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Session 1: Impact of stocking and introductions on the environment


3. Stocking and introduction of non-native species are widespread management techniques. Such manipulations of fish populations frequently aim at improving the quality and diversity of angling. There has been increasing demand in the United Kingdom for large non-native fish while stocking has been developed in Spain to support trout (Salmo trutta) populations in the face of increasing angling intensity. Creation of new angling opportunities, increased variety, different challenges and larger trophy fish are expected benefits of stocking and introductions in Newfoundland and Labrador.

4. Another objective of manipulation can be to influence food webs in order to restore water quality and ecosystem health.

5. Some introductions are also made illegally because they are not authorized because of policy constraints although angling demand is high and commercial benefits override the small penalties incurred. It is also likely that some established non-native populations could be attributed to discarded or escaped live-bait used by anglers during recreational angling.


6. Species-poor fish communities, such as those of European freshwater fish, favour the establishment of non-native populations and are vulnerable to invaders. The potential for non-native species to become established in European waters is confirmed by the importance of these species in fish communities of French reservoirs. Direct assessment of their impact is seldom possible but the local increase in species richness is likely to increase niche overlap, and therefore inter-specific interactive relationships. Interactions could result from competition with indigenous fish (food, cover or spawning sites) or direct predation on native populations.

7. Competitive impacts of roach (Rutilus rutilus) on vendace (Coregonus albula) and perch (Perca fluviatilis) on Arctic charr (Salvelinus alpinus) were assessed in lakes of the English Lake District on the basis of spatial distribution and diet analysis. It was concluded that roach did not constitute a competitive threat to native species but this conclusion may need to be revised following recent increases in abundance of the species. In the same area, predation by ruffe (Gymnocephalus cernuus) and dace (Leuciscus leuciscus) on vendace eggs or larvae are currently being analysed. Under some situations, the colonization of British waters by zander (Stizostedion lucioperca) can more or less annihilate the juvenile components of prey population. In Newfoundland waters, predation by brown and rainbow trout has impacted forage fish populations and also affected species diversity and invertebrate communities. Moreover, these exotic salmonids appear to have the competitive advantage over the native brook trout in some disturbed systems.

8. Genetic effects of introductions and stocking are also significant. Goldfish (Carassius auratus) introduced in rivers and lakes in southern England hybridize and compete directly with native crucian carp (Carassius carassius). In Spain, where rivers have been highly stocked with non-native trout (Salmo trutta), introgression of foreign genes was measured. Introgression and reduction of the number or range of native populations has also been described in natural populations of salmonids in Newfoundland.

9. Co-introduction of parasites is another risk associated with stocking or introduction of fish. This has proved especially severe in the case of Anguillicola crassus, a parasite of eel (Anguilla anguilla) that has become widespread in Europe.

10. Despite these negative aspects, successful introductions of whitefish (Coregonus lavaretus) and arctic charr (Salvelinus alpinus) have been made in large alpine lakes. No detrimental effect was registered while the introduced species now support sustainable fisheries.

11. Fish introductions should not be systematically assumed to be negative but risks are higher with exotic species than those made with transplants.

12. There is often insufficient information on impacts of introductions and stocking programmes particularly because there is no network for monitoring. Causal relationships are, therefore, difficult to distinguish from indirect correlation with environmental parameters. Managers have a double role in that they have to maintain, improve and develop fishing at the same time as having to protect the environment. Such a situation can lead to conflicts.

13. Nevertheless, the threat posed by fish introductions is particularly insidious because restoration management tools are not available. Therefore, the precautionary approach should be adopted with regard to the introduction of species, particularly in the case of non-native fishes.

14. In the English Lake District, preventative management of communities is based on new legislation banning the use of live and dead freshwater fish, salmonids and eel for bait. At a national scale, management solutions proposed by the Environment Agency can be presented in five points: education through articles in the angling press, Web site, etc., legislation, policies, enforcement and audit in order to check the success or failure of management operations.

15. Strong policy and guidelines have also to be developed in order to protect wild fisheries in Spain, and in Canada in Newfoundland and Labrador.

16. Increasing scientific understanding of fish and their habitat will be required to support legislation. More research is needed on behaviour and the mechanism involved in the spread of non-native species. There is also a need for more information on the ecological and dynamic impacts of stocking, the economic evaluation of inland fisheries and the impact of introduced parasites on fish stocks.

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