Growth rate of captive Gharials Gavialis gangeticus (Gmelin, 1789) (Reptilia: Crocodylia: Gavialidae) in Chitwan National Park, Nepal

Gharials Gavialis gangeticus have been reared in ex situ facilities in the Gharial Conservation and Breeding Center (GCBC) in Chitwan National Park of Nepal since the 1980s. There remains a paucity of detailed information concerning their growth rates, particularly with respect to season. We randomly selected 20 gharials (45 months old) in the GCBC, tagged them, and recorded total length (TL) and weight over three warm (April–September) and two cold (October–March) seasons between 01 April 2013 and 30 September 2015. We also recorded amounts of fish consumed by these gharials every month over the 30-month period. On average per season, the gharials grew by 9.48±3.63 cm (1.58cm/month) in length and gained 2.61±1.14 kg (0.43kg/month). Growth rates were significantly higher during warm seasons. The highest increase in both length (mean= 21.2±8.61 cm) and weight (mean =5.59±2.12 kg) occurred during the first warm season (April 2013–September 2013) of the study, and annual growth rate was also highest during the first year. Our data indicated strong correlation between mean length and body weight. A total of 2,103.9kg fish was consumed by 20 gharials over 30 months, for a mean consumption of 3.5kg fish per individual per month. Mean fish consumption was also significantly higher during warm (96.99±37.35 kg) versus cold (29.83±17.09 kg) seasons. Survival rate was 100%. Our findings establish baseline data for growth and feeding rates of captive gharials that will be useful in making management decisions in captive breeding and rearing facilities.


INTRODUCTION
Gharials Gavialis gangeticus are Critically Endangered crocodilians currently distributed in among 14 widelyspaced locations in India and Nepal (Lang et al. 2019). In Nepal this species occurs in the Karnali and Babai rivers in Bardia National Park (NP) and the Narayani and Rapti rivers in Chitwan NP (Fig. 1). Major threats are habitat destruction and alteration, water extraction, construction of dams and barrages, mortality in fishing nets and pollution (Lang et al. 2019). Gharial populations were reduced by more than 90% throughout their range, including Nepal, between 1930 and1980 (Stevenson & Whitaker 2010). In response to this crisis, the Government of Nepal initiated an ex situ conservation program in Chitwan NP by establishing the Gharial Conservation and Breeding Center (GCBC) in 1978 (Fig.  1). The main goal of the GCBC has been to reinforce and maintain viable populations of gharials in situ (Maskey 1989;Khadka 2010). Scientific information on every aspect of captive breeding, incubating and rearing is critical to ensure efficient conservation programs (Maskey 1989;Ballouard et al. 2010). Although updated information is available on some aspects of nesting and reproduction of gharials in the GCBC (Ballouard et al. 2010;Khadka 2010Khadka , 2013, there is a knowledge gap concerning growth and feeding rates of captive animals in Nepal. Historic information on survival, growth and feeding of hatchlings was reported by Maskey (1989), but there is no recent information available. To address this knowledge gap and inform management actions, we investigated and established baseline on growth and feeding rates of captive gharials in GCBC (Images 1 & 2).

Study Area
This study was conducted at GCBC facilities in Chitwan NP (27.400-27.813 0 N and 83.880-84.830 0 E; datum= WGS84; Fig. 1). Chitwan NP covers an area of 953km 2 and is located in Chitwan and Nawalparasi districts in south-central Nepal. It has a sub-tropical climate that can be broadly divided into three seasons: warm (March-May), monsoon (June-September), and cold (October-February) (Maskey 1989). Chitwan NP is drained by two major rivers (the Narayani and the Rapti), in which both Gharials and Mugger Crocodiles Crocodylus palustris occur.

Methods
GCBC collects Gharial eggs from the Narayani and Rapti rivers annually for incubation in semi-natural conditions. Hatchlings are reared for up to five years  We randomly selected 20 Gharials which had hatched between the first and second week of June 2009, and tagged them with uniquely numbered plastic cattle tags attached to one of the vertical tail scutes. Since our objective was not to estimate growth rates between sexes, we did not identify sex of these Gharials. Gharials were thus 45 months old when the study started on 01 April 2013. We reared the selected Gharials together in a separate enclosure of 9 x 7 m. The enclosure was bedded with sand, and contained a pond (6m long, 3m wide and 1.25m deep) and did not have any shade. For the purposes of this study, and taking into account the effect of temperature on crocodilian metabolism, we divided 12 month-periods into two seasons-warm (April-September) and cold (October-March). Fish, primarily comprised of Oreochromis spp., Puntius spp., Cirrhinus mrigala, and Gudusia chapra was fed to the experimental group for the duration of the study (01 April 2013-30 September 2015). The amount of fish offered to the group was based on decades of experience of rearing Gharials in GCBC. The amount of fish fed out was weighed, and amount that was uneaten was deduced to estimate amount of fish consumed by Gharials. We determined average fish consumption over a month by dividing the weight of consumed fish in that month by total number of Gharials. In April and September each year, we measured TL using a flexible measuring tape to the nearest cm, and recorded body weight using a spring balance to the nearest gram.
We performed data analysis using Deducer package (Fellows 2012) in R (R Core Team 2018) and presented mean values along with standard deviation. We performed paired t-test and student's t-test wherever applicable to test for statistical significance. We also performed linear regression analysis to test for association between TL and weight of Gharials. We prepared the map on ArcGIS 10.3.

RESULTS
At the start of the study (01 April 2013) Gharials ranged 140-167 cm in TL (mean= 150.3±8.09 cm) and 5.6-10.5 kg in weight (mean= 7.49±1.35 kg) (  2  Fig. 2) and they had thus grown around 29-62 cm in TL (mean= 47.40±18.18 cm) and 5.9-19.5 kg in weight (mean= 13.06±5.71 kg) in 30 months duration (Table 2), equivalent to mean growth rates of around 1.58 cm/month and 0.43 kg/month in TL and weight, respectively. We measured seasonal growth in Gharials in unequal numbers of warm (n=3) and cold (n=2) seasons. On   (Table  2) and mean increase in TL was higher during warm than cold season (One tailed t-test; t= 9.53, df = 19, P<0.001). On average, Gharials gained weight by 2.61±1.14 kg per season (Table 2) and gained more weight during warm than cold season (One-tailed t-test; t= 10.64, df= 19, P<0.001). Our data also allowed annual growth to be calculated for at least two different 12-month periods (Table 2), each including one warm and one cold season. The annual growth rate was 1.98 cm/month and 0.51 kg/month for length and weight respectively for 2013-2014. Similarly, annual growth rate was 0.85 cm/month and 0.17 kg/month for length and weight respectively for 2014-2015. Mean change in TL of Gharials either increased or remained constant between seasons, while mean change in weight decreased in some Gharials during cold seasons (Table 2). There was a strong correlation between mean TL and body weight (r 2 = 0.95; F 1, 18 = 327.1, P<0.001). Twenty Gharials consumed a total of 2,103.9kg fish over 30 months period with a mean of 3.5kg fish consumed per Gharial per month (Table 3). Mean fish consumption was significantly higher (One tailed t-test; t=6.65, df= 25.47, P<0.001) for warm than cold periods; fish consumption during warm season was more than threefold higher than for cold. While Gharials exhibited considerable variation in their growth, survival rate was 100% throughout the duration of the study.

DISCUSSION
Growth rates have been studied on some species of crocodilians, but similar information on Gharials is largely lacking. Although historic accounts on growth and feeding rates for captive Gharials in Nepal is provided by Maskey (1989), we did not find any similar studies on Gharials in India, to the best of our knowledge. Maskey (1989) reported growth rate in both weight and TL of Gharial hatchlings in captivity was higher in warm than in cold periods. The same study also reported a strong correlation between the rates of increase in body weight and TL of Gharial hatchlings (Maskey 1989). Similarly, fish consumption rates of Gharials were reported to be up to 2.5 times greater in warm than in cold by Maskey (1989). These findings on growth and feeding rates of Gharials corroborate with our finding. Growth is a product of food intake and bioenergetic needs which in turn is mediated by temperature (Maskey 1989).
Various studies have suggested that growth rate in crocodilians can vary among species as well as within the same species from different geographical regions, age groups and sex (Thorbjarnarson 1988;Gorzula & Seijas 1989;Saalfeld et al. 2008;Barrios-Quiroz et al. 2012;Balaguera-Reina et al. 2015). We estimated growth rate in TL between 0.73-2.76 cm/month (mean= 1.58 cm/ month) for Gharials. Growth rate in TL of Spectacled Caimans Caiman crocodilus was reported between 2-2.6 cm/month during the first year of life (Gorzula & Seijas 1989). Growth rate in TL of American Crocodile Crocodylus acutus hatchlings in wild was reported to vary between Haiti (3.9 cm/month) (Thorbjarnarson 1988) and in Panama (0.9-4.8 cm/month) (Balaguera-Reina et al. 2015). Similarly, growth rate in TL of American Alligators Alligator mississippiensis was also reported to vary between 2.7 cm/month (for individuals with TL<50 cm when captured) and 2.3 cm/month (for individuals with TL between 50-125 cm when captured) in wild (Saalfeld et al. 2008). Growth rate is also observed to vary between different age groups in Gharials in captivity (Bed Khadka Unpub.; Maskey 1989). One month old Gharial hatchlings (mean TL= 43.71±1.40 cm; range=40.1-47 cm; n=100) showed mean growth rate of 2.7cm/month over the period of 109 days (Bed Khadka Unpub.) which was higher than the mean growth rate reported in this study. It should, however, also be noted that growth rates in crocodilians are typically higher in captivity than in wild. Relation between temperature and growth rate is reported in other species of crocodilians as well. For instance, Joanen & McNease (1987) showed that growth rates in American Alligators could be doubled by optimizing temperature throughout the year. Similarly, Webb et al. (1978) showed that growth rate in Saltwater Crocodiles Crocodylus porosus was higher in the wethot season than in the dry-cold season in Australia and growth rate decreased as body size increased.
Gharials are reared in captive facilities across Nepal and India and held at zoos throughout the world. Our findings establish baseline growth and feeding rates for captive Gharials. Such information will be helpful in optimizing feeding and rearing practice for Gharials in captivity.