The crocodiles of Danau Sentarum, West Kalimantan.
Danau (=lake) Sentarum National Park (00[degrees]51'N 112[degrees]06'E), a Wetland of International Importance (under the Ramsar Convention), is located in the province of Kalimantan Barat (West Kalimantan), over 700 km upstream from the lower South China Sea (Giesen and Aglionby, this volume).
The wetlands of Danau Sentarum (including the lake Danau Sentarum proper) are a complex of seasonal freshwater lakes, connecting rivers and swamp forest located in the catchment of Indonesia's longest river, the Kapuas. Danau Sentarum and environs experience the fullest extremes of seasonal fluctuations in water level. These wetlands form a distinct and important hydrological unit absorbing floodwaters from Sungai (=river) Kapuas during the wet season and contributing up to 50% of the river's downstream discharge in the dry season (Klepper, 1994).
Giesen (1987), in his initial study of Danau Sentarum, presented a cursory review of the few previous exploratory or scientific accounts that included the area (then known as the Kapuas Lakes). Some of these older accounts make mention of crocodiles in and around D. Sentarum. However, Beccari (1904) commented that he "never had the good fortune" to see Tomistoma schlegelii (one of two crocodilian species known to inhabit the D. Sentarum region), while collecting biological specimens there in 1867. The other crocodile known to occur at D. Sentarum is Crocodylus porosus, to which Beccari lost several of his travel companions elsewhere in Borneo.
While additional older references likely exist, an exhaustive literature search for historical information on the distribution of crocodiles in Borneo was not undertaken for this article. However there have been several recent investigations touching upon Bornean crocodile distribution. (These include Cox and Gombek 1985, in Sarawak; Cox et al. 1993, pan-Kalimantan; Frazier and Maturbongs 1990, in East and Central Kalimantan; Mum and Romono 1994, in E. Kalimantan; Ross et al. 1998, pan-Kalimantan; Stuebing et al. 1998, in Sarawak; and Whitaker 1984, in Sabah). For Danau Sentarum specifically, several researchers (e.g. Giesen 1987 and A. Sebastian in litt. 1994) have casually (but relatively infrequently) observed either or both species of aforementioned crocodile. Frazier (1994) conducted the only structured crocodile survey of D. Sentarum. Additionally, teams in 1995 (Ross et al. 1998) conducted a couple of brief night surveys there but did not sight any wild crocodiles.
Determination of crocodile status at Danau Sentarum is complicated by the system's unique hydrology, and by the fact that the taxonomy of palustrine crocodiles in the Indopacific region including Borneo (and specifically including the Danau Sentarum region) is unclear (Ross 1990; Cox et al. 1993; Ross et al. 1996).
Conventional methods to determine the occurrence and species of crocodiles, and their relative status within an area, are a combination of field survey (looking for wild specimens and spoor), examination of captive specimens, animal parts and artifacts, and interview of informants (e.g. hunters, fisherfolk and other local residents, traders, etc). Going further to estimate crocodile population numbers requires that replication be an integral part of an extended survey regime. For example, a single pass along a river during one night (or day) in a season may well reveal the existence of a crocodilian(s) and even their species. It will not provide a population estimate, although the level of observations may infer a paucity or abundance of individuals. No replicate crocodile studies have been performed at Danau Sentarum.
Frazier (1994) employed nocturnal spotlighting from boats and on foot as his survey method. Spotlighting crocodiles is a simple and relatively inexpensive way to verify crocodile presence and oftentimes, species. Therefore in a preliminary assessment of an area, it is a practical initial approach. Spotlighting involves the smooth, systematic and comprehensive scanning of exposed land-water interfaces (e.g. banks, bars and fallen trees) as well the open water to the fore of the survey craft. A single beam of light is used taking care not to illuminate the boat (or spotter's hands and face). A search beam reflects crocodilian eye-shine with a characteristic brilliance, in a range of hues from yellow to amber to red. It is this eye-shine that many crocodile hunters around the world still exploit in pursuit of their prey. It doesn't take long for most observers to gain the ability to separate other types of reflected eye-shine--there are many (e.g. invertebrates, frogs, birds)--from those of the crocodile. Given dark conditions and slow, fluid and quiet movement, it is often possible to approach crocodiles (especially young ones), close enough to allow for species identification and size estimation. Specimens seem mesmerized (or perhaps blinded) by the search beam and (if not too large) can often be hand-captured by the experienced. Captured crocodiles are then inspected (species, vigor, color and markings, sex, etc) measured, marked and released.
Such parameters as time, compass bearings and/or landmarks, weather, water level (tidal phase), lunar phase, habitat and human activity are routinely recorded, with a view to replicate studies. In 1994, survey distances were subsequently calculated by measuring courses against landmarks on 1:50,000 scale base maps that had been prepared by the ODA/PHPA GIS unit of a joint UK-Indonesia government project (UK-ITFMP) then operating (Frazier 1994), with D. Sentarum as a major study area (Giesen and Aglionby, this volume).
It is important to note the limitations of spotlighting, and to have a clear understanding of the value of any such observations gained by it. Spotlighting is ineffective in heavily vegetated swamps and thicket or forest. Resident crocodiles more often than not hide in or behind vegetation or debris. The search beam is rendered impotent under such conditions. Spotlighting reaches its most effective along an exposed land-water interface, that is to say primarily along bare river banks (or lake shore). With the coming of the dry season, rivers recede to expose their banks, and crocodiles which do not move to larger rivers or other wetlands farther away, concentrate in the diminishing waters (or at least this has always been the prevailing view).
A prime consideration in evaluating spotlight observations is the level of human activity in an area, especially hunting. Crocodiles under hunting pressure are likely to be wary and may submerge in response to the noise of an outboard motor or the cast of a search beam. It is likely that older crocodiles are more wary than younger ones. The author has seen exceptions to this seemingly logical conclusion, but wariness is an unknown and immeasurable quantity. There is always the chance that individual crocodiles are already submerged or have their eyes oriented away at the precise moment the search beam is cast. Given such unfathomable variables, spotlight observations cannot be used as a definite measure of population numbers, but they may be able to provide the aforementioned information on species identity as well as some information on size class (i.e., age class).
Low yet navigable water levels are conventionally considered a boon to crocodile surveys since remaining crocodiles theoretically concentrate near and in persistent water. In the main Danau Sentarum study (Frazier 1994) however, water levels were often so low as to preclude navigation. Those river courses still able to accommodate boat traffic were invariably choked with people fishing, and their diverse assemblage of vessels and implements.
Three distinct types of habitat were cited by local residents as dry season haunts of the crocodile in the Danau Sentarum area (Frazier 1994). These were hulu sungai (upper reaches of rivers), lubuk (the sometime dry-season remnants of the wider and deeper pools of a contiguous river course) and kerinan (isolated dry season pools in forest or dry lake bed). One source, however, stated that C. porosus also retreat to Sungai Kapuas in the dry season. Typically all three habitats (with the exclusion of some lubuk) could only be reached on foot under those conditions during the 1994 survey. The same water-borne spotlighting principles were applied to the foot searches, but maintaining a low level of noise, and use of a single beam of light (for more than one person) proved difficult. Whitaker, however, found on-foot searches along upstream forested segments in the Bintuni Bay region of Irian Jaya to be an effective method of surveying C. novaeguineae (Frazier, 1990).
Anecdotal information is sometimes the only "data" that can be collected. In areas where crocodile encounters are infrequent (owing to reduced numbers) it is typically older men who make the best informants. Interviews should occur under relaxed conditions so that a measure of rapport can be established. Often mystical powers or actions are attributed to crocodiles, and attentive listening maintains the harmony of the interview. Obviously information obtained in interviews is of unknown and varying quality, and often colored by the perceptions and prejudices of the informant. The questions posed should never provide an answer for the informant. It is often advantageous to ask an informant to serve as a guide, thereby gaining an investment of time in support of his or her story.
There are reportedly four crocodile species described from Kalimantan/Borneo (Cox et al. 1993; Ross et al. 1998). Three of these species are still known to be extant in the wild. The status of the extant species according to The 2000 IUCN Red List of Threatened Animals (Hilton-Taylor 2000), the IUCN/SSC Crocodile Specialist Group (CSG), (Ross 1998) and CITES (CITES 2000), are summarized in Table 1. A fourth species of Bornean crocodile, C. raninus, was recently resurrected by Ross (1990) on the basis of an examination of museum material and literature review. Subsequently the type locality for C. raninus was restricted to Pontianak, West Kalimantan (Ross 1992; Ross et al. 1998), which is situated near the mouth of Sungai Kapuas, the river catchment of Danau Sentarum. However no observations have been confirmed as C. raninus since the museum specimens were collected over 100 years ago. A search of the "CITES Species Checklist" (http://www.unep-wcmc.org/CITES/common/dbase/fauna/index.shtwl) on the Crocodylidae, does include an entry for C. raninus (access date: October 29, 2000), but it is not otherwise listed under the CITES appendices (same source).
Two of the three known extant Bornean crocodiles occur in and around Danau Sentarum (in apparently very low densities). While a plethora of local names exist for the crocodilians of Borneo (Ross et al. 996), the species confirmed at D. Sentarum are locally referred to as buaya for Tomistoma schlegelii and rabin for Crocodylus porosus [buaya is the generic bahasa Indonesia name for "crocodile"] (Frazier 1994). Giesen (1987) and Frazier (1994) observed both wild and captive Tomistoma at Danau Sentarum. In 1996, several additional captive Tomistoma and C. porosus were observed in villages in or near D. Sentarum (Ross et al. 1998). Giesen (1987) observed an estimated 2-m total length (TL) C. porosus in the field. Several credible accounts and photographs of captives of both species exist from Danau Sentarum. A deteriorating photograph of a very large, perhaps 4-5 m TL probable C. porosus (accidentally captured and drowned in a fishing net in Sungai Kenelang in 1989) was observed in 1994 (with copies since obtaine d) by the author. Local informants recounted numerous anecdotes of wild encounters with one or the other crocodile species to the author at D. Sentarum (Frazier 1994). In 1994, a 4 meter-long Tomistoma was accidentally drowned in a jermal fishing net near Bukit Tekenang; its skeleton was collected and deposited at the Zoological Museum in Bogor (W. Giesen in litt. 2000).
Immediately after the 1994 survey, the author was provided with undeveloped film said to contain crocodile images from the Danau Sentarum region (taken in August 1994). Immediately upon return from the field, the film was processed. It yielded four photographs of a captive yearling Crocodylus sp., which resembled C. porosus except that it otherwise possessed twin pairs of enlarged bilaterally symmetrical post-occipital scutes. Post-occipital (PO) scutes are (enlarged) dorsally positioned scales occurring immediately behind the skull platform, or on the back of the "neck" of the crocodile. (The absence of, or vestigial, enlarged PO scales is one quick field method used to differentiate C. porosus from other Crocodylus sp.). The photographed specimen was said to have been caught in Sungai Tengkidap (J. Aglionby pers. comm. 1994), a river on the southern bounds of the lake complex. Enlarged post-occipital squamation is relatively rare in the estuarine crocodile. Ross et al. (1996) found mean occurrence of enlarg ed PO scales to be less than 1 scale in 32 West Kalimantan and 23 East Kalimantan specimens of wild-caught captive crocodiles. Ross and Mayer (1983) mention that Deraniyagala (Deraniyagala 1939) reported C. porosus specimens with up to 4 enlarged PO scutes, but the former authors only rarely observed up to 2 distinct scutes in their study. A photograph of a C. porosus captured subsequently in a bubu (fish trap) at Danau Sentarum (J. Aglionby in litt. 1994), revealed typical PO C. porosus squamation (i.e. no enlarged scutes). In 1995, a captive C. porosus from Leboyan, (D. Sentarum) with 3 larger PO scutes was photographed (Photograph 4 in Ross et al. 1996), but these were distinctly asymmetrical in size and shape, and therefore did not resemble the regular pattern displayed by the Sungai Tengkidap specimen. Likewise, this author cannot recall ever observing C. porosus with 4 post-occipital scutes that were arrayed as large distinct bilaterally symmetrical twin sets (as manifested in the S. Tengkidap Crocodylu s sp.).
Owing to its PO scale pattern, it was initially thought that the photographed specimen could possibly represent C. siamensis, which has recently been confirmed as present in the wild in East Kalimantan (Ross et al. 1998). However this preliminary conjecture was easily rejected upon comparison of photographs of the S. Tengkidap specimen with those of captive yearling C. siamensis from Thailand and East Kalimantan. C. siamensis has a strikingly rough appearance and a comparatively broad and blunt snout, and notwithstanding the PO squamation, the Sungai Tengkidap specimen more closely resembles the "smoother" and "sharper snouted" C. porosus.
Neill (1971) recounted how in 1935, Schmidt suggested that a freshwater Crocodylus sp. should be found on Borneo, and when it was, it was identified as C. siamensis. Neill's map showing Bornean distribution (p. 398) puts known C. siamensis records in West Kalimantan and these are on or about the lower Sungai Kapuas. Interesting though is the placement of a question mark for the species' distribution in the vicinity of the Kapuas Lakes, i.e. Danau Sentarum (as well as in Central/South Kalimantan).
In numerous interviews, local residents did not even hint at the presence of (or perhaps did not recognize) more than two crocodile species during the main Danau Sentarum survey (Frazier 1994). In both East and Central Kalimantan the author found widespread consensus among residents that there were at least 3 crocodile species present (Frazier and Maturbongs, 1990). Anecdotal support for a third resident species of crocodilian from other parts of West Kalimantan (and Central Kalimantan) was not corroborated by (High-water-impacted) survey efforts in 1995 and 1996 (Ross et at. 1998).
In terms of PO squamation, the aforementioned Sungai Tengkidap photographic specimen was probably "not typical of C. porosus or C. siamensis but may be referable to C. raninus" (Ross et at. 1996), however certain more definitive diagnostic features were not visible in the photographs. The dorsal PO squamation of the S. Tengkidap photographic specimen also resembles that in photographs (in Ross et at. 1998 as Figures 1-4) of two captive specimens taken at a crocodile farm in South Kalimantan in 1995 (pers. obs.). These latter two farm specimens were attributed to the "raninus group" of crocodiles by Ross eta?. (1996) on the basis of their "ventral squamation," the character of which is a central diagnostic morphological feature for this group (Ross 1990). The raninus group or "large scale group" of crocodiles has 22-26 rows of transverse ventral scales (Ross et at. 1996). Unfortunately the unidentified Crocodylus sp. from S. Tengkidap was only photographed from the dorsal perspective, and its P0 squamation whi le intriguing and perhaps suggestive, is itself not diagnostic. An effort was mounted to find, re-photograph and retrieve this unidentified crocodile after the exposed film was developed and sparked interest. W. Giesen traveled to the area three months later (W. Giesen in litt. 1994). There he learned that there had originally been two crocodiles that had been caught in rapid succession. One of these had escaped two days later. Then, shortly after the remaining crocodile was photographed, a dukun (soothsayer) ordered that the specimen be released. It seems that the child of the captor of the crocodile had fallen ill, and the dukun concluded that the crocodile had caused this illness, perhaps in retaliation for being held captive.
With the exception of two brief night surveys (at Danau Pengembung and D. Semati), conducted in 1995 by Ross et al. (1996) under unseasonably high water levels (and yielding negative observations), only one structured crocodile survey regime has taken place at Danau Sentarum. This was a dry-season survey conducted by Frazier (1994). The following details relate to that survey regime.
Three water-borne night surveys over an estimated (one-way) distance of 134 km of river and three night-searches on-foot along tributaries to water holes, covering an estimated 25 km (one-way) were undertaken during this preliminary investigation of crocodiles at Danau Sentarum. In addition, other similar travel in and near the reserve in excess of300 km provided supplementary opportunities for observation. Crocodilian eye-shine was sought whenever travelling at night, and daylight afforded the opportunity to look for crocodile tracks and slides, or individuals floating or basking. Table 2 lists surveys and major field travel during the 1994 preliminary investigation along with summarized crocodile observations (from Frazier 1994).
During the entire survey regime, only 6 crocodiles were spotted, all on a 7.6 km span of the same river (Sungai Embaluh Leboyan, Survey 7), resulting in an observed density of 0.8 crocodiles/km over that segment (which fell completely outside of the then current reserve boundaries). The density observed for the entire Survey 7 was a near negligible 0.1 crocodiles/km. Two of the six crocodiles were identified as Tomistoma schlegelii and one of these false gharials was captured, examined and measured, marked and released. The remaining four crocodiles were recorded as EO (eyes only) observations. It was clear however that they were all sub-adult and probably yearling class, as were the two animals that were identified to species.
Just outside of Nanga Pengembung, along the shrunken course of Sungai Pengembung (en route to Survey 2a), the party was searching for and found what appeared to be two distinct sets of juvenile-adult crocodile tracks (disclosed earlier during an interview in the village). However on two occasions, hikes to lubuk ended without observations. An unidentified track in mud surrounded by a peat deposit was noted at Kerinan Suakuri (Surveys 6a and b). Other intriguing information about crocodile inhabitation of kerinan, including crocodile nesting, could not be verified in the allocated time especially due to the unavailability of willing guides.
Anecdotal information abounds on crocodiles at Danau Sentarum. Interviews should ideally help to elicit information useful in narrowing and focusing search area. In the 1994 study (Frazier 1994), one interview led to observations of two individual sets of crocodile tracks (Survey 2a). The informant had said that an adult Tomistoma had recently (June 1994) been seen in this area during daylight hours. Other interviews didn't prove so fruitful but nonetheless provide some possible clues for future study of crocodiles at Danau Sentarum. A selection of anecdotes and observations follows.
On Aug 7, 1994, Pak (sir) Sahalan (of Nanga Kenelang), a former crocodile hunter provided information on Tomistoma schlegelii (known in the D. Sentarum area as buaya, which simply means crocodile throughout most of Indonesia). He related that the dry season sees a retreat of buaya into more secluded areas because with the lowering of water comes an increase in fishing. Buaya were said to seek the deeper, wider pools (lubuk) of the shallow river courses and small isolated ponds (kerinan). The informant said that small crocodiles (formerly) remained in the remnant river courses, and (1-5) adults would seek shelter in lubuks. [Later Pak Sahalan served as guide to a couple of alleged lubuks (Survey 3a) where he had captured adult buaya years before, but these "wide places" appeared incapable of accommodating even one juvenile crocodile].
Pak Sahalan also related that the Tomistoma nesting season was September/October and that the species typically lays a clutch of 29-35 eggs in a nest of litter constructed at the base of a tree. (Informants from D. Sentarum told Ross et al. (1996) that the nesting season was July/August and that clutch size was 25-40. Three wild Tomistoma nests discovered in East Kalimantan in 1996 by Ross et al. (1998) had clutch sizes ranging from 23-37, and were found both in forest and on a floating mat of vegetation. Informants put the nesting season at July-October, or usually the dry season). Another informant, Pak Abdul Muin (of Pulau Majang), said both rabin and buaya made nests in July; lay eggs in August; and that their eggs hatch in September or October. When asked where crocodiles nested in the area, the informant described a place he called "Kerinan Mensait." He said he had observed a high density of nesting rabin (C. porosus) at this location (some years before). He described the area's vegetation as being domi nated by clumps of Pandanus sp. While his characterization of nesting phases was too compressed, his description of mounded nests, nesting material and surrounding vegetation seemed to indicate familiarity with crocodile nests. The informant enumerated three reasons that people were not currently exploiting this area of (prolific) crocodile nesting: 1) the area was hard to find, 2) skin prices were too low, and 3) hunters needed to be brave to enter a rabin nesting area. The author's several attempts (at various villages) to hire guides to the area were unsuccessful (Frazier 1994).
The informant Pak Sahalan contended that both species of known D. Sentarum crocodile will regurgitate partially-digested prey to attract game, laying in wait with its mouth open ready to seize inquisitive pigs or other animals investigating the smell. The informant stated that the preferred bait for hooking crocodiles was monkey flesh. In 1994, a drowned adult Tomistoma (mentioned in the Introduction) was found to contain several "furballs" which closely resembled the fur of Macaca fascicularis, W. Giesen in litt. 2000). Frazier (1994) didn't see any signs or hear of any current crocodile hooking when he was in the D. Sentarum area.
When asked about local residents' attitudes toward crocodiles, the general consensus among another group of informants was fear, although they did not express any overt malice toward the animals. They explained that people do not actively seek out crocodiles but that the animals sometimes drown in fishing nets. This was supported by E. Widjanarti (pers. comm. 1994), a researcher who had seen the skin of the (abovementioned) adult Tomistoma that had drowned in a jermal fishing net. This event had been reported in a local newsletter published by the conservation project (Edition 14, June 1994 of Suara Bakakak) at Danau Sentarum.
The paucity of wild crocodile observations from the few surveys at Danau Sentarum means that descriptions of crocodilian habitat must rely primarily on anecdotal information. The relative importance of these habitats cannot be estimated given this dearth of data. Many basic habitat descriptions have necessarily been included in the foregoing to illustrate survey methods and characterize species occurrence. The following summarizes the available information on the (dynamic) crocodile habitats of Danau Sentarum.
By some standards, crocodiles are possibly more difficult to observe in the dry season than in the wet season at Danau Sentarum. This is in contrast to the conventional view wherein the dry season is the time to observe crocodiles (presumably this applies to crocodile haunts elsewhere which are less disturbed than those at Danau Sentarum). The distinctive hydrological regime at D. Sentarum is one of polar extremes. In the typical dry season, water drains into the Kapuas River to such an extent that vast areas of dry, sparsely vegetated (but formerly submerged) land remain. During the wet season this floodplain receives backflow from Sungai Kapuas, filling up to form a vast flooded complex. In May of 1867, Beccari (1904) was travelling on Danau Seriang (presumably just west of Danau Sentarum proper) and wrote this description: "The surface of the lake, clear and free from arboreal vegetation, extends only a few miles, but nowhere could we see a trace of dry land."
Local informants universally contended that the dry season sees the departure of crocodiles from larger river courses of Danau Sentarum. Surveys on these larger rivers indeed yielded no crocodile observations (pers. obs.). Crocodiles are said to move to hulu sungai (upper river courses), lubuk (pools in intermittent or shallow rivers) and kerinan (isolated persistent water holes) habitats (and in one account, to the Kapuas River). Giesen et al. (1994) also suggested that crocodiles might also make a seasonal retreat to Sungai Tawang (however no sightings were made along this heavily disturbed river during the 1994 study). Crocodiles would presumably migrate to the aforementioned more secluded areas in response to the radical intensification in fishing and peripheral activities that are encouraged and facilitated by falling water levels. However, this could not be confirmed in the 1994 study (Frazier 1994). Only 6 wild crocodiles were observed during the entire, albeit brief, 1994 survey regime. These surveys included a variety of habitats, but not many replicates of the hulu sungai, lubuk and kerinan types. All of the observed crocodiles were sighted in a single sector on Sungai Embaluh Leboyan (a relatively narrow floodplain river bounded alternatively by riverine forest and Pandanus sp.). This site was formerly outside of the eastern reserve boundary. Ross et al. (1996) reported that Danau Semati (upriver from the S. Embaluh Leboyan site just mentioned), is a small Pandanus sp. and grass fringed lake, with some Hanguana malyana, said to be a Tomistoma stronghold (but this was not confirmed during a brief night survey). In the same vicinity are also D. Merbong (bordered by swamp forest) and D. Lintang (fringed by Pandanus sp.).
Additional specific as yet unsurveyed (or scarcely surveyed) locales said to be crocodile habitat include: 1) Kerinan Mensait (west-northwest of Temukup). This is purportedly an area of C. porosus nesting, characterized by Pandanus sp. vegetation; 2) The lakes Danau Batu(k), (hulu Sungai Kulan); Hulu Uangtapa; and Danau Semanuk, (hulu Danau Belida) were known refuges of either or both buaya (Tomistoma) and rabin (C. porosus) in the past; and 3) the permanent lakes southeast of Danau Luar and, as mentioned by Ross et al. 1996, the lakes north of Sungai Embaluh Leboyan (Giesen et al. 1994).
The author knows of no scientific research including the particulars of crocodile nesting at Danau Sentarum. However, recent records of Tomistoma nests from East Kalimantan (in Ross et al. 1998) and Sarawak (in Steubing et al. 1998) may provide clues for additional study on the availability of suitable nesting habitat at Danau Sentarum. In East Kalimantan (N=4 nests) three types of nesting habitat were noted: 1) mixed freshwater swamp, on a floating mat of vegetation, 2) secondary forest, and 3) peat swamp, at base of tree (all inspected in August 1996). In Sarawak, a nest was also discovered at the base of a tree in disturbed peat swamp forest (observed in July 1994).
During the course of the 1994 survey it became readily apparent that crocodiles at Danau Sentarum face a host of adverse factors, both natural and anthropogenic (Frazier 1994). Danau Sentarum undergoes seasonal fluctuations in water-level, spectacular in amplitude; from large open lakes and flooded forest in the wet season to a mosaic of bone-dry and bare lake-bed moonscapes dissected by tepid, stagnant and near-desiccated streams in the dry. While nothing can ameliorate the natural (crocodiles have adapted to these conditions), the man-induced threats can certainly exacerbate the precarious position that crocodiles occupy at Danau Sentarum in the dry season. Adverse factors have been treated individually below, in the current order of the magnitude of negative impact as perceived by the author.
Jermals and other fishing nets and barriers
Danau Sentarum is a bountiful fishery that has been heavily exploited for over a century (Giesen 1987; Dudley this volume). Frequently enough, crocodiles inadvertently fall victim to a range of nets, traps and other fishing apparatus. Often these are small to medium-size specimens which are sometimes pulled from the implements still alive, e.g. a small C. porosus was caught alive inside a bubu (fish trap) shortly after the current survey regime (J. Aglionby in litt., with photo of specimen, 1994). This is also mentioned as a widespread phenomenon throughout Kalimantan in Ross et al (1998). Other times crocodiles drown or are perhaps killed in order to extract them. A. Sebastian (in litt. 1994) came upon a dead juvenile crocodile (probably Tomistoma) along Sungai Embaluh Leboyan that appeared to have had part of its snout hacked off (perhaps whilst extricating it from a fishing net). Other captive, presumably-netted crocodiles were recently observed or reported to have been held at Nanga Kenalang (a 2 m TL C. porosus, W. Giesen pers. comm., 1994), Nanga Pengembung (3 small Tomistoma, E. Widjanarti pers. comm. 1994) and Sekolat (a juvenile Tomistoma, pers. obs.). The effect of this scale of incidental removal in healthy crocodile populations would normally be negligible, as the majority of crocodiles don't survive long enough to breed anyway.
It is the jermal however and other large gill nets and barriers which pose the greatest danger to crocodiles. A jermal is a very long V-shaped apparatus composed of a (finemeshed) net secured to a series of long fixed poles. It is often used in concert with empang (a fish barrier; a bamboo/stick fence placed across a channel typically with a central slot where a net is connected), together presenting an extremely effective (destructive) method of capturing fish, which sometimes inadvertently snares crocodiles.
Jermals (with or without empang) are often dominating structures in relation to the channels where they are placed. Jermals not only net very large catches of fish, but also occasionally ensnare (sometimes large) crocodiles. As reported elsewhere in this article, a large Tomistoma drowned in a jermal near Genting in June 1994. The large C. porosus accidentally netted in 1989 might well have been caught in a jermal or perhaps a pukat (a type of gill net). Informants at Nanga Pengembung reported that large crocodiles sometimes drown in fishing nets although the frequency of occurrence was described as "not every year."
As with other reptiles, typically only a very small percentage of a total crocodile population will survive to adulthood. This breeding cohort occupies that narrow position at the top of the population pyramid. Perhaps as many as 90-95% of the viable embryos deposited as eggs in nests will not survive to reach breeding-age, falling victim to a host of predators and natural or anthropogenic disasters. In populations as disturbed as those at Danau Sentarum, the loss of each breeding-size crocodile (e.g. to a jermal) has an even greater disproportionate effect on the viability of those populations.
Giesen (1987) found "net density" to be "very great in some areas" and described pukat densities as "extremely high" in Danau Sekawi. During the 1994 study the author found that the same situation prevailed. While Sungai Kenalang was chock full of small gill nets and floated hooks and trot lines, 9 active jermal were encountered between Danau Sekawi and Sekolat along Sungai Belitung. Eight of these were located upstream from the confluence of Sungai Bekuan which is roughly equivalent to 1 jermal erected every 1.2 km (over approximately 9.5 km). Jermals were also observed on Sungai Embaluh Leboyan especially below Nanga Leboyan and above Semanggit. Likewise a jermal was also seen in the vicinity of Lubuk Pengael on Sungai Sumpak. Sungai Tawang was very heavily fished, as exemplified by an assortment of anchoring poles, trot lines, and nets and traps with empang running its entire length.
Giesen (1987) reported that suspected intentional burning was consuming "large areas of low-lying inundated forest" in the Danau Sentarum area, and cited literature mentioning area fires in the latter half of the previous century. More recently, Luttrell (1994) investigated contemporary forest burning at D. Sentarum (April-June 1994) and compiled a preliminary list of major burn events over the last 30 years based on informant interviews. Both authors mentioned the opening up of habitat for fishing, as one of the reasons behind the burning. Luttrell listed 20 locations as key fire sites. Like her, this author found deliberate forest burning to be extremely prevalent, encountering smoke and fire on every outing. Two on-foot surveys also abutted previous burn sites. A detailed study on burning at Danau Sentarum is provided by Dennis et al. (this volume). A return to Kalimantan Barat in 1996 by the author (Ross et al. 1998) did not include Danau Sentarum, but it was clear that burning has become all too common o n a massive scale. While this phenomenon was not examined per se in 1994, the author feels that this practice must be considered detrimental to crocodiles in the absence of information to the contrary.
Kottelat (1993) emphasized the singular importance of the forest as an exogenous nutrient source for the lakes of Danau Sentarum. This can be inferred from the almost total absence of submerged vegetation in the lakes. Extrapolating from other similar Southeast Asia locales, he also suggested that forest habitat was important for fish reproduction. Furthermore, in this study of fish, Kottelat pointed out that forest loss removes "shelters and sources of habitat complexity," causes a loss of food sources, increases water temperature thereby lowering dissolved oxygen capacity, and increases siltation destroying spawning grounds and larval habitats for some species. This has an obvious potential implication for the piscine component of crocodile diet.
But perhaps of more immediate concern is the potential direct impact that forest burning has on crocodile populations. With the opening up of more forest the likelihood of frequent human incursion increases. This might simply force crocodiles to seek other more secluded areas or it might result in an increase in the level of accidental net mortality through increased fishing opportunity. Finally, by virtue of its scale and extent, burning of forest habitats probably has already destroyed some crocodile nesting areas. This obviously can have a drastic impact on crocodile populations.
Giesen (1987) discussed fish poisons and the history of their use in West Kalimantan and the Danau Sentarum area. In Giesen's account, Schadee (1913) is cited as reporting that "most monitor lizards, turtles and crocodiles are killed as well during the [poisoning] operations." This author in fact observed monitor lizards eating dead fish (from among many) perhaps killed by poison along Sungai Tawang (Frazier 1994).
Two major fish kills occurred just before and during the author's brief visit and this testifies to another real threat to the crocodiles of Danau Sentarum. It is highly likely that both kills were the result of deliberate poisoning. Whatever the genesis and formula, it appears that fish poisoning is occurring regularly in the dry season in hulu sungai (upriver) locations. There are two immediate potential adverse effects for the crocodile: 1) the aforementioned secondary poisoning from consumption of contaminated prey and 2) site abandonment due to collapse of prey fish populations and/or irritation caused by the toxic agent. Such forced migration itself would put crocodiles at risk from other threats. One major poisoning event merits additional detail here.
In the first hours of 8 August 1994, the author's survey party returned to base camp to find Tekenang village alive with commotion (Frazier 1994). Local residents discovered that their entire stock of fish, was dead and dying inside keramba (fish cages) moored along the edge of Sungai Tawang. Later that afternoon, during further survey travel the party noted a great number of dead fish at Sumpak, but little mortality was observed at Nanga Pemerak upon passing (15:45) and upon return (17:15). However the author observed numerous large fish breaking the river's surface in gentle rolling arcs. Later the survey team returned to Nanga Pemerak from a foot survey sometime after 23:00. Here the party was met with a surreal scene where the river paralleling the village was completely carpeted with a seething mass of dying and dead fish.
News of this fish kill travelled fast throughout the Danau Sentarum area. A group of informants placed the blame for this event squarely on people living in hulu sungai locations. Many residents felt that this fish kill stemmed from use of a commercial piscicide or other industrial agent (rather than traditional tuba akar, "root poison") owing to its devastating efficacy.
Hunting crocodiles for hides had apparently abated at Danau Sentarum (as at 1994) owing to depressed world skin prices. (Ross et al. 1998 and Steubing et al. 1998 commented more recently on the relatively low prices for the lower quality hides of Tomistoma). No evidence whatsoever concerning current deliberate crocodile hunting was uncovered by Frazier (1994). However inadvertently captured/drowned crocodiles are sometimes skinned for hide sale. Small to medium-size crocodiles captured live from nets and traps are sometimes kept as pets or curiosities (pers. obs.; J. Aglionby pers. comm. 1994; W. Giesen pers. comm. 1994) or are sold to traders (J. Aglionby in litt. 1994; Giesen, 1987).
Should skin trading become profitable once again, hunting of crocodiles in the already heavily disturbed environs of Danau Sentarum will represent a significant impact to local and perhaps regional crocodile populations.
A paucity of scientific data exists on all aspects of crocodiles at Danau Sentarum. This dearth of knowledge extends not only to ecological and population aspects, but also to the taxonomic status of its crocodiles. It is clear however, that dry season phenomena of natural and anthropogenic origin operate in a perverse synergy to adversely affect the crocodiles that are there. The local population of seasonal fisherfolk explodes as the waters of Danau Sentarum recede, and fish density increases. The use of indiscriminate fish capturing methods, especially jermals and poisoning, directly and at least indirectly in the case of poisoning, have an adverse impact on crocodiles. Forest burning without doubt exerts negative pressure on crocodiles, by opening secluded habitats to human exploitation and by probably destroying nesting habitat. Danau Sentarum would on its surface, appear eminently suitable crocodile habitat given its plentiful sources of food and its semi-remote character. However the manifold large-sca le anthropogenic impacts described above are likely significantly depressing this natural potential. Danau Sentarum National Park, irrespective of its park status is in fact a multiple use area. One of these uses is obviously conservation. For all land uses to coexist harmoniously, they should be sustainable. However, jermal fishing, use of industrial piscicides or surrogate chemicals, and forest burning are not sustainable and will ultimately adversely affect all land uses, in one time scale or another.
Table 1 Status of Borneo's known extant crocodiles. IUCN Species Red List CSG CITES 2000 1998 2000 Crocodylus porosus LRlc H I/II * Estuarine crocodile Crocodylus siamensis CR A lac H + I Siamese crocodile Tomistoma schlegelii EN C1 H + I False gharial/gavial [Descriptions of the various codes used herein refer only to the data presented in Table 1. For complete descriptions of the RL, CSG and CITES categories, see the original sources]. RED LIST CR=critically endangered; A lac= An observed, estimated, inferred or suspected population reduction of at least 80% over the last 10 years or three generations, whichever is the longer, based on (and specifying) direct observation and a decline in area of occupancy, extent of occurrence and/or quality of habitat. E=endangered; C1=Population estimated to number less than 2500 mature individuals and an estimated continuing decline of at least 20% within five years or two generations, whichever is longer. LRlc=[globally] Low risk, lest concern. CSG H-high need for wild population recovery. H + = highest need for wild population recovery. CITES I=Appendix I, II * = Appendix IL under annual quota criterion (for Indonesia, 6000 skins in 2000) Table 2 List of surveys and field travel to survey destinations, with observations. No. Starting Survey Course (or Travel crocodile km Date Route and Destination) observ. 1 06.08.94 Daytime travel from the mouth of nil 49.6 sungai Tawang to Bukit Tekenang (en route to base camp) 2a 07.08.94 Danau Genting to mouth of Sungai 2 9.3 Pengembung (return only; on foot) tracks 2b 08.08.94 Pengembung to Bukit Tekenang nil 6.4 (Sungai Tawang) 3a 08.08.94 Lubuk Penyerang Burung and Hulu nil (8?) Sungai Pemerak to Nanga Pemerak (return only; on foot) 3b 08.08.94 Nanga Pemerak to Bukit Tekenang nil 13.1 (Sungai Tawang) 4a 09.08.94 Daytime travel Bukit Tekenang nil 26.2 (Sungai Tawang) to Sekolat (Sungai Belitung) 4b 09.08.94 Sekolat to Danan Sekawi (Sungai nil 13.5 Belitung) 4c 09.08.94 Danau Sekawi (Sungai Belitung) to nil 39.7 Bukit Tekenang (Sungai Tawang) 5 10.08.94 Daytime travel from Bukit Tekenang nil 29.3 to Semanggit (Sungai Embaluh Leboyan) 6a 10.08.94 Kerinan Suakuri (off Sungai Embaluh 1? (8?) Leboyan in the vicinity of Bukit (track) Semujan; on foot) 6b 11.08.94 Kerinan Suakuri (off Sungai Leboyan (8?) in the vicinity of Bukit Semujan; on foot) daytime return visit 7 11.08.94 Temperau (Sungai Embaluh Leboyan) 6 [all 6 51.9 to Bukit Tekenang (Sungai Tawang) within] 7.6] 8a 12.08.94 Daytime travel from Bukit Tekenang nil 36.3 to Pulau Majang (Sungai Tawang, (+6?) Sungai Kenelang and on foot) 8b 13.08.94 Daytime travel from Pulau Majang to nil 70.1 Temukup (on foot, Sungai Kenelang, Sungai Tawang and Sungai Sumpak) 9 13.08.94 Temukup to Sumpak (Sungai Sumpak) nil 42.0
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