Water Hyacinth and Overfishing Problems on an Indonesian Lake

Water hyacinths, Eichhornia crassipes, and floating grass islands have been a serious problem for more than 50 years on Rawa Pening, a 2500-ha lake in central Java. Fish harvest increased with control of the plants in 1932-42 and 1951-56 but decreased as plants again increased in abundance. Control measures since 1962 have maintained open water but have not eliminated the problem. Hydrilla verticillata becomes abundant as the surface is cleared of water hyacinths. Hydrilla shelters Macrobrachium shrimp and other aquatic organisms, important to fish, and also is harvested for pig food. Annual fish yield dropped from about 530 kg/ha in 1972 to 178 in 1975 and to 18 kg/ha in 1978. Most species of fish averaged smaller in 1978 than in 1975. Overfishing is indicated, but the low catches are probably the result of other factors also