Vietnam Primate Conservation Status Review 2002 - Hoang Lien ...

Vietnam Primate Conservation Status Review Part 2: LEAF MONKEYS
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3.3.5 Status
The historical (original) density of Cat Ba langurs on the island is unknown. According to older
villagers, the Cat Ba langur occurred in high numbers in the past.
By comparing T. p. poliocephalus to the relatively well-studied and closely related White-headed
langur (T. p. leucocephalus), a rough estimation on the historical population density can be obtained.
The density of White-headed langurs in optimal habitat is 17.17 individuals per km 2 (Huang Chengming
et al., 1998) to 19.71 individuals per km 2 (Li Zhaoyuan et al., in press.). It can be assumed that the
entire island of Cat Ba had previously been optimal habitat for the langurs. With a land surface area
of 140km 2 , Cat Ba could have supported at least 2,400-2,700 individuals.
Most likely the langur population began to decrease in the 1960s, and this decrease was further
accelerated by an increase in human settlements in the following decade. However, there is not
enough data to accurately assess the demise of the Cat Ba langur population over the years. All
previous census data seems generally incorrect. A more accurate and also more impressive number
originates from local interviews about hunted langurs: Between 1970 and 1986, some 500 to 800
individuals were killed (Nadler & Ha Thang Long, 2000).
Another rough estimation of the population decline since 1990 pertains to group size, which is
independent of the individuals in the whole population. The group size of Cat Ba langurs in healthy
habitat is most probably the same or very similar to the group size of the White-headed langurs. In
optimal habitat the mean group size of T. p. leucocephalus is 9.12 individuals per group (Li Zhaoyuan
et al., in press.). For the Cat Ba langur, Nguyen Cu and Nguyen Van Quang (1990) indicate a group
size of 9.45 (8.6 to 10.3) individuals per group.
Surprisingly, observations of the White-headed langur show that a drop in the overall population
density to 55.5% and a drop in group density to 48.5% do not correspond with the group size (individuals
per group) (Li Zhaoyuan et al., in press.). Despite the drastic reduction in population density, for
instance, group size has stayed the same. This means that langurs form rather stable social systems
and that groups live relatively close to one another. In this way, groups that have experienced a loss
of some members will either accept new animals to replenish their numbers or consolidate into new
groups. Consequently, group size is not seriously affected as long as individuals interact to form the
species’ typical social unit (9.12 individuals per group for leucocephalus).
Assuming that in 1990, most groups of Cat Ba langurs still maintained a species-specific group size,
and considering that leucocephalus and poliocephalus are very similar in their social behaviour, it
can be concluded that most langur groups were still in contact with each other a decade ago.
However, we know there has been a drastic decline in the Cat Ba langur population from 1990 to 2000.
There has also been a decrease in group size: 9.45 individuals per group in 1990 to 6.15 individuals per
group in 2000 (Nadler & Ha Thang Long, 2000). This indicates already that many of the groups are
most probably no longer in contact. An average loss of 3.3 members per group reflects an overall loss of
60 to 70 individuals during the past decade from existing groups. Additionally some groups were totally
eradicated. The minimum number of hunted and trapped langurs in this period is 90 to 100. Thus from
1990 to 2000, hunting pressure reduced the Cat Ba langur population by about 50%.
There are reports that during the FZS survey and immediately afterwards several langurs were killed
(about 10 are known), and in addition one langur group with 7 individuals was erased (Nadler & Ha
Thang Long, 2000). Another group encompassing 10 animals was captured in January 2000 (Baker,
2000). The sole survivor of this group is now kept at the EPRC and involved in a breeding programme.
Hunters killed two additional langurs in 2001 and sadly one of these was a pregnant female (Stenke,
pers. comm.).