Life in Borneo's waters
Such incidents have come at a cost. There is evidence that some populations of freshwater fish have declined over the last few decades in many Borneo rivers, such as in the Barito. Lakes which are bearing the brunt of logging and conversion of forests and new settlements.
However, new species of freshwater fish continue to be discovered.
The major waterways of BorneoBorneo’s major rivers are a vivid example of how dynamic the island’s ecosystems can be.
Some of the major waterways that stand out in the Borneo landscape include the Kapuas river (in Kalimantan, Indonesian Borneo), which is 1,143 km,only slightly shorter than the Rhine. The Kapuas flows to the west coast, draining some 2/3 of West Kalimantan province - a watershed of 100,000 km2 (an area slightly larger than Hungary).
Other major rivers in Kalimantan include the Barito (900 km), which flows south, and the Mahakam (775 km), which empties into the Makassar Strait to the east of Borneo.
In Borneo’s inland basins and in the lowlands, several major rivers form extensive lake systems. The Mahakam, Barito, Kapuas and Baram rivers form oxbow and seasonal lakes in their lowland reaches, some of which are of economic importance for fisheries.
The journeys of Borneo’s riversLike sponges, Borneo’s tropical rainforests regulate the quantity of water that drains into streams in the uplands. These merge with others to form larger waterways, which change water volume depending on the seasons. During the rainy season, water levels can change very fast. For example, much of the Kapuas can rise 10-12 metres overnight during heavy rainfall.
From trickles to roaring torrents
In these streams, evolution has endowed invertebrates with useful adaptations such as flattened bodies and the presence of hookers, to avoid being swept away by the strong current. One fish genus in particular has evolved a handy ‘tool’ - the sucker fish (Gastromyzon species) has suckers to attach itself to the bottom of fast-flowing waters.
Compared to montane streams, upland streams havemore activity. Fish found here are often strong swimmers, with well-adapted streamlined bodies. There may be 20 species each of dragonflies, mayflies and stoneflies.
Borneo’s freshwater wildlife
Seemingly insignificant factors can affect species distribution. For example, the location of freshwater prawns and crayfish can be determined by how big the sediment particles are.
More obvious factors also define the ecological make-up of a particular freshwater habitat. Plant and invertebrate food supply, shade provided by overhanging vegetation and the presence or absence of riffles (shallower, faster moving sections of a stream) and pools are factors that are all considered when a species seeks a home.
In particular, top predators such as the false gharial (Tomistoma schlegelii) and the estuarine crocodile (Crocodylus porosus), river turtles, monitor lizards and otters shape the ecology of other species lower in the food web.
The endangered otter civet (Cynogale bennettii) is a very competent fisher in forest rivers. Webbed feet, and valve like nostrils and ears allow it to swim freely in search of fish, molluscs and crayfish.