Mongolian gazelle population assessment 763


FIG. 2 Distance sampling line transects (survey effort = 13,895 km), survey strata and locations of Mongolian gazelle groups encountered in the central and eastern Mongolia survey region during the May–June 2020 survey. (Readers of the print journal are referred to the online article for a colour version of this figure.)


Results


Population surveys in Mongolia The 2019 and 2020 survey data included 58 and 1,047 group observations (totalling 1,719 and 151,691 individuals), re- spectively. Mean group size was 30 (range = 1–600)in 2019 and 160 (range = 1–16,200)in 2020 (Table 1). Detection probabilities were 0.57 (95%CI = 0.39–0.85) and 0.34 (95%CI = 0.30–0.38; Fig. 3), with effective strip widths of 200 m(95%CI = 136–296 m) and 412 m(95%CI = 370– 459 m) for the 2019 and 2020 surveys, respectively; the dif- ferences are in large part because of the different right trun- cation regimes. There was an indication of size bias in estimations of mean group size for the 2020 survey data but not for the 2019 survey data because of the more severe right truncation of these earlier survey data. The gazelle group encounter rates during the 2020 survey


were highest in the south-west followed by the north Kherlen, west, Menen and central strata, and the lowest en- counter rate was in 2019 in the southern Gobi (Table 2). Overall, the encounter rate in the central and eastern steppe regions of Mongolia was substantially higher than in the


southern Gobi (0.71 vs 0.01 groups/km2). In all cases the ex- pected group size estimates were smaller than the mean group size estimates (in north Kherlen and Menen, the expected group size was less than half the mean group size), indicating that a greater proportion of larger groups tended to be seen farther from the transect line, with smaller groups more likely to be missed with increasing distance from the observers (Table 2). For the 2020 survey, estimated group density was highest


in the Menen stratum and estimated individual density was highest in the central stratum, whereas the highest abun- dance estimates were obtained for the central and north Kherlen strata. In contrast, the 2019 survey in southern Gobi yielded the lowest estimates for all of the parameters; individual density, for instance, was approximately one order of magnitude lower than the overall density for the 2020 survey (Table 3). Overall individual density estimates from the 2019 and


2020 surveys were 0.52 and 4.72 gazelles/km2, respectively, with total population estimates of 40,899 (95%CI = 16,307– 102,580) individuals in southern Gobi and 1,991,300 (95% CI = 1,464,900–2,706,700) in central and eastern Mongolia (Table 3). In 2019, most of the variance in the abundance


Oryx, 2024, 58(6), 759–768 © The Author(s), 2024. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605323001515


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