-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.

Depending on their reproductive strategy, different fish species aim to aggregate or disperse eggs and larvae in their reproductive habitat. Many pelagic species disperse their eggs widely around the potential nursery areas. Larval dispersion or aggregation affects population sub‐structuring, which has important implications in fisheries management and conservation of the natural spatial diversity in populations. The dispersion of larval vendace (Coregonus albula) was quantified in two oligotrophic Finnish lakes, and effects of density and environmental variables on the inter‐annual variation in the larval distribution were examined by analysing spatial abundance data from the lakes from 1999 to 2017. A 3‐D hydrodynamic egg distribution model was used to simulate the larval transport after hatching. Vendace larvae dispersed lake‐wide to both littoral and pelagic zones but, in some littoral hot spots, more larvae aggregated year after year. However, in years of high larval number, the densities increased not only in the hot spots, but generally at all sampling plots. An overall increase in abundance was observed at all sampling sites. The simulations of the egg distribution model supported the hypothesis that the dispersion of the eggs occurs by spawners, i.e. by spawning at several different spawning sites, which are located all around the lake. The dispersion of vendace eggs and larvae can be seen as a bet‐hedging strategy in space and time since in boreal oligotrophic large lakes with fragmented morphology, weather and other environmental factors in spring during hatching varies from year to year spatially in unpredictable manner. Lake‐wide larval dispersion suggests that the subpopulations of adjacent lake deeps may swap considerable amounts of individuals during early life and may not be closed units. Conservation of particular habitats seems unnecessary for Finnish vendace populations where large potential spawning areas in lakes are available.

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