(Chairmen: D. Barthelmes, J. Sarvala)
This session dealt with the responses of fish populations to high fishing intensities. It was agreed that it is possible to increase the fishing pressure so that a substantial part of the stock is removed, which in turn results in faster growth of the remaining stock (Barthelmes, Klemetsen, Sarvala). An increase in the growth rate and change in age distribution can be used to check on the efficiency of the intensive fishing (Sarvala). Faster growth of fish implies increased zooplankton and benthos biomass and indicates that the biomanipulation approach using fish has been successful.
Different removal methods based on decreasing CPUE (Helminen) as well as virtual population analysis (VPA) were used to assess the success of intensive fishing in two Finnish lakes.
The session contributions showed that, in general, a very high fishing effort is required to achieve substantial reduction in fish population biomass (Sarvala). The self-regulatory potential of coarse fish is very high as the improved growth allows for more effective reproduction. Fishing also shifts the age distribution towards younger ages that have a higher growth efficiency than older age groups.
The Roundtable discussion on mass removal of fish in tropical and subtropical countries (Chairmen: T. Petr, M. Gophen) met on 10 June 1991 and it was suggested that:
methods for mass removal of fish developed in temperate and cold temperate countries of the northern hemisphere are now available. These could be tested and applied in developing warmwater countries where mass removal of fish is required for purposes such as efficient harvesting of natural populations or stocked fish, removal of unwanted fish, selective fishing for different fish size, etc. The Roundtable discussion pointed out that such methods still require testing in new conditions. EIFAC member countries could offer advice, and, subject to availability of funds, also assistance in setting up research, technology transfer, testing, pilot studies and full-scale application.
the possibility of using fish for enhancing water quality in tropical and subtropical countries is poorly known, as this approach has been developed predominantly for its use in industrialized countries of the northern hemisphere. In developing countries increasing living standard and population pressure lead to rising demands for clean water supply. An ecosystem approach, including the use of fish for enhancing water quality, could be transferred to developing countries for testing under local conditions. Because of the high rates of dynamic processes in tropical aquatic ecosystems, it was emphasized that before implementation of new fisheries management operations such as removal of unwanted species, limnological measures should be considered. The Roundtable suggested that assistance be offered to the FAO member countries through EIFAC, which should communicate this offer via the EIFAC secretariat to other FAO regional bodies, i.e., COPESCAL, CIFA and IPFC.
The Roundtable discussion on “Zandermudigkeit” (Chairman: Dr. D. Barthelmes) discussed the strong decrease of normally fluctuating pike-perch catches from levels of over 10 kg/ha/y to about 1 kg in a period of 1 to 2 years in waters where fishing of zander was moderate and the pike-perch population could therefore increase. In this period of 1 to 2 years the submerged vegetations and pike-tench populations reestablished, and water cleared.
During the collapse of the pike-perch catches zander proved to be very vulnerable to angling, because of shortage of food. It was also noted that cannibalism of pike-perch strengthens this mechanism. It proved difficult to predict the time lapse between the two peaks. The pike-perch populations are difficult to manage.
During the discussion other examples were mentioned. It was pointed out that an increase in water transparency can have a negative influence on the survival of pikeperch fry. To understand the phenomenon as well the implications of “Zandermudigkeit” for the fishery and water management more research is necessary.
