(Chairmen: L. Persson, T. Backiel).
It was shown that along the productivity gradient of temperate European lake ecosystems there is a shift in the fish community from a dominance of Salmoniformes to a dominance of percids and finally to a dominance of cyprinids in highly productive lakes (Persson). This shift involves greater changes in the trophic structure in systems dominated by percids as compared to those in systems dominated by Salmoniformes or cyprinids. The dominance of cyprinids in highly productive systems was suggested to be primarily due to the low structural complexity of such systems. It was suggested that highly productive lakes dominated by submerged macrophytes and perch may only be a transient stage.
An outline of optimal pike habitat was presented. The recruitment of 0+ pike is determined at two stages during the first growing season (Grimm). The surface area of inundated emergent vegetation determines the recruitment of 0–12 cm pike. The area of emergent vegetation and ingrowing shrubs determines the total pike standing stock. Standing pike stocks of 50–75 kg pike/ha are obtained in lakes with 5–15% of the surface area overgrown with emergent and 30–50% with submerged weeds. It was suggested that the water colour of the system does not directly affect standing stocks of pike.
The role of predation and competition in determining the outcome of biomanipulation in two lakes showed that in the first lake, despite the stocking of 0+ pike the numbers of 0+ cyprinids and 0+ perch increased. In lake two, competition between 0+ pike and 0+ perch was not exclusive, allowing co-existence in the diverse macrophyte growth. It was concluded that interactions between pike and perch can be enhanced in the presence of suitable macrophytes. It is important to determine the optimal quality and quantity of macrophytes for perch and pike which in turn requires quantitative experiments and habitat suitability data (Walker).
The fish community was modelled at aiming to solve problems of biomanipulation (Kazanskij). The model involved fish species and respective life history parameters, as well as intra- and interspecific relations. Lower trophic levels were also included.
In Lake Wolderwijd (2 700 ha) in the Netherlands, mass removal of fish resulted in an increase of water transparency from less than 0.5 m to 2 m (Hosper). The cost effectiveness of this approach is promising (see paragraphs 3, 4 and 5).
The general discussion raised several items. It was suggested that the impact of water quality on fish needs to be considered in any mass removal/biomanipulation activities. Zooplankton may find a refuge from predation in deep lakes and this may lead to a change from planktivory to benthivory (Gophen). However, in Sweden, zooplankton is eaten by fish at all depths in some lakes (Hamrin). The need for considering external loading alongside the dissolved and sediment-bound phosphorus was stressed and P concentration measurements in water and fish are therefore not a measure of success or failure (Hamrin). In the Netherlands no conclusive results in studies of phosphorus in three lakes were reached: removal of fish resulted either in decrease, no change or increase in phosphorus and it was concluded that fish such as perch do not seem to contribute to (or remove) phosphorus loading (Grimm). The function of chironomids in internal loading of phosphorus from the bottom was also questioned: roach removed four times more phosphorus from the bottom than chironomids (Hamrin). Gophen suggested that biomanipulation could be a successful means of control if one could control phosphorus.
The seminar concluded with the following recommendations addressed to the seventeenth session of EIFAC:
that the proceedings of the seminar on Mass Removal of (Unwanted) Fish in Large Inland Waters be published in a scientific journal
that a book on Integrated Water Management of Shallow Lakes for Water and Fishery Managers, produced in the Netherlands, be considered for translation into English and published in one of the EIFAC document series
that attention be paid to the development of fishing methods for mass removal of fish from large inland waters and to the assessment of the subsequent impact on stocks. A preparation of a manual on this subject was recommended
that methods for processing of mass-removed trash fish be investigated.
