Investigations on the Ecology and Fish Fauna of the Mahakam River in East Kalimantan (Borneo), Indonesia

Water quality was determined on the Mahakam River floodplain from 1980 to 1987. Five water-body types were distinguished, i.e. slow-flowing and faster-flowing rivers, lakes, small waterbodies and swamps. Water quality in large rivers was stable in the short but not the long term. In all other ecotypes variability was higher, with marked fluctuations being recorded in small waterbodies. A total of 147 indigenous freshwater fish species have so far been identified from the Mahakam. The zoogeographical consequences are discussed. Migratory patterns were inferred for four fish species, Helostoma temmincki, Leptobarbus hoe-venii, Puntius schwanenfeldii and Thynnichthys vaillanti. Juvenile H. temmincki (< 3 cm SL) migrate at the surface in schools of 40-80 individuals at speeds of 0.48-1.08 km · h⁻¹, travelling 3.8-8.6 km · h⁻¹. Fecundities for P. schwanenfeldii (7,900–16,000 eggs), T. vaillanti (2,900–24,000 eggs) and H. temmincki (6,200–9,100 eggs) were determined.