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Variations in the year-class strength of perch (Perca fluviatilis) were analyzed among 23 populations in Baltic coastal areas distributed between 57 and 66°N using the age distribution in catches. In spite of large differences in abiotic and biotic factors, the variations in year-class strength were similar in 14 of these areas. The similarities could be attributed to large-scale weather variations influencing water temperature. Year-class strength was shown to be correlated with an index based on temperature and day length during the whole first year of life. In two of the nine deviating populations, perch nursery areas are situated in small nearshore freshwaters, where fluctuations in the water level may be more important for recruitment than temperature variations. The other deviating areas were exposed to environmental disturbances, i.e. acidification, oxygen deficits, pulp mill effluents, and thermal discharge, which obviously affected recruitment in a decisive way.

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SynopsisSeveral freshwater species use the Kyronjoki River estuary as a spawning and nursery area. The main reasons for this seem to be the morphology of the estuary, the abundance of shelter provided by aquatic macrophytes, high food production and favourable temperature conditions. Acidification of the estuary due to drainage from acidic soils has made part of the estuary unsuitable for fish reproduction. In addition, year to year fluctuations in the acidity of the estuarine water have affected the reproductive success of several species. The severity of the effects of the acidification at the population level is determined by the spatial and temporal distribution of the larvae and juveniles.

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SynopsisThe burbot population off the River Kyronjoki has suffered from acidification in its reproduction area, causing a reduction of the population throughout the 1980s . During the same period of time the mean length-atage of burbot was found to increase, with a significant increase in length-at-age for age groups 3-8 from 1979 to 1993 . The length-weight relationship however was not found to have changed during this same period . The decreasing burbot population was found to influence the changed length-at-age . An increase in potential prey species population was also documented during this period of increasing length-at-age . The increased lengths were associated to the changed availability of food caused by the simultaneous fluctuations in both burbot and potential prey species populations .

Due to discharge from acid sulfate (a.s.) soils, watercourses and coastal areas in the Gulf of Bothnia are periodically heavily acidified with high concentrations of potentially toxic metals. Data on water quality from 2005 to 2014 in an embanked lake, an estuary of four rivers in western Finland, showed repeated events with acidic water (pH < 5.5) with high concentrations of Al. Size fractionation and species modeling of Al showed that a significant part of the Al occurred as highly toxic small-size fractions (dissolved < 1 kDa and colloidal 1 kDa—0.45 µm) as free ions and complexed to sulfate. The larval abundance of the burbot (Lota lota L.) was shown to be sensitive to acidity during the wintertime spawning migration and spawning. Bearing in mind the importance of estuaries of the northern Baltic Sea as spawning and nursery areas of fish, the reoccurring failure in the reproduction of fish may cause a more serious threat for the lake and adjacent coastal fish stocks than the spectacular, but less frequent, mass kills of adult fish. This demonstrates the close relationship between climate, hydrology, water geochemistry and the aquatic coastal ecosystem in areas affected by a.s. soils. As the current forecast of climate chance indicates warmer winters with more continuous runoff, the effects can become even more prominent. This study also shows that the annual larvae abundance of burbot may be used as a bioindicator and an instrument for the fisheries for obtaining more comprehensive knowledge of the ecological effects of acidic metal discharge from a.s. soils.

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