(Chairmen: T. Petr, B. Steinmetz)
A questionnaire concerning mass removal of fish in EIFAC countries was analysed by Steinmetz for 15 countries. The results indicate that in most countries there is a need for such interventions, especially to deal with stunting fish populations. Other aspects requiring special attention are the planning for restoration, the fishery itself, and the overall management of water bodies.
The results of the mass removal of fish from Lake Wolderwijd (2 670 ha, the Netherlands) confirmed the need for careful planning. This may require considerable time, especially in large water bodies where knowledge and experience are still limited. The major objective was the reduction of the fish stock (biomass estimate 200–250 kg/ha) to approximately 50 kg/ha (Grimm).
In late 1990 commercial fishery on Lake Wolderwijd was initiated. The following gear was applied: seven trawls, three seines (1 200, 750 and 180 m long respectively), four pair trawls, two beam trawls and 550 fyke nets. By May 1991, 440 000 kg of fish were removed, equal to the planned target catch. The cost of the project was over 2 million Dutch Guilders.
The possibilities of application of electric fishing were reviewed by Cowx. Large-scale gear, including boom mounted electrode arrays and electrified seines were considered the most suitable. The efficiency of this equipment can be improved when used in conjunction with high frequency (400 Hz), low duty cycle (10% on time) pulsed currents. The need for safety regulations was also highlighted.
The following publications were directly referred to for further information:
Cowx, I.G. and P. Lamargue (1990). Fishing with Electricity. Fishing News Books, Blackwell Scientific Publications, Oxford.
Cowx, I.G. (1990). Developments in Electric Fishing. Fishing News Books, Blackwell Scientific Publications, Oxford.
In discussion, the long-term impact of the applied mass removal techniques was questioned, especially regarding coarse fish and the possibility of the re-establishment of fish stocks at their original levels within 2–3 years. Relatively high phosphorus concentrations could have an important role here.
Trawling proved to be a useful tool for mass removal of cyprinids in Swedish lakes (Hamrin). An understanding of seasonal movements and of preferred habitats of targeted fish assists in improving the selectivity of the method. As a result, the majority of valuable fish is not removed. Initial catch per haul of 20 minutes was 200–1 000 kg, but only about 5% were represented by valuable fish. Transport and processing of the landed cyprinids proved to be a problem and needs to be solved.
The fish management technique using rotenone was discussed by Mr. Livingston. In Lake Delvan (Wisconsin, USA), 2 000 acres of surface area was treated in late autumn using 50 000 gallons of rotenone at a total cost of US$ 6 million. 800 tons of dead fish were collected, about 65% along the shore, and the rest from boats. Some fish sunk to the bottom where they decomposed, and some small fish were carried away by birds who are not affected by rotenone. In the discussion Hamrin noted that rotenone is used in enclosed areas in some Swedish lakes for the same purpose.
During the general discussion to this session it was noted that local conditions have already led to adaptation of gear such as seines. Some electrofishing gear seems to be efficient for only a short time as fish adapt to it and start avoiding it. Large seines may lead to overfishing of valuable fish, which leads to long periods of recolonization.
