Changes in water pH and colour since the late 1980s were studied in 35 small boreal lakes of varying hydrological and landscape settings but similar climate and acid deposition. The data was collected during the autumnal overturn on the annual basis except in lake with weekly sampling during the ice-free period. In addition to the deposition data information about catchment soil types as well as local meteorological and hydrological conditions were used for the long-term data interpretation. The lakes are situated in a small area in southern Finland, 130 km north from Helsinki, where sulphate deposition declined by [60% in one decade since the mid1980s. The results showed that water colour increased in most lakes while pH did not. In lakes dominated by surface runoff there was a distinct upward shift in colour, with an initial increase after the mid-1990s and a second increase in 2004. The first shift appeared when the sulphate deposition reached a level ca. 25% of that in 1988. However, the upward shift in colour also coincided with a change in hydrological conditions after several dry summers. In contrast, the second shift in colour clearly coincided with a switch in hydrology due to the abnormally wet summer of 2004 after severe drought in 2002-2003. Although the hydrological conditions indisputably had a key role in determining the annual variability in colour, a distinct negative relationship between acid deposition and water colour in 90% of the lakes strongly suggested that reduction in sulphate deposition fostered the leaching of coloured organic substances from the catchment soils. Increase in colour, in turn, strongly influenced lake water pH, and the present day higher organic matter concentrations seemingly depress pH values more than in the 1980s, before the reduction in acid deposition.
The effects of tank bottom substrates and presence of shelter on growth, survival and condition of cheliped in the juvenile (stage 2) signal crayfish, Pacifastacus leniusculus (Dana), with an initial stocking density of 200 individuals m−2 were investigated. In the first trial, three different tank bottom substrates with or without shelter were used. The three bottom types were loose gravel, glued gravel and bare bottom. The fastest growth both in terms of wet weight and carapace length was observed on loose gravel bottom. Also survival (range 30.1% to 49.8%) was affected by bottom type being higher on gravel bottoms than on bare bottom. Shelter improved survival only on loose gravel bottom. Bottom type influenced also the number of animals with cheliped injuries. The highest proportion of animals with no injuries (59.5%) was found on bare bottom and the proportion was about 12% and 13% units less on loose gravel and on glued gravel bottom. The presence of shelter had a marginal probability in increasing the number of animals without injuries. In the second trial, the substitution of ordinary gravel as tank bottom substrate with crushed limestone was studied. The mean final wet weights were 0.517 g on ordinary gravel bottom and 0.481 g on limestone bottom. The survivals were about 74% and 80% respectively. Limestone bottom decreased marginally final wet weight, and increased survival but did not affect the final carapace length that had a mean value of 13.6 mm. We conclude from the present experiments that the type of tanks and the rearing system used supported good growth of juvenile signal crayfish. The best growth results could be obtained by using loose gravel (or loose limestone) on the bottom of the tanks with a sufficient quantity of hiding places. However, there appears to be an increased risk for cheliped injuries on bottoms with loose materials.
-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.
E¡ects of size grading and stocking density on growth and survival of 3.5-month-old signal cray¢sh (Pacifastacus leniusculus Dana) juveniles were investigated in a 3 Â 2 factorial design with size group (small, large and mixed) and stocking density (25 and 200 individuals m À 2 ) as factors for 90 days. Carapace length and wet weight growth of the juveniles were a¡ected by the presence or absence of other size groups, stocking density and animal size. All factors had statistically signi¢cant interactions, which make the interpretation of the results more complicated. The smaller cray¢sh grew faster in the absence of large animals and larger animals bene¢ted from the presence of smaller cospecies. Increasing density decreased the growth in length and weight. Survival (82^100%) was good in all groups. Subsequently, the within population dynamics was studied by selectively removing the largest individuals (selectively removing (SR) group). E¡ects on growth and survival of the remaining juveniles were evaluated on 6-monthold P. leniusculus juveniles reared in plastic tanks (100 juveniles m À 2 ) for 122 days. Size distribution manipulation was performed at days 34 and 70. Juveniles in SR group grew signi¢cantly faster in comparison with a control group (RR), from which juveniles were removed randomly. Also, the survival rate was signi¢cantly higher in the SR group than in the RR group. In conclusion, size grading of juvenile P. leniusculus either at the start of the experiment or repeatedly during the experiment increased growth performance of smaller animals in length and weight.
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