during the dry season in these highly seasonal, monsoonal rivers.  Courting and mating occurs in mid–February followed by nesting and egg-laying in mid–March to early–April. Female gharial nest in seasonally exposed sandbanks along slow moving sections of medium- to large-sized rivers, and lay an average of 40 eggs. When concentrated in these areas, gharial are highly vulnerable to impacts from fishing and malicious killing. The eggs are also vulnerable because in some areas they are sought for food/medicine. Incubation takes 2-2.5 months with mostly females guarding nest sites.  Hatching occurs in late May through mid-June.  Adult care is well-documented. Females open the nests at the time of hatching but do not transport the young to the water.  Hatchlings from multiple nests aggregate in crèches numbering from hundreds to a thousand or more.  Females, and typically a single large male, guard hatchling crèches from potential predators for 1-2 months. Once monsoon waters start to rise, the guarding adults make long distance seasonal movements, and the crèches break up with hatchlings dispersing widely into aquatic shoreline habitats.  Adult gharial are seasonal migrants in large, open river drainages, with movements of 50-200+ km recorded in Chambal. Upon release, non-resident, captive-reared individuals appear highly mobile, and some have moved in excess of 1,000 km.

In the Chambal subpopulation, most large females nest every year (reproductive frequency >90%; Thorbjarbnarson 1996, Lang and Kumar 2016). Similar values appear to apply to other subpopulations, e.g. Katerniaghat subpopulation, but may not apply to the most northerly subpopulation, e.g. at Chitwan. The Nepalese subpopulations may show higher rates of biennial nesting.

A number of gharial subpopulations now exist in lentic, rather than lotic, environments, principally in reservoirs behind dams and/or barrages, formed in flooded water courses that were formerly river channels. These include the Katerniaghat subpopulation and the Corbett subpopulation.  The Chitwan subpopulation is possibly impacted by the Tribeni Dam on the Gandak (=Narayani) River, but the distances between gharial concentrations and the dams in all three locations vary.  Saikia et al. (2010) sighted gharial at natural lake localities on the Brahmaputra drainage, with presence confirmed by local inhabitants.  A gharial photographed by local forest department staff, was caught there and translocated into a permanent channel (Saikia 2012).

Additional references on gharial habitats and ecology are noted in Groombridge (1982), Maskey (1989), Rao et al. (1995), Wildlife Institute of India (1999), Choudhury et al. (2007) and Stevenson (2015). 

Historically, prior to 1943, Gharial were common and abundant in the main rivers and tributaries of the Indus, Gangetic and Brahmaputra drainages, and also inhabited the Mahanadi-Brahmini system in north central India. The species may have occurred in other rivers in peninsular India, such as the Godavari-Indravati rivers, as well as in the rivers and tributaries of the Irrawaddy in Myanmar, but supporting data are scant, equivocal or lacking (Singh 1991, Ranjitsinh and Singh 2002, Thorbjarnarson et al. 2006, KoKo and Platt 2012).

The species is now limited to only 14 widely spaced, restricted localities in north India and lowland Nepal. Five subpopulations [Chambal River, Katerniaghat Reservoir (on Girwa River), Chitwan National Park (Nepal), Corbett National Park and Gandak River] exhibit recent reproduction/recruitment (Table 1 in the Supporting Information). Levels of nesting are commensurate with the estimated number of mature females occurring at Chambal River and at Katerniaghat Reservoir. Limited nesting, relative to the number of adult females, occurs at Chitwan, Corbett and on the Gandak River. Reproduction is not evident currently in the Babai subpopulation at Bardia NP, despite an apparently, a small number of mature females (~5-10+) inhabiting this protected stretch of river, where nesting may occur in future years. Taken together, these five locations (excluding Chambal) account for <15% of the global population, and < 14% of the known nesting abundance. Eight minor sites with no breeding evident account for less than 25 mature adults, or the remaining <5% of the gharial metapopulation.

Only one locality—National Chambal Sanctuary—is an open, protected riverine habitat, with strong water flow, normal seasonal migration and natural unobstructed movements 625 km river km (A; Table 1 in the Supporting Information). The Katerniaghat locality (B; Table 1 in the Supporting Information) is very small in comparison (<20 km) and is utilised by gharial living in a reservoir behind a barrage on the Girwa River. The other four potential breeding river locations are either limited in size, unprotected or subject to disturbances from variable water flow, fishing, sand mining, cultivation: they exhibit limited or no natural recruitment.  Remnant subpopulations are known at eight minor localities (G-M; Table 2 in the Supporting Information), characterized by low numbers of adults, and unfavourable, mostly unprotected habitats. Currently, in 2018, gharial are extirpated in Pakistan, Bhutan, Myanmar, and possibly Bangladesh where only few, vagrant individuals may persist.

For nearly half a century, serious concerns have been expressed about the status and continued survival of gharial populations globally (Biswas 1970; Bustard 1975, 1999; Whitaker 1975, 1987; Maskey 1999, 2008; Sharma and Basu 2004; Choudhury et al. 2007).  In a recent commentary and review, Stevenson (2015) outlined a brief history of these conservation initiatives for gharial, and evaluates successes and failures.  These efforts have included ex-situ activities focused on enhancing existing populations and re-establishing populations in places where the species has been extirpated.  Ex-situ activities include production of juveniles through conservation ranching and captive breeding, with releases and introductions into wild habitats.  In-situ activities, focused on enhancing and strengthening existing populations, include habitat protection, direct species protection measures, and local community programs aimed at species’ protection and continued survival.

Head-starting: Conservation ranching of wild eggs, with rear and release programs, were at the center of initial conservation actions in the 1970s and 1980s, in both India (Bustard 1999) and Nepal (Maskey 1999). To date, in excess of 6,000 gharial have been reared and released in both countries (India: ~5,000; Stevenson 2015; Nepal: ~1,000; Khadka et al. 2013, Lang 2016b; Acharya et al. 2017). However, the success of these programs is open to question, and remains largely untested.  The initial restocking of suitable habitats such as the Chambal and Katerniaghat in India contributed to curbing the rapid rate of population decline prior to 1970s, helped stabilize existing numbers, and led to eventual population increases in selected localities.  In fact many of breeding adults now living in the National Chambal Sanctuary may be animals released as juveniles 10 to 30 years ago.  But a lack of any robust marking protocols or systematic follow-up on the fate of such releases, preclude objective evaluation of their effectiveness. Elsewhere, repeated releases of head-started juveniles have not resulted in appreciable population increases, and the fate of released individuals are either not known (Stevenson 2015) or known to have not succeeded in boosting the wild population (Shah and Paudel 2016; Acharya et al. 2017; Lang 2017).  [In general, relative to other species such as alligators, gharial eggs have proved difficult to transport and incubate successfully under artificial conditions.  Likewise, developing suitable techniques for rearing gharial hatchlings (which prefer to feed on live fish) that thrive and grow in captivity has proved challenging.  The head starting of gharials has proven to be more difficult than anticipated, expensive and has not become the panacea for gharial conservation anticipated, particularly when resources and manpower are in short supply.

Captive breeding: In India, successful captive breeding programs were established for gharial in the 1980s and 1990s, notably at Nandan Kanan in Odisha, at the Madras Crocodile Bank Trust in Tamil Nadu and at the Kukrail Breeding Center in Uttar Pradesh (Bustard 1984, 1999; Whitaker 1987).  These facilities, in turn, produced young gharial for the various release programs, as well as for exchanges with Asian and international zoos worldwide.  In Nepal, the major sources of gharial for the head-start programs have been ex-situ incubation of eggs from wild nests, but this strategy has been augmented in recent years by captive breeding (Khadka et al. 2013). Outside of south Asia, success in the captive breeding of gharial in zoos has lagged behind successful programs for most other crocodilian species but recently there have been some notable successes in Europe and in the USA (e.g., Crocodile Zoo Protivin in Czech Republic; Saint Augustine Alligator Farm, Florida).

Release strategies: Within a decade after the start of Project Crocodile in India, more than a thousand raised juvenile gharial, 2 to 5 years of age, were released back into protected habitats (Whitaker 1987). In addition to releases in the National Chambal Sanctuary, gharial were released in the Girwa, Sharda, Ramganga, main Ganges, Betwa, Ghaghara, and Suheli Rivers (Bustard 1999).  In the intervening years, prior to the species’ listing to Critically Endangered in 2007, releases have continued apace, but as Bustard (1999) noted earlier, “no systematic monitoring is being carried out”. Since 2007, releases have continued, but are poorly documented by individual state Forest Departments within India. These have occurred at localities inhabited by gharial at Chambal, Girwa (=Katerniaghat), Ghaghara, Gandak, and Ken Rivers.  In addition, head-started gharial have been released in previously occupied habitats, including the Upper Ganges in protected sanctuaries, at Hastinapur in Uttar Pradesh, and at Haricke, in a reservoir on the Beas River, a tributary of the Indus in the Punjab, India.  These latter two releases are re-introductions of juvenile gharial back into riverine habitats from which gharial had been extirpated previously. Here the objective is to re-establish gharial where they used to occur historically, but were eliminated in recent times.  Similar efforts were made in the Koshi River, Nepal on repeated occasions in the 1980s and 1990s, but these earlier attempts to re-establish gharial failed (Shah and Paudel 2016). Recent releases in Nepal (2007 to 2016) at Bardia and at Chitwan have not resulted in appreciable population increases (Acharya et al. 2017).