Getting an accurate idea of the number of any wild animal is no trivial task and relies on a solid understanding of their distribution and density (i.e. how many are likely to be found per unit area of any given habitat). If the target species is quite large and conspicuous, active during the day, and happens not to move very far, then the job is a little simpler. Much more complicated to census are the smaller species that spend a lot of their time lurking in tall vegetation and can be easily disturbed by human activity. These problems with estimating local numbers are multiplied when trying to come up with a national, or worse global, population figure, and are why scientists tend to shy away from giving absolute numbers, speaking instead in terms of densities - a certain number per hectare or acre.
In some contexts, such as deer management in agricultural and forestry settings or for conservation programs, it's important to have some idea of population size and demography (the split of ages within the population). Local deer managers achieve a “good feel” for numbers on their ground through weeks of monitoring and night counts with thermal and night-vision scopes, but there's no national network to link these datasets together. Consequently, estimates of deer populations tend to be calculated based on areas of suitable habitat and known densities within it and, hence, values are given as ranges or rounded up, rather than providing a precise figure.
When considering population estimates for water deer, it is important to recognise they mature early for a cervid and can produce large litters. The result is a species with significant potential for rapid population growth and equally rapid population collapse. This is reflected by the census data for Whipsnade, which show how the population increased just over six-fold in only three years; from the 32 deer released into an area of undeveloped pasture in 1930 to the 200 animals estimated in 1933. The Whipsnade population subsequently crashed. A similar rapid population decline, driven by substantial mortality and high foetal/fawn loss, has also been observed in the wild population at Woodwalton Fen, Cambridgeshire. Indeed, in their 1981 report for the National Conservancy Council, Arnold Cooke and Lynne Farrell wrote:
“Perhaps the most significant and conclusive point to come from these calculations is that survival rates for both young and adult deer fluctuate markedly from year to year. This has led to appreciable fluctuations in population level, especially when years of high adult mortality in spring had low recruitment.”
For in-depth discussion on the censusing and monitoring of water deer populations, the reader is directed to Arnold Cooke's 2019 book Muntjac and Chinese Water Deer.
The demography of the population varies according to habitat and local management but again data from the wild are scarce. At Woodwalton Fen, Cooke and Farrell found that the number of first year deer in the population was typically about 20% per year, corresponding to 0.5 young per surviving pair and suggesting most of the fawns either died or dispersed away during autumn. At Whipsnade, Stefan Stadler found the number of first year deer in the population varied between 27% and 41%, while Helin Sheng and Houji Lu reported the population structure of one exceptionally heavily hunted population on the Zhoushan Islands to be about 48% first year deer, 36% two-year-olds, 8% three-year-olds, 4% four-year-olds, with five- and six-year-olds each accounting for 2%.