Approach
17. Several biomanipulation approaches are used in Europe:
(a) The traditional approach to biomanipulation by enhancement of predators and reduction of planktivorous and benthivorous fishes without participation of local stakeholders, and
(b) Advanced approaches:
(i) Biomanipulation considering water quality and the socio-economic demand of anglers and commercial fishermen;
(ii) Biomanipulation conducted mainly by means of the fishery.
18. The German Saidenbach water supply reservoir is an example of the classic approach. Lake trout (Salmo trutta f. lacustris) of less than 1 kg (15 - 20 ind./ha) was chosen as top predator in this reservoir as summer temperature rarely exceeds 20°C. However, because the proportion of fish in the diet during one summer was only 11 percent, the trout only consumed 2-3 percent of the roach and perch biomass.
19. Fish stocks are not always the cause of algal blooms. For example, during summer, the hyper-eutrophicated Salford Quays, a freshwater system in the north of England, used to be anoxic with frequent mass fish kills. The Quays were de-stratified using a mixing system and stocked with a variety of fish species. In this case, de-stratification was an effective tool against algal blooms. The introduction of fish has not been detrimental to water quality here.
20. A newer approach to biomanipulation includes the socio-economic interests of recreational and commercial fisheries. This involves stakeholder analysis. Based on a literature review of the key parameters, a step-by-step guideline for lake restoration by biomanipulation described, inter alia, as maximum thresholds for external P-load and internal P-concentrations, and the critical maximum fish biomass necessary for a significant reduction of phytoplankton. Successful biomanipulation can be better achieved by using the experience and the motivation of the recreational and commercial fishermen.
21. Specialized carp anglers can effectively reduce carp populations. A mail and internet survey found that experienced German carp anglers catch on average 332 kg per year. There was a positive correlation between catch and amount of bait used, effort and experience. The amount of bait used was high, about 215 kg of "boilies", cereals and other substances per angler year for ground baiting. This corresponds to a gross P-input of 1.018 kg P per angler year. This gross input would become balanced at a harvest of 212 kg carp per year. This is an unrealistic figure where catch-and-release is practised, indicating that specialized carp angling may contribute to anthropogenic eutrophication, especially in smaller, nutrient poor water bodies at high angler densities. Specialized carp angling on the one hand may reduce carp biomass and improve water quality, while at the same time it may lower water quality through excessive ground-baiting.
22. In Denmark there are no substantial differences between lake fish management for biomanipulation and for the fishery. The fishery uses a whole set of fishing gears suitable for effective control of fish stocks, thereby assuring the integration of the fishermen in the biomanipulation process. This ensures that at least 80 percent of planktivorous fish biomass can be removed within one to two years. Furthermore, heavy stocking with pike fry (1 000 - 1 500 ind./ha) will help to reduce the numbers of 0+ cyprinids and, after 3 to 5 years, can significantly affect the reproductive potential of the cyprinids.
Evaluation
23. The classic approach using small brown trout at low stocking densities as a predator for deep and cool reservoirs does not seem to be effective for biomanipulation in central Europe. The influence of higher stocking densities and higher individual weight of trout on the proportion of fish consumed needs to be further investigated.
24. The example of Salford Quays showed that fish stocks are not always the cause of algal blooms. Whether or not there are mechanisms for effective phosphorous fixing in the sediment, such as high iron content, should be taken into account in planning a biomanipulation project. In such cases, aeration and de-stratification may be more suitable than fish stock manipulation.
25. The proposed step-by-step guideline for lake restoration by biomanipulation is a valuable attempt at reconciling fisheries and water quality management by integrating all stakeholders interests. It provides advice for the use of biomanipulation as a regular tool in water quality management. It has to be tested in practice, and the technological parameters have to be completed. The power of the approach lies in the integration of traditional fisheries management measures such as stocking of piscivorous fish with ecosystem-based management. The continuity of interventions is paramount for the long-term success of biomanipulation programmes.
26. The inclusion of experienced carp anglers in biomanipulation projects can be recommended in cases where dense carp stocks lead to a resurgence of eutrophication. Catch and release should be avoided in biomanipulation projects.
27. Data on the amount of baiting should be included in further projects. Bait seems to be a substantial P-source in some cases but more experienced anglers increase their catch without increasing the amount of ground-bait used. Further investigations on P-reduced baits are necessary. Furthermore, specific management guidelines for ground- and pre-baiting are needed because ground-baiting is generally common among coarse fish anglers.
28. The long-term inclusion of fishermen seems to be necessary for the success of biomanipulation projects that depend on continuous fishing pressure and should be adopted especially in countries with significant commercial fisheries such as Poland, eastern Germany and other east European States.
