The introduction of exotic fish in Sri Lanka with special reference to Tilapia
... In rice fields, T. pectoralis controls pest larvae and algae (Fernando 1989), and is believed to be beneficial for rice plants (Ardiwinata 1957;de Zylva 1999) because through grazing on aufwusch algae, it contributes to the release of inorganic nutrients, thereby enriching the habitat for primary producers (Heckman 1979). ...
... There is a local trade and industry for drying and salting this fish and within Southeast Asia T. pectoralis is an important export product (Soong 1948;Tan, Chong, Sier & Moulton 1973;Davidson 1975;Frimodt 1995;Yoonpundh & Little 1997;FAO 1992) because of its tasty meat (Khoa & Huong 1993). T. pectoralis controls pest larvae and algae (Fernando 1989), and is believed to be beneficial for rice plants (Ardiwinata 1957;Cagauan 1991;de Zylva 1999) because through grazing on aufwusch algae, this species contributes to the release of inorganic nutrients, thereby enriching the habitat for primary producers (Heckman 1979). It's only competitors for the aufwusch are snails, so it can be raised along with other useful fish species without decreasing their yield (Heckman 1979). ...
In this thesis, the interactions between fish and rice in concurrent rice-fish systems are discussed. The overall objective was to increase the economic performance and viability of rice-fish farming. The following specific objectives were selected: (1) investigate whether fish have any effect on the rice yield, (2) investigate whether fish have any effect on the rice arthropod pests, (3) get a better understanding of the water ecology in the rice-fish system, in order to (4) evaluate four fish species: Barbodes gonionotus (Bleeker), Cyprinus carpio L., Oreochromis niloticus (L.), and Trichogaster pectoralis Regan.
The rice-fish system studied is a concurrent system in which rice and fish are grown at the same time. Fish species most often cultured in these systems are B. gonionotus, C. carpio and O. niloticus, usually grown in polyculture. Such systems are said to have many advantages over rice monoculture: (1) increase in rice yield per unit area, (2) control of arthropod pests, (3) control of weeds, and (4) better nutrient composition of oxidized surface soil. However, if all this is true, why, then, are farmers not eager to invest in rice-fish culture?
This thesis consists of 3 parts. Part A (chapter 1 to 4) treats the specific objectives 1 and 2. From the data presented in chapter 1 it was clear that rice-fish culture did not increase the rice yield per unit area as compared to rice monoculture. Increasing the water level resulted in lowered rice yields. As a consequence, farmers swapping from rice monoculture to rice-fish culture should expect a decrease in rice yield per unit area. High fish yields – and fruit and vegetable production on the dikes surrounding the system – should, therefore, compensate for the loss in rice yield to guarantee a profitable rice-fish enterprise. In the chapters 2, 3 and 4 the topic of pest control was treated. It is widely accepted – although with little prove – that fish stocked in rice fields can control rice pests. The results presented in chapter 2 & 4 confirm that fish are able to control rice caseworm Nymphula depunctalis in rice fields. However, control of N. depunctalis by fish should not be overrated, as N. depunctalis does not cause much yield reduction. Except for N. depunctalis, fish had no impact on other arthropod pests like rice leafhoppers, rice planthoppers and Cnaphalocrocis medinalis (chapter 3 & 4). This is not surprising as fish are general predators that have no direct contact with the arthropod pests they are supposed to control. Nearly all important rice arthropod pests live on the rice plant where fish cannot reach them. It was concluded that introduction of fish in rice fields could not be regarded as a fully-fledged pest control measure. However, as rice-fish farmers are inclined to spray fewer pesticides to avoid killing the fish crop, introduction of fish in rice fields could be regarded as complementary with integrated pest management (IPM) programs.
Part B (chapters 5 & 6) treats specific objective 3. Rice-fish systems are characterized by intensive rice cropping and extensive fish cropping. This means that fish largely depend on primary production in the aquatic system for their growth and survival. Therefore, a thorough understanding of the floodwater ecology in both trench and field of the rice-fish system is necessary. From the results presented, it was clear that the rice seeding rate had a considerable impact on the aquatic ecology of the floodwater of field and trench. Higher seeding rates went together with higher rice biomass and decreased ammonium
availability, resulting in reduced aquatic photosynthesis, lower pH and dissolved oxygen values in the afternoon. Presence of fish also had a considerable impact on the floodwater ecology of trench and field. In the field floodwater, ammonium concentrations quadrupled in the presence of fish, while ortho-phosphate levels decreased. Rice biomass and water turbidity seriously increased in the presence of fish, hence reducing light penetration and aquatic photosynthesis in the floodwater. All this has serious implications towards the cropping of fish in rice fields. It is clear that trench and field have to be considered as two different habitats of the rice-fish field. The field should be seen as a fish feeding bank as it
can provide food to fish. The trench enables and supports fish growth when the conditions in the field are far from optimal. The trench provides phyto- and zooplankton for plankton feeding fish, and benthic organisms for benthic feeders. Through fertilization and manuring a farmer can improve the primary production in the trench.
PART C (chapter 7 to 10) addresses specific objective 4. While discussing the performance of B. gonionotus, C. carpio, O. niloticus and T. pectoralis it became clear that the rice-fish system was easily overloaded, resulting in intraspecific competition for all tested species as well as interspecific competition between B. gonionotus & O. niloticus, C. carpio & O. niloticus, and B. gonionotus & T. pectoralis. Fish mainly competed for food. Lowering the fish stocking densities, and manuring or fertilizing the trench were found to be appropriate methodologies to remedy this. Furthermore, farmers should try to prevent wild fish from entering the rice field. The data presented in PART C clearly show that there are opportunities for fish yield increase.
It was concluded that rice-fish culture deserves and needs further attention and research.
... The expansion of inland fisheries in Sri Lanka following the establishment of tilapias, mainly Oreochromis mossambicus (Peters), is an example of the beneficial effects of exotics on human livelihoods. The Sri Lankan reservoir fisheries are among the most productive in the world, and there is no evidence that the introduction of tilapias has adversely affected indigenous species (Fernando, 1991(Fernando, , 2000de Zylva, 1999;Fernando et al., 2000). Two native species of Labeo have declined in Sri Lankan reservoirs since the 1950s, but this effect has been attributed to stocking of exotic (and non-breeding) major carps rather than to O. mossambicus (Fernando, 1991;de Silva and de Silva, 1994). ...
1. Development of effective conservation strategies for freshwater biodiversity must take account of the trade-off between species preservation and human use of ecosystem goods and services. The latter cannot be prevented, and attempts to manage ecosystems that focus solely on maximizing biodiversity will fail.
2. A compromise position of management for ecosystem functioning and human livelihoods — rather than preservation of every species — will provide a better basis for biodiversity conservation. This has implications for the management of exotic species.
3. There are some situations, in lentic habitats in Sri Lanka, for example, where the establishment of exotics has increased fishery yields without apparent detriment to native biodiversity. The Sepik River in Papua New Guinea provides another illustration of potential compromises between human livelihoods and biodiversity conservation.
4. The Sepik supports a relatively unproductive fishery. Two fish stocking projects (in 1987–93 and 1993–97), representing a partnership between the Papua New Guinea Government and the United Nations (UNDP/FAO), led to the introduction of a suite of exotic fish into the Sepik. Species were selected on the basis of their potential to occupy niches not filled by native fish. Unfortunately, the outcomes of these introductions are poorly documented, although there is preliminary evidence both of increased human use of exotic fish as well as declines of some native species associated with the spread of exotics.
5. Better understanding of the results of the Sepik fish introductions is important, because the pressures of burgeoning human populations in most of tropical Asia make it impossible to preserve near-pristine environments such as the Sepik. While attempts to conserve natural or near-natural systems must remain a priority, there is a need to develop strategies for the management of damaged or degraded ecosystems, which may contain exotics, with the aim of maintaining ecosystem functioning and, if possible, maximizing the persistence of native biodiversity. Copyright
... There is a local trade and industry for drying and salting this ®sh and, within South-east Asia, T. pectoralis is an important export product (Soong 1948;Tan, Chong, Sier & Moulton 1973;Davidson 1975;Frimodt 1995;Yoonpundh & Little 1997;FAO 1992) because of its tasty meat (Khoa & Huong 1993). T. pectoralis controls pest larvae and algae (Fernando 1989), and is believed to be bene®cial for rice plants (Ardiwinata 1957;Cagauan 1991;de Zylva 1999) because, through grazing on aufwusch algae, this species contributes to the release of inorganic nutrients, thereby enriching the habitat for primary producers (Heckman 1979). The only competitors for the aufwusch are snails, so it can be raised along with other useful ®sh species without decreasing their yield (Heckman 1979). ...
Trichogaster pectoralis Regan is often cultured successfully in rice fields in South-east Asia. However, since the advent of high-yielding rice varieties and intensive rice cropping, several reports suggest poorer growth performance of the species. In this paper, we investigated the growth performance of T. pectoralis in rice–fish systems characterized by intensive rice culture and extensive to semi-intensive polyculture of Oreochromis niloticus (L.), Cyprinus carpio L. and Barbodes gonionotus (Bleeker). In these systems, T. pectoralis recorded negative net productions. This was mainly the result of a negative specific growth rate (SGR) when fish had to stay in the trench (−0.519% body weight day−1) or when fish had free access to the rice crop (−0.081). Only when harvested rice plants were allowed to generate new tillers (ratooning) did T. pectoralis perform slightly better (0.453% body weight day−1). The SGR of T. pectoralis was low owing to a lack of food in intensive rice systems. The other species, all with SGR values higher than 1% body weight day−1, clearly had a competitive advantage over T. pectoralis. We concluded that intensification of rice cropping did indeed result in poor T. pectoralis growth. Successful T. pectoralis culture is only possible in areas of traditional rice cropping.
This datasheet on Trichogaster pectoralis covers Identity, Overview, Associated Diseases, Pests or Pathogens, Distribution, Dispersal, Biology & Ecology, Environmental Requirements, Impacts, Uses, Management, Genetics and Breeding, Economics, Further Information.
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