5. Following the recommendation of the twelfth session of EIFAC, a Symposium on Habitat Modification and Freshwater Fisheries was held in the period immediately preceding the thirteenth session of EIFAC, from 23 to 25 May 1984. The list of participants is given in Appendix C. The Symposium was convened by Dr J. S. Alabaster. The proceedings of the Symposium will be edited by the convenor and it is proposed that they be published by Butterworths, United Kingdom. These will consist of papers presented at the Symposium and summary of discussions and general conclusions.
6. The papers presented to the Symposium, and the subsequent discussions, reinforced the importance of man-made aquatic habitat changes as a major factor which can have an adverse effect on fish stocks; the causes of such habitat changes, and possible remedial actions, are summarized in the following sections.
7. The effect of damming and canalization of large areas was exemplified by the situations on the lower Danube and the Vistula and by the review of work on the Columbia river. Effects experienced on a smaller system in England were also described. In both larger river systems habitat modifications are reflected in changes in commercial fisheries.
8. Considering migratory fish, the dams on the Danube (Iron Gates I and II) affected sturgeon adversely but not the shad (Alosa sp.) which spawns downstream. Problems with dams affecting the upstream migration of shad were, however, mentioned for rivers in France (Rhône, Garonne and Loire). Dams on some tributaries of the Vistula were considered responsible for the total elimination of Atlantic salmon from the system and the decline of sea trout and Vimba vimba. Satisfactory fish passes for shad are not yet available, although the appropriate technology for salmonids is well documented including the use of tunnels.
9. With regard to non-migratory fish, profound changes in catch composition have occurred in the Danube, including the delta. Fish ponds have been constructed as compensatory measures in the Danube and thus the total fish production was maintained at a high level.
10. The creation of reservoirs was generally found to be beneficial to non-migratory species, although stocking with pike-perch and tench was only successful initially and the species fare less well in later years of the reservoirs existence. Theoretically, less pronounced changes in the abiotic habitat could probably be adapted to by indigenous local population. Where significant biotic changes occur, management policies designed to utilize the new conditions had the best theoretical chance of success, although where the original fish population was rich in species, it was difficult to predict the most successful course of action.
11. Land drainage, gravel extraction and other similar habitat changes cause reduction in the standing crop or yield of fish stocks or both. A diversity of remedial measures has been tested throughout their practical application in a number of the EIFAC Member States. Some of them have cost-benefit estimates, and this information may already be built into a scheme in the design stage. It was felt that a wider use of currently available measures for land drainage, gravel extraction and other habitat changes would be beneficial for habitat enhancement. For major engineering schemes, measures to minimize the impact on fisheries should be incorporated at the design stage. There is a need for improved information flow between individual EIFAC Member States, especially of the successful novel ameliorative measures of significant value for fish habitat preservation and enhancement. The importance of effective management of fishery resources after restoration works have been completed was stressed, and resource management of whole catchments was considered necessary to achieve this in most cases.
12. Crayfish were a special case because of their vulnerability to habitat disturbances such as dredging at times of low flow, high temperature and, particularly, during the moulting period. A variety of remedial measures were available including temporary removal and creation of artificial niches. In many European countries populations were severely affected by plague and one solution was to import disease-resistant species.
13. It was believed that careful measures can counterbalance the negative impacts of lake regulation, such as on the migration of fish, food organisms, and winter spawning areas. In assessing the impact of habitat modifications on fisheries it was suggested that it is more important to focus on the fish community than on the type of modifications. Opinion was expressed that in situations where habitat has been changed by a number of uses, it may be more beneficial to follow the trends of restructuring fish communities and fish food organisms, instead of attempting preservation of the previous communities.
14. While the present Symposium concentrated on engineering modifications of water resources, especially their regulation, it was felt that there is a need to pay an increased attention to gradual cumulative impacts on fishery resources by other users of water, such as intensive recreation on now-regulated lake shores, and water sports. Littoral destruction which usually follows is accompanied by drastic decline of some fish species. The high demand for lake-side recreation is in direct conflict with the need to protect fish habitats in the lake margins. The Symposium felt that because of the common occurrence of such situations in EIFAC countries, there is an urgent need for more studies to find appropriate remedial measures.
15. The issue is complex and could be approached for example through comprehensive studies of environmental, including socio-economic, impacts on such threatened systems.
16. Although in most member countries afforestation is considered beneficial to water habitats, in those with acid rain problems and the resulting acidification of soils, it may aggravate the situation. The problem should be reviewed from time to time and, if deemed necessary, more attention should be paid to it. However, siltation of rivers affected by afforestation and deforestation practices may be a major problem.
17. The Symposium has drawn attention to the persisting difficulty with forecasting impacts on fish and fisheries of physical modifications of the habitat. Some environmental impact statement (EIS) have proved incorrect, and often there is no checking on their validity during the post-construction phase of a project. New problems may arise which are not considered in the original EIS. In spite of such shortcomings, the Symposium felt that the principles of EIS are valuable and EIS should be prepared, as far as practicable, in connection with all future proposed work affecting fisheries.
18. Quantitative data on cause-effect relationships are essential and those obtained from specific sites should be tested in other situations to assess their general usefulness. EIFAC's major contribution could be in collating such information, including annotated bibliographies and case histories of management procedures both successful as well as those which have failed.
19. The question of Code of Practice for works with potential environmental impact was raised. Some of the EIFAC Member States have Codes of Practice which aim at minimizing the effects on aquatic biota, but the suggestion that a single Code of Practice could be made generally applicable appears unrealistic as legal obstacles in some Member States would prevent its acceptance. It was suggested that as an initial stage the legal situation in each Member State should be examined as a preparatory step to assess whether formulation of a Code of Practice would be suitable.
20. The Symposium recommends that, in view of increasing pressures on fish communities and the quality of fishery resources arising from multiple uses of freshwater ecosystems and associated catchment areas, EIFAC Member States should give high priority in their research and management programmes to :
forecasting and monitoring the impact on fish of man-made habitat modification and of any necessary remedial measures;
testing and improving existing and new methods of predicting the impact of habitat modification and the effectiveness of remedial measures.