Zooplankton, zooplanktivorous fish and their interactions in Southeast Asian waterbodies with special reference to Sri Lanka: a review

Many countries in Southeast Asia have no natural lakes but reservoirs, flood-plains and rivers. The reservoirs are mainly inhabited by riverine fish species and introduced pelagic exotics. Benthivores, herbivores and detritivores dominate, whereas zooplanktivores are relatively rare (generally 4.6-9.5% of the fish community). Therefore, zooplanktivorours fish are not dominant in most Sri Lankan reservoirs and play only a minor role in the food web. In the Southeast Asian region, only small, often pelagic, zooplanktivorous fish species occur. They are either of riverine or marine origin. Observations were made that only six predominantly zooplanktivorous species viz a cyprinid from riverine origin Rasbora daniconius, a half beak from marine origin Hemiramphus limbatus, the glass perchlet Ambassis urotaenia and three freshwater clupeids from marine origin ( Clupeichthys aesarnensis, Ehirava fluviatilis, Sardinella tawilis ) inhabit the reservoirs in Southeast Asian countries such as Sri Lanka and Thailand and a lake in the Philippines. The highest percentages of zooplanktivory in the tropical region are observed in waterbodies which contain zooplanktivores of marine origin. These small pelagics are very productive, i.e., have relative high Annual Production/Biomass (P/B) ratios. P/B ratios of the small Southeast Asian pelagics ranged generally from 3-6, small clupeids showed the highest P/B ratios. Of the riverine species, R. daniconius is the most successful; it is a common species in the Sri Lankan reservoirs. This species is, however, not able to catch the small cladocerans and the cyclopoid copepods efficiently and fails completely to catch the fast moving calanoid copepods. Instead of feeding on larger zooplankton, they often feed on adult winged midges which are floating on the water surface. H. limbatus is not abundant in Sri Lankan reservoirs and its distribution is limited where only occasionally its densities are high enough to permit a profitable fisheries. Although the freshwater clupeids seem to be superior in their efficiency of collecting food organisms, their distribution is often limited. Despite their ability to build up stable and large populations in a handful of water bodies in the Philippines and Sri Lanka, they have a problem in dispersing into other freshwater lakes and reservoirs. In Thailand, the situation is different because C. aesarnensis is well adapted to riverine conditions and, therefore, present in all reservoirs with inflowing riverine habitats, but in most cases these stocks are still unexploited.