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Session 3: Rehabilitation of inland fisheries


29. In lakes, fish such as salmonids require satisfactory DO concentrations. Deep-water salmonid habitat can be expanded in lakes suffering from depleted summer oxygen levels by artificial mixing and raising hypolimnic DO levels by oxygenation to at least 4-5 mg/l.

30. Utilizing cutaway bogs in areas where peat extraction has been terminated can create new fishing opportunities. The size and shape of the created lakes is dictated by the intended purpose of the fishery.

31. Experience in Ireland demonstrates the importance of having an overview of the entire catchment in establishing programmes for the restoration of salmonid riverine catchments. Baseline studies are needed for an understanding of the relative importance of imbalances in individual channels relative to the overall catchment.

32. Stocking programmes represent a major element of fisheries management in the United Kingdom. The relative merits and cost effectiveness of stocking rivers with different life stages and at different times of the year could be useful in determining if stocking contributes to stock enhancement.

33. No net loss of productive capacity of habitat can be achieved through a wide variety of actions that protect fish habitat yet still allow engineering works to take place in and around water. Focusing on redesign and relocation of projects and mitigation of harmful effects can result in a net gain of fish habitat.

34. Artificial ponds (borrow pits) with limited habitat in the Czech Republic could be improved by managed flooding during the spring and summer to act as nursery areas for flood plain species.

35. Rehabilitation and enhancement activities are often unsuccessful due to lack of understanding of the biotic and abiotic factors influencing the fish populations under study. Rehabilitation schemes often fail to address the wider catchment problems and issues affecting fish communities.


36. Rehabilitation of lakes by artificial oxygenation has shown varied results. Oxygen levels can be elevated resulting in recolonization by fish and invertebrates. This also reduces the chance of fish kills from upwelling of anoxic hypolimnic water. Experience has shown however, that continuing input of nutrients to lakes continues the eutrophic condition and the chance of fish kills from toxic algae production persists. Furthermore, sediments continue to be anoxic and mortality of whitefish eggs will continue. Experience from three lakes in Switzerland has shown that as long as lakes stay eutrophic, oxygenation will be necessary to maintain 4 mg/l DO levels.

37. Research has shown that water quality in newly created lakes in cutaway bogs is excellent and that plant colonization in the newly created lakes was nearly 95 percent within three years. Plant colonization is essential for creating a basis for a productive food chain. The explosion of invertebrate populations is coincident with the establishment of the macrophyte community. Although self-sustaining populations have not yet been observed, it is estimated that a stocking rate of 200 kg/ha can maintain a healthy population and provide excellent catch and release fishery for tench and carp. These artificial fisheries can have a considerable amenity value.

38. Baseline studies of salmonid catchments in preparation for river restoration have identified problem areas, allowing the generation of habitat enhancement procedures that will restore a natural balance. Key problem areas include arterial drainage programmes for flood relief, removal of riparian vegetation to extend grazing opportunities and sheep overgrazing. A combination of use of natural materials, fencing out cattle and riparian plantings have been successful in mimicking natural conditions in all three categories of damaged channels. It is recommended to leave one subcatchment aside as a control to monitor effectiveness of restoration.

39. When stocking rivers with fed salmon fry, a net gain can be realized when natural survival rates from egg to smolt are in the region of 1 percent or less. At high survival rates in the wild, the advantage of rearing and stocking fish is diminished. For coarse fish, chub, dace and roach, stocking either 1+ or 2+ is unlikely to make a significant difference to catches. In river restoration situations, however, with no existing population, stocking older fish of 2+ years is likely to be more cost effective and lead to faster establishment of self-sustaining population. Knowledge of survival rates is still lacking.

40. Reduction of impacts to fish habitat by development and resource extraction can be achieved by a number of activities but can not be made by government intervention alone. Agency partnerships as well as legislation enforcement and compliance are necessary but greater emphasis must be made on advice to clients, public education and outreach activities to engender stakeholder participation in conserving habitat.

41. Long term flooding of floodplain ponds (borrow pits) increased species richness and relative abundance of fishes showing that borrow pits represent valuable spawning and nursery habitat. However, nursery habitat appears to be limited due to untimely recession of the floodwaters, which could negate any positive effects. Improved control of duration of flooding could be a useful tool but requires further study.

42. Often fisheries do not improve as expected following rehabilitation works. Additionally, improvements achieved often do not warrant the expense of the scheme. Restoration works in many cases can not achieve the desired results because fisheries managers’ expectations are too high. Furthermore, identification of bottlenecks to viable fish populations in the whole catchment is essential for the planning of individual restoration projects and should be undertaken before their initiation. Post project monitoring of rehabilitation works is essential if unsuccessful schemes are not to be duplicated.

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