Carlin tagging data for 1980–1991 were used to examine the influence of smolt size and feeding conditions on the post‐smolt survival of ranched River Neva salmon, (Salmo salar L.), in the the Gulf of Finland, which is the native feeding area of the stock, and in the Bothnian Sea, where the stock has been introduced. Because of better feeding conditions, the survival rates were higher and less variable in the Gulf of Finland than in the Bothnian Sea. In the Bothnian Sea, the annual variability in survival decreased and the mean value increased with increasing smolt size from the smallest (14–16 cm) to the largest (28–30 cm) size classes. The survival was positively correlated with growth rates, food resources and sea‐surface temperatures. This suggests that in the Bothnian Sea the annual variability in survival is mainly because variable marine conditions affect growth rates, and, thus, the vulnerability of the post‐smolts to size‐dependent predation. In the Gulf of Finland, the survival advantage of large initial size and rapid growth was counteracted by size‐selective post‐smolt mortality from fishing. The increase in the survival rate with increasing smolt size levelled off at 22 cm, and the correlations between survival, growth and the indices of feeding conditions were mostly insignificant. For large smolts, some negative correlations were recorded, suggesting that the relative significance of mortality from fishing may even exceed that of size‐dependent natural mortality. The implications of the results for management are discussed.

Until July, post-smolt salmon Salmo salar (n=337; 129-375 mm L T , mean 225 mm) in the Bothnian Sea relied largely on surface fauna (mainly terrestrial insects). From August onwards, fish was the principal food type. The smallest piscivorous post-smolts were <200 mm, but the main shift to piscivory occurred at sizes of 240-320 mm. Piscivory was promoted by a large smolt size. Almost all one-sea-winter (1-SW) salmon (n=316; 278-524 mm, mean 397 mm) were piscivorous. Over 70% of the post-smolt and 96% of the 1-SW salmon with identifiable fish species in their stomachs had preyed on herring Clupea harengus. Other fish prey included the ten-spined Pungitius pungitius and three-spined sticklebacks Gasterosteus aculeatus but no sprats Sprattus sprattus. The results support earlier observations of a close relationship between recruitment of herring and production of salmon in the Bothnian Sea, and of the crucial role of smolt size in determining the ability of feeding salmon for utilizing the food resources of the area. 2001 The Fisheries Society of the British Isles

-In Finland, massive signal crayfish introductions started towards the end of 1980s, with an estimated total of 2.2 million signal crayfish been stocked before year 2016. During that period, Finnish fisheries authorities have implemented three national management strategies setting guidelines for the crayfish introductions. The main aims of the strategies have been conservation of native noble crayfish stocks and a controlled spreading of the alien signal crayfish within a designated region. In this study, we report the current distribution of signal crayfish in Finland in comparison to the guidelines set in these three national strategies. The present distribution area of the signal crayfish covers most of the Southern Finland. The signal crayfish has been introduced with a stocking permits to over 480 water bodies. In addition, there have been numerous stockings without permits, which are often next to the region designated for signal crayfish. Based on the results, we conclude that crayfisheries strategies adopted in Finland have only had limited effect on the spread of signal crayfish. We presume that main causes for the uncontrolled spreading of the signal crayfish in Finland have been lack of strict official supervision and general lack of awareness about the risks associated with the alien species spreading.

The introduced North American signal crayfish Pacifastacus leniusculus (Dana) has replaced the native noble crayfish (Astacus astacus) in many European freshwaters and can be considered a new component of these ecosystems. The 2 species are apparently similar in many respects, but their ecological equivalence is uncertain and has been little investigated, especially at the whole-lake scale. We compared the effects of the 2 species on the abundance, species richness, and composition of littoral macroinvertebrate assemblages in a set of small-and mediumsized boreal lakes, which included 8 lakes with noble crayfish, 8 lakes with signal crayfish, and 8 lakes without crayfish. We collected semiquantitative littoral macroinvertebrate samples with a kick net from 3 replicate sites in each lake. The abundance of invertebrates did not differ significantly among the 3 lake categories, but lakes with crayfish had lower species richness than lakes without crayfish. Mollusk taxa, in particular, were fewer in lakes with crayfish. Assemblage composition also differed between lakes with and without crayfish. However, macroinvertebrate species richness and composition did not differ between lakes with signal or noble crayfish, indicating that the 2 crayfish species are ecologically equivalent with respect to their effects on shallow, littoral invertebrate assemblages of boreal lakes.

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