Ecoregional Assessment of Biological Diversity in East Kalimantan

East Kalimantan Ecoregional Assessment: Chapter 2 – Target Ecological Systems
Pesut Conservation Issues:
Danielle Kreb (pers. com.) states that a range of factors threatens the surviving population of
Pesut in the Mahakam River. She lists the following as being the most important to Pesut, but
they also impact many other fauna in this uniquely rich area:
1. Direct mortality in gill nets is by far the most serious threat and accounts for 80% of the
mortality of the Pesut. They get tangled up in nets, particularly at night. During the day,
Pesut die when they are being released from nets by fishermen- who generally empty the
nets twice a day. Kreb and Rahadi (2002) estimate the number of newborn Pesut in the
Mahakam per year varies between three and five, whereas mortality per year is minimally
five Pesut on average. From 1995 until 2001, 37 deaths were recorded based on
interviews (30 deaths resulted from gillnet entanglement, 3 included deliberate kills, 2
because of vessel collisions). During 1997-98 at least seven Pesut were illegally livecaptured
from the river and taken to oceanaria.
2. Habitat displacement and direct strikes by boats which use the same areas often preferred
by Pesut - consequently the dolphins move to other less favorable areas. This causes the
animals to dive more than they might otherwise and is thought to generally contribute to
higher stress, lower productivity (Kreb and Rahadi 2002) and direct mortality.
3. Reduction in their food supply (a range of fishes - including the large Patin, Pangasius
spp.-and prawns). This occurs by:
‣ Direct destruction of the habitat of their prey species by commercial boats. This
happens by these boats striking the banks of the rivers and directly destroying the
bank root systems where many fish spawn;
‣ Riparian forest loss due to legal and illegal logging activities as well as forest fires
have also caused a decrease in available animal and plant detritus from overhanging
vegetation. This detritus forms the basis of the food chain for many invertebrates and
fish;
‣ High sedimentation and turbidity have also contributed to a decrease of available fish
resources. This is due to: limited light penetration affecting aquatic plant life;
accumulation of suspended sediments on gills of certain fish and causing death by
suffocation; silt settling on the bottom of slow flowing river parts or lakes smothering
food resources, eggs and spawning grounds;
‣ Logging activities and forest fires also affect swamp forests where residual
populations of blackfish remain in isolated refuges, which do not dry out during the
dry season and provide the spawners for the next flood period. Also, juvenile fish and
small fishes of other species shelter beneath floating mats of vegetation in the
swamps; and
54