Differential response to disturbance factors for the population of sympatric crocodilians (Gavialis gangeticus and Crocodylus palustris) in Katarniaghat Wildlife Sanctuary, India

Abstract

1. Katarniaghat Wildlife Sanctuary along the Indo-Nepal border in India harbours a well-known breeding gharial (Gavialis gangeticus) population in its global distribution range together with a substantial population of breeding muggers (Crocodylus palustris). However, no systematic information on size and structure is available for either of the species' populations in the protected area.2. This study was undertaken in winter and the ensuing summer of 2010-2011 to estimate the relative population density and structure of the two crocodilian species and to examine the effect of various disturbance factors, with the main focus on the effects of tourism and illegal fishing on the behavioural attributes of the two species.3. Five daytime surveys were conducted from December 2010 to April 2011. Relative density based on encounter rate (number per 20 km) was highest for gharial juveniles followed by gharial adults and gharial sub-adults in all the five surveys. Muggers, on the other hand, showed the opposite trend, with the lowest encounter rate being for juveniles, followed by sub-adults and adults. The encounter rate of both species declined with increase in the mean ambient temperature from December to April. 4. The river habitat was divided into 2 km segments and disturbance factors were recorded at intervals of 100 m on both river banks. Wariness was taken as an indicator of response to disturbance caused by human beings and was measured from mechanized boats used for tourism and non-mechanized boats used for illegal fishing. 5. Segments with sandbars, in spite of moderate to high disturbance rate, were preferred for basking by both species. In conclusion, with suitable habitat for basking, gharials and muggers were observed to tolerate moderate levels of disturbance. Wariness resulting from disturbances from the non-mechanized boats was higher than that from the mechanized boats. Gharials tend to avoid humans, possibly with age/size acquired experience and knowledge, whereas muggers become more tolerant to human presence with increased age/size. 6. Despite many disturbance factors, the crocodilian population in Katarniaghat Wildlife Sanctuary is doing relatively well compared with populations in other habitats in India. With the involvement of local stakeholders and strict implementation of forest laws, the habitat can be further improved and a healthy gharial population can be ensured.

Spatio‐temporal partitioning between two sympatric crocodilians (Gavialis gangeticus & Crocodylus palustris) in Katarniaghat Wildlife Sanctuary, India

“…In total, 928 sightings (718 for gharial and 210 for mugger) were recorded during the study period. Based on five extensive day surveys, the mean encounter rates (the number of total sightings/total distance covered) were: 49.2 ± 8.5 for gharial adults; 37.4 ± 5.4 for gharial sub‐adults; 57.0 ± 10.3 for gharial juveniles; 24.4 ± 4.9 for mugger adults; 15.8 ± 4.08 for mugger sub‐adults; and 1.8 ± 0.7 for mugger juveniles (Choudhary et al, ). The time taken for data collection was as follows: 459 h in the winter season and 468 h in the summer season for basking sites; 39 h for nest sites; and 120 h for temporal separation.…”

“…The preference for shallower slopes and lower height by gharial is because it is difficult for them to negotiate places that are steep and elevated, owing to their weak legs (Bustard & Singh, ; Whitaker & Basu, ). Current land use (disturbances) influences the selection of a site for basking for both species (Choudhary et al, ), as continuous disturbance will increase the frequency of shunting between land and water – a waste of time and energy that could otherwise have been used for thermoregulation.…”

“…Distance to water is important in summer as body heating takes place quickly, and crocodiles descend into the water more often to avoid overheating. A large extent of river habitat may seem to be available but was not used by crocodiles as the maximum encounter rate was confined only to a few segments (Choudhary et al, ).…”

“…Four outermost segments (two upstream and two downstream) of 2 km each were excluded from the sampling area because the river was very shallow upstream and was difficult to reach, even with a non‐mechanized boat. Downstream areas are auctioned to local fishermen and so the levels of disturbance are very high (Choudhary, Choudhury & Gopi, ). Some parts of the tributaries were also not included because of inaccessibility.…”

Gharial nesting in a reservoir is limited by reduced river flow and by increased bank vegetation

“…In the KWS, the small remnant gharial population is vulnerable to limited habitat size, lack of suitable nesting sites, hatchling predation (including egg poaching by local people) and poor hatchling recruitment. Status reports, primarily winter basking counts of adults and nest numbers, provide snapshots of population numbers and composition 46 – 55 (Table 1 ). The total population size and nest numbers have gradually increased since 1975, but the increase is almost negligible compared to numbers of captive-reared, released gharial added periodically to the KWS population.…”