SummaryThis paper summarizes the objectives, activities initiated and approaches used in developing Atlantic salmon stock rebuilding programmes in three large river systems in northern Finland in order to identify the priorities for research and management for further restoration and rehabilitation steps. Until recently, these historically renowned major salmon rivers have been harnessed for hydropower for 40-60 years without provisions for fish passage. Hydropower companies are obliged to compensate for the losses caused by dam construction by annual fish releases, including juveniles of Atlantic salmon and migratory brown trout. Returning fish are harvested in the designated Ôterminal fishery areasÕ at river mouths. Recently, activities aimed at restoring salmonid stocks have started in all these rivers. First fishways at the dams closest to the river mouths were constructed on two rivers, and preliminary projects were completed to review the preconditions for restoration, options for bypassing the dams, and to study the performance of these fishways. New projects incorporating further detailed fishway planning are underway. The various projects have proved successful in bringing together authorities, hydropower companies, local organizations, and expertise from various institutions for a joint effort to tackle these multifaceted and multidisciplinary problems.
Many recent studies have stressed density-dependent regulation mechanisms in oscillating vendace (Coregonus albula) populations. Long-term studies on the population dynamics of vendace have been hampered by the difficulty of obtaining reliable estimates of year-class sizes. Data collected from Lake Pyhäjärvi (Southwest Finland) during 1971–91 allowed relatively accurate analysis of vendace abundance changes. In the winters of 1979–91, daily catch statistics were collected from professional fishermen. Species, age, and size composition were analysed from regular catch samples. We estimated the size of the vendace population from the decrease in the CPUE during 10 winters. For the remaining 10 years, estimates were based on the relationship between the numbers of a year-class in autumn and mean individual growth rate. A 2-yr cycle was the most conspicuous feature of the data implying density-dependent regulation. We suggest that in Lake Pyhäjärvi the basic model producing a persistent 2-yr cycle is the asymmetrical food competition between age groups.
The efficiency of mass removal of fishes can potentially be assessed using catch statistics collected during intensive fishing periods. The calculations are based on the general assumption that a decline in population size will produce a decline in catch per unit effort (cpue). When the removal is efficient it is possible to estimate the population size both at the beginning of the fishing period and after the removal.
Two examples are based on the winter seine net fishing of vendace (Coregonus albula) in Lake Pyhäjärvi, SW Finland in (1) 1983–1984 and (2) 1989–1990. The effects of the error in catch composition samples and random changes in probabilities of capture during the removal period on the final estimates of initial stock sizes are also examined.
The precision of the population estimates shown here was greatly influenced by the violation of the underlying assumption that the probability of capture is equal for all members of the target age group. In any case, particular attention should be paid to optimizing the sampling programme so that it will reveal the best information on the exploited stock with the resources in hand.
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