CBA PRACTICES IN RELATION TO DISEASE, ENVIRONMENT AND BIODIVERSITY

CBA practices need to be considered in relation to disease transfer and environmental impacts including on species diversity. Although these issues are also relevant to HBA, there are certain considerations specifically or indirectly pertinent to CBA practices because some of the impacts on biodiversity are negative (Beveridge, Ross and Kelly, 1994).

There are no clear positive impacts on biodiversity yet noted in relation to CBA. Clearly, both CBA and HBA practices are associated with a number of problems such as water pollution and environmental damage, wich are exacerbated where CBA is extensively practised simply because CBA means higher volumes of animal under culture (Tables 3 and 4). With the extensive development of CBA practices and increasing transport and trade of wild seed both regionally and globally, problems of disease and genetic pollution associated with transfers and escapes of wild seeds may be a matter for concern. Below are some examples that illustrate the problem.
• In the case of Epinephelus groupers, a Vibrio strain in Epinephelus bleekeri was transferred from Thailand to China Hong Kong SAR in wild caught seeds and resulted in the elimination of almost all cultured groupers in China Hong Kong SAR in the late 1990s, a serious blow to the industry at the time which took several years to recover (Sadovy, 2000).

• Environmental impacts from CBA practices need to be addressed. Low environmental impacts from CBA practice for Anguilla eels are assumed because artificial feed rather than natural feed is provided and because land requirement are low for intensive culture practices in both Asia and Europe (Ottolenghi et al., 2004). However, unregulated use of groundwater for eel culture in China and Taiwan Province of China (Taiwan PC) has caused severe land subsidence (Chen et al., 2006). Moreover, the high demand of fish-meal for eel feed and the use of chemicals for disease treatment and prevention during eel culture in China need to be addressed.
• All crustaceans and a significant number of finfishes in CBA are carnivorous and require feed input that includes wild-caught fish (i.e. fresh feed input). While these are also relevant to HBA species, the extensive use for CBA, especially for carnivorous species, can add significantly to the problems that such practices cause. Uneaten feed, faecal and urinary wastes may have negative environmental impacts and lead to local water quality degradation and sediment accumulation (Wu, 1995). For example, in a tuna (Thunnus thynnus) fattening culture farm in the Mediterranean Sea producing 800 tonnes of tuna a year, the use of defrosted fish was shown to affect the benthic environment over an area 400 m diameter, an impact considerably greater than other fish culture practices in the same area (Vita and Marin, 2007). Study of carrying capacity of the local environment (i.e. the maximum numbers of animals or biomass that can be supported by a given ecosystem for a given time) is particularly important for aquaculture practices of this sort which, although they can produce a valued product, can also cause more wide-ranging negative impacts on the natural environment.
• Possible adverse biodiversity impacts from CBA practices in relation to global Anguilla eel seed trade are of interest. The introduced European eels (for CBA and restocking proposes in Japan) have been found free in Japanese natural waters in recent years with the silver stage eels migrating downstream at the same time as native Japanese eels form downstream migrations (Miyai et al., 2004). The potential impacts of inbreeding between the two species and on local aquatic biodiversity should be examined, since eels are important predators in freshwater benthic habitats.
• The HBA culture of a number of tropical marine fish species will continue to depend to some degree on wild broodstock to maintain genetic diversity, for feed, and, in some cases, continued contribution of wild seed. In many cases, not only is the target species removed but also a heavy bycatch component. For instance, for 1 kilogram of shrimp post-larvae collected, an estimated 10 kg of larvae and juveniles of other species may be discarded (Beveridge, Ross and Kelly, 1994). More extreme ratios are likely in certain fisheries for grouper juveniles (Mous et
24 Capture-based aquaculture: global overview
al., 2006). Heavy exploitation for target species can mean extremely high levels of associated bycatch with the potential for negative impacts on biodiversity. More generally, high densities of farmed fish and food attract predators that could, conceivably have an impact on local species, while the heavy demand for wild fish feed and fishmeal, intensified by CBA, is exerting growing pressures on such species; in extreme cases this could affect local biological diversity (Beveridge, Ross and Kelly, 1994; Naylor et al., 2000).