8210 - Laserfiche WebLink

Home Browse Search

Direct Control of Fauna Table 20.1 Impacts of hatchery-reared sportfish on wild fish populations (from Mamell 1986 Introduction of pathogens and parasites Hybridization, introgression, intergradation Trophic alterations: ,Predation, cannibalism Food competition Spatial alterations Territorial shifts, altered dominance hierarchies Stress and aggression Altered movements, temporal changes in activities Altered growth and survival characteristics Altered growth rates 'Changes in natural mortality 'Changes in vulnerability to angling Displacement, replacement, extinction native stock by adversely affecting the genetic diversity of the native gene pools. The potential disadvantages of this practice range from minor interruptions in resident fish communities to drastic changes in the aquatic systems including major genetic shifts and even extinction (Table 20.1 ~ ,The debate about this issue is causing concern amongst fisheries professionals in the USA. Some scientists see the debate as counterproductive and consuming so much energy that political and economic decisions are being made without ad- equate biological input, because scientists are spending so much time debating hatcheries and their effects on wild stocks (Martin et al 1992. Again, the manager must balance many aspects of fish production and riverine management be- fore deciding if supplemental stocking is worth the risk. In some cases environmental changes have precluded natural reproduction of the native fish so that the remnant population must be sustained by plantings of hatchery fish. Moreover, in an economic and social climate where fish are essential for human survival, planting hatchery fish over native stocks must happen when no viable alternatives exist to provide the needed protein. Another risk, potentially lethal, is also associ- ated with intensive aquaculture especially in subtropical climates. Scholtissek (1992 has postu- lated that the mixing of birds, swine and fish farming sets in motion a process whereby swine 391 act as mixing vessels in which lethal new forms of human Type A influenza viruses are produced. In summary, stocking can be an effective tool in protecting and rehabilitating existing native fish communities, and can provide increased social and economic benefits. However, stocking fish species which are more adaptable to surviving in degraded aquatic ecosystems, without parallel programmes to restore the degraded systems, and supplemental stocking that degrades existing native populations are two practices which are incompatible with the objective of providing sus- tainablebenefits from healthy aquatic ecosystems. Stocking strategies Several strategies are available to fisheries man- agersusing stocking as a management tool. These include selection of species, strains and life stages which are best suited to meet management objec- tives. An ecosystem approach to fisheries man- agement requires that fish stocking only be used as a tool to achieve balanced fish com- munities. Fish community objectives should, therefore, form the basis for identifying stocking requirements. Fisheries managers considering a stocking pro- gramme should first evaluate the possibility of using other management options. Increased en- forcement, habitat rehabilitation, and harvest control techniques are just three examples of other management tools which are often cheaper, less damaging to native stocks, and more effective than stocking. Species selection Fish culture has played a major role in the intro- duction or range extension of fish species into waters in which the species did not occui na- turally. While some of these introductions have created naturally reproducing populations which are generally deemed as beneficial to society, others have resulted in irreversible damage to native fish communities. For example, rainbow trout (Oncorynchus mykissj, introduced to tri- butaries to the Great Lakes, have provided very large, self-sustaining fisheries, but in the process have significantly reduced the range and numbers