We've already seen that water deer occupy a significantly smaller range today than they have historically, and it's difficult to imagine this is not also the case in terms of their population. From Robert Swinhoe's early accounts, we know that the species was already heavily hunted, some parties returning with 30 or more carcasses, but seemingly still reasonably abundant. In 1872, Swinhoe described that the deer were “yearly becoming more numerous” around Shanghai and, the following year, wrote of how all season the market had “been perfectly glutted with them; and numbers rot for want of consumers”. Chinese population data have, nonetheless, been difficult to come by, although across the literature with which I'm familiar, densities seem typically between 0.005-0.22 animals per hectare (0.01-0.54 per acre), although may reach 1.64 does per ha (2.5 acres) on high quality feeding grounds. This recalculates to densities of between one animal per 200 ha (500 acres) and one per 4.5 ha (11 acres), with exceptional feeding allowing an animal to survive in as little as 0.6 ha (1.5 acres).
In their chapter on the status of deer in China, published in 1993 as part of the Deer of China compendium, Helin Sheng at East China Normal University and Noriyuki Ohtaishi at Hokkaido University in Japan estimated the population of wild water deer in China to be between 10,000 and 30,000 animals. Even at this juncture, however, Helin Sheng noted that some 10,000 animals were taken annually by hunters during the 1980s and, in his 2019 Muntjac and Water Deer, Arnold Cooke wrote that the species is thought to have declined in both number and range owing to habitat destruction and hunting. Indeed, in their 2014 paper to Biochemical Systems Ecology, Weiwei Shi and colleagues at Zhejiang Normal University gave a total wild population in China of about 3,500 as of 2007, with the largest group living in the Zhoushan Archipelago in China's Zhejiang Province. Genetic analysis pointing to high levels of diversity, despite relatively low numbers of present-day deer, supports the idea that there has been a rapid decline in numbers recently, although reintroduction programmes of captive-born individuals from Zhoushan stock to two areas around Shanghai and additional legal protections are aimed at tackling this decline.
While the Zhoushan Island population is probably the largest, it also appears to be genetically rather isolated. In their 2014 paper, Shi and her co-workers found that while there were no signs of inbreeding within the Zhoushan populations, genetic differentiation increased with geographical separation. In other words, populations on Zhoushan differed more from those on Daishan, which is further away, than to animals on nearby Xiushan, suggesting limited movement between islands. Furthermore, reanalysis of genetic data from a global sample of water deer by Rory Putman and colleagues strongly suggests there's no, or very little, gene flow between mainland populations in China and those on the Zhoushan islands.
In Korea we know virtually nothing about the population in the north, and have no recent data because, unlike in the south, there are no regular wildlife surveys. In his 1968 Korean book, The Mammals of Korea, Hong-Ku Won, a professor at Kim Il-Sung University in North Korea's capital Pyongyang, commented on the limited distribution of the species along the west coast. Despite having been historically relatively abundant and widely distributed through the north of the peninsula, hunting for fur, meat and use in traditional medicine resulted in a steep decline of the species. The deer are nonetheless regarded as a valuable natural resource and Won recounted how, in 1959, the North Korean government translocated 16 water deer from Moonchen in Kangwon Province to what is now the Mount Guwol Biosphere Reserve in South Hamgyong Province in the northeast of the country, and imposed a ban on hunting for the following five years. A further reintroduction was made in 2012. These reintroductions appear to have boosted the population, but I know of no recent data on the distribution or abundance there. Both Yeong-Seok Jo and Junha Kim told me that they consider the population of water deer in the north to be low, owing to the rate of habitat destruction and food shortages experienced by much of the north.
Contrary to the paucity of information in the north, water deer appear to be thriving in South Korea, to the point where some sources suggest they are considered a pest. In 2011, using data collected by the Ministry of the Environment of Korea between 2000 and 2005, Baek-Jun Kim and colleagues calculated average densities in lowland areas of South Korea at about 0.07 per ha, 0.02 per ha in mountainous habitat above 300m (1000 ft.), and 0.01 per ha in urban areas. It should be noted, however, that there was substantial variation in the estimates provided by this analysis - in lowland habitat, for example, the standard deviation was higher than the mean. Overall, the range Kim and his co-workers arrived at was between 0.0008 and 0.241 per ha, broken down as follows: 0.004-0.241 per ha in lowland; 0.004-0.053 per ha in the mountains; and 0.0008-0.024 per ha in urban areas.
The latest wildlife survey report published by the National Institute of Biological Resources of Korea, suggests the “national density” of water deer in South Korea in 2020 was 0.077 per ha, down from 0.079 in 2019 and 0.082 per ha in 2018. Looking back at earlier reports, it appears that the population in South Korea has been steadily increasing from 0.057 per ha in 2001. Jihyang Jung and colleagues, in their 2016 paper to Animal Cells and Systems, gave the average population density of Korean water deer as 0.078 per ha, varying from 0.037 to 0.11, based on the 2015 wildlife survey. In terms of national estimates for South Korea we have only a guestimate that suggests substantial abundance. In an article to the Korean news site Society during January 2018, Kwon-pil Chun reported that there are thought to be more than half a million water deer in the country (500,000 to 700,000 are the figures given in the article). It should be noted, however, that these figures were arrived at by the Ministry for the Environment of Korea by multiplying the wildlife survey densities by the area of the country, and so should be treated with caution.
While half a million or more water deer may seem almost unbelievable, Professor Gap-soo of Yeungnam University reported that 113,000 deer were killed by hunters or farmers in the country during 2016 alone. Similarly, in a 2016 paper to Ecology and Resilient Infrastructure, Tae-Young Choi presented data showing that nearly 28,100 water deer were reported killed on South Korean roads between 2011 and 2015; 2015 was the highest, with almost 8,400 casualties. Extrapolating across the country and considering deer wounded by cars, Choi estimated that as many as 60,000 may be involved in road traffic accidents each year. If the annual mortality is, indeed, some 170,000 deer and the species is still sufficiently abundant to be considered a nuisance, this would seem to add credence to the high population estimate.