Population Size and Structure of the Nile Crocodile Crocodylus Niloticus in the Lower Zambezi Valley
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Population size and structure of the Nile crocodile Crocodylus niloticus in the lower Zambezi valley K EVIN M. WALLACE,ALISON J. LESLIE,TIM C OULSON and A UDREY S. WALLACE Abstract Concern has been raised about the lack of resilience to harvesting (Velasco et al., 2003). Habitat population data for the Nile crocodile Crocodylus niloticus loss (Thorbjarnarson et al., 2002) and human–crocodile in the lower/middle Zambezi valley. This area is important conflict (McGregor, 2005) are also exerting pressure on for conservation as well as being a source of crocodile eggs crocodilians. The multitude of threats is a serious concern and adults for the ranching industry. Two spotlight surveys, and monitoring of populations is a critical process in the in 2006 and 2009, were used to estimate population size, management of any species. Despite this, of the many structure and trends. A stage-structured matrix model was surveys of crocodilians throughout Africa only a small parameterized from existing literature and the expected percentage facilitate estimation of population trends predictions were compared to those observed. The survey because of temporal, spatial and methodological incon- data suggests a population increase since 2006. Crocodile sistencies (Lainez, 2008), and therefore the status of −1 density was greatest (3.1 km ) in the areas of increased crocodiles in Africa is not well-known. −1 wildlife and habitat protection and lowest (1.4 km ) in areas The Nile crocodile Crocodylus niloticus in Zambia and of increased human presence. The predicted population Zimbabwe is categorized as Lower Risk/least concern on the stage structure differed to that observed, suggestive of a IUCN Red List (Crocodile Specialist Group, 1996). It is listed population not at equilibrium. Data on offtakes of crocodile on Appendix II of CITES, which denotes a species not eggs and adults would be useful for examining why this is necessarily threatened with extinction but one that may the case. Continued monitoring of the wild population is become so unless trade is closely controlled. The wild necessary, to evaluate the trend of an increasing crocodile crocodile population in Zambia and Zimbabwe remains an population, and additional demographic data for modelling important resource for the crocodile farming and ranching purposes would be desirable. industry through the harvesting of eggs and adult breeding stock. It has been noted that continuation of this practice Keywords Conservation, Crocodylus niloticus, manage- alongside a lack of baseline scientific data may have ment, matrix model, Nile crocodile, spotlight survey, a detrimental effect on the wild population (Siamudaala Zambezi valley et al., 2004). This paper contains supplementary material that can be If the size and structure of the wild population can be found online at http://journals.cambridge.org estimated, this information can be used to compare the results with a theoretical model, as this can provide insight into whether the study population is typical of other populations of the same species. Such insight can help Introduction inform management plans. Population models have been developed for numerous crocodilian species (Smith & fi rocodilians ful l an essential ecosystem role (Mazzotti Webb, 1985; Craig et al., 1992; Tucker, 2001; Gallegos et al., 2009 Cet al., ) and have inherent commercial value for 2008). One of the simplest models used to predict growth 2002 both the leather industry (Hutton et al., ) and tourism rates and stable stage structure is a discrete stage-structured 2008 (Telleria et al., ). Overexploitation can cause population model (Caswell, 2001), which facilitates predictions of crashes, leaving some populations potentially vulnerable the growth rate, stage structure, reproductive input and 2009 (Bishop et al., ) or critically threatened (Ballouard sensitivity analysis (Caswell, 2001). This demographic 2010 ff et al., ). Conservation e orts have allowed some analysis can offer valuable information for the management 2000 populations to recover (Webb et al., ). Sustainable util- of species. The models are used to simulate the impact of ization of crocodilians is possible and some species show intervention scenarios, such as the effect of harvesting animals of a certain size or increasing the survival of KEVIN M. WALLACE (Corresponding author) TIM COULSON and AUDREY individuals. Analyses conclude that perturbations to the S. WALLACE Imperial College London, Division of Ecology and Evolution, survival of the mature breeding size class (e.g. by trophy Silwood Park, Ascot, Berkshire, SL5 7PY, UK. ff E-mail [email protected] hunting) have a greater e ect on population growth than that of perturbations at the egg stage or smaller size classes ALISON J. LESLIE Department of Conservation Ecology and Entomology, Faculty 1992 2001 of Agrisciences, University of Stellenbosch, South Africa (Craig et al., ; Tucker, ). This is useful as fi Received 16 June 2011. Revision requested 17 November 2011. conservation funds are often limited and speci cally Accepted 24 November 2011. targeting such funds at a part of the life cycle that results © 2013 Fauna & Flora International, Oryx, 47(3), 457–465 doi:10.1017/S0030605311001712 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.14, on 29 Sep 2021 at 09:01:58, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605311001712 458 K. M. Wallace et al. in the desired outcome in the most cost-effective way is surveyed up to 20 km from the confluence, where shallow important. rapids prevented further upstream travel. Nocturnal spotlight surveys are the most commonly There are four levels of wildlife/habitat protection: used technique for evaluating crocodile populations and open areas, game management areas, safari areas and trends over time. Difficulties encountered during such protected areas. Open areas allow human settlement, surveys can range from physical access to areas, and technical development and agriculture. Game management areas bias such as observer skill and boat speed (Cherkiss et al., allow settlement and agriculture but also act as a buffer zone 2006), to environmental variables such as water level for protected areas. Safari areas do not allow human and temperature (Hutton & Woolhouse, 1989). The settlement or agriculture but permit wildlife hunting and technique can underestimate populations because of these harvesting quotas (as do open areas and game management difficulties and because of the cryptic nature of crocodiles areas). The two protected areas (Lower Zambezi National (Bayliss et al., 1986; Hutton & Woolhouse, 1989). Therefore Park and Mana Pools National Park) have the highest the results of spotlight surveys are often interpreted as levels of wildlife/habitat protection and are no-take zones a population index. The real population size is often for wildlife. The human population is highest along the unknown as complete counts of an entire study site are Zambian riverbank, being most concentrated in the Chiawa rarely possible. Population indices are estimated from Game Management Area and the Siavunga Open Area, incomplete counts that may incorporate correction factors although there are numerous tourist and hunting lodges (Hutton & Woolhouse, 1989; Fergusson, 2006). Repeated along the entire length of the Zambezi River in both Zambia standardized spotlight surveys are the most common form and Zimbabwe. To maintain consistency with the 2006 of population monitoring for crocodiles (Seijas & Chavez, survey the study site was separated into two main sections, 2000). As long as a bias is consistent, the population the eastern section, Kanyemba to Ruckomechi (the upstream index will remain relative to the true population count border of Mana Pools), which includes the National Parks, and inferences can be made from the population index and the western section, Ruckomechi to Nyamumba, which in subsequent years of surveys to establish population has a lower overall level of wildlife/habitat protection and trends. The estimates should be interpreted carefully but are a higher level of human population and agricultural still the primary indicator of population status for development. crocodilians. This study is a follow-up survey to that of Fergusson Methods (2006). Our objectives were to (1) compare our 2009 spotlight survey with that of 2006,(2) parameterize a stage- Research protocols were similar to that of Fergusson’s structured matrix model for C. niloticus, and (3) compare (2006) survey, to facilitate comparisons. A nocturnal the predicted population structure from the model to that spotlight survey was used to census the population from a observed in the wild. We discuss the results in terms of 5-m swamp cruiser boat powered by a 60-hp outboard motor management recommendations. during September, the same month as the 2006 spotlight survey. The survey was timed to coincide with the new Study area moon phase (the darkest period of the month). The weather was cloudless with calm river conditions, good visibility The stretch of the Zambezi River between Kariba dam and the mean air temperature (27.3 ± SE 0.1°C) exceeded and Cahora Bassa has been termed both the lower and that of the temperature (24.7 ± SE 0.1°C), providing optimal middle Zambezi, hereafter we refer to this area as the lower spotlighting conditions (Hutton & Woolhouse, 1989). The Zambezi. The area has a distinct wet season during water level was low prior to the wet season (November– November–April, followed by a cool dry season (May– April), when the water level peaks. The same experienced July) and a hot dry season (August–October). The lower spotlighter and coxswain were used for each night shift Zambezi study area (Fig. 1) extends from Nyamumba at the during the survey. western extreme to the confluence of the Zambezi/Luangwa Only one survey boat and crew were available and River (Kanyemba) in the east and is c. 270 km long, therefore only one bank could be surveyed at a time because following the main Zambezi channel. The lower Zambezi of the width of the river (c.