Shrimp Farming and the Environment - Library

(Raa 1996) indicates that it is possible to stimulate a shrimp’s immune system to withstand certaindiseases.Harvest of broodstock and wild post-larvaeMany farmers in South and Central America, Bangladesh, and India still depend on wild-caught postlarvae,usually caught by local fishermen. Some shrimp farmers prefer wild-caught PL because theybelieve that these are more resistant to disease, and in general more robust, than artificially producedPL.The intensive collection of wild PL may lead to a decline in wild stocks. If this happens, the same localfishermen who collect wild PL are the ones most likely to suffer. However, the impact of collectingwild broodstock and post-larvae is controversial. Some authors report that intensive seed-stock fishinghas reduced the capture of mature shrimp and other species (Clay 1996), while others claim that thereis no convincing evidence that shrimp farming has depleted the native shrimp in any country (Boyd1997).Using wild broodstock or post-larvae makes it difficult to control their disease status. By comparison,the Norwegian salmon industry has succeeded in reducing disease outbreaks by controlling the healthstatus of broodstock, eggs, and smolts, as well as by developing and using effective vaccines.Dependence on wild seed is therefore undesirable in the longer term. However, the seed captureindustry provides income and employment for poor coastal people in many parts of the world, and anystrategy to promote replacement with hatchery-reared seed must take this into account.Introduction of exotic speciesIn general, introduction of exotic or non-native species into an area is considered undesirable becauseof the risk of competition with native species and because non-natives may transfer pathogens andparasites to which native organisms are not adapted. In a number of countries, this threat is consideredso serious that introduction of non-native species is prohibited by law, for example in Norway andsome other European countries.While it is documented that importing living non-native shrimp has introduced new pathogens (importof P. monodon to Latin America brought White Spot Virus), there does not appear to be anydocumentation of uncontrolled proliferation of new shrimp species resulting from importation. Thespread of new diseases and uncontrolled reproduction of new species should be regarded as ecologicalrisks unless it is firmly demonstrated that no harmful effects on native shrimp or other populations willresult from introduction of a non-native species. In general, the export and import of non-native shrimpacross borders and continents should be discouraged and international protocols strictly followed.Abandonment of pondsSignificant areas in some of the most important producer countries have been abandoned by shrimpfarmers. Usually, disease has made production unprofitable in these locations, so the farmers have beenforced to quit. In many cases, these ponds will be put into shrimp production again, as has happened inThailand and China, when disease incidence declines or when improvements in management practicesmake production profitable once again. Sometimes the resumed production intensity is lower and, insome instances, shrimp are grown in polyculture.In Thailand, the National Economic and Social Development Board reports that 24% of the shrimpfarms established in mangrove areas are abandoned after 2–4 years because of disease and productionproblems, and that these sites are unsuitable for other purposes such as agriculture (Anon. 1997b).Other estimates for pond life expectancy range from 5–15 years (Flaherty & Karnjanakesorn 1995). Ithas been suggested that the life span of shrimp culture ponds depends on the stocking density, food andfeeding regime, quality of the bottom soil, and water temperature (Matsusato 1993). In certaininstances of extensive shrimp culture, farm life spans of up to 50 years may be observed.33