Recovery of brown trout populations in streams exposed to atmospheric acidification in the Bohemian Forest

Abstract. Water chemistry and fish occurrence in outflows from two lakes recovering from acidification were studied during 2005–2010. The two lakes represented the regional extremes: while Lake Laka (LA) was the least acidified among the Bohemian Forest lakes, Lake Čertovo (CT) was the most acidified. Water chemistry improved downstream in either outlet due to mixing with less acidic water from tributaries. While pH of the LA stream was circumneutral and concentrations of ionic aluminium (Ali) were low (< 50 μg.l-1) except for temporary acidic episodes during snowmelt, the CT stream remained permanently acidified with Ali concentrations (> 200 μg.l-1) throughout the year and was considered as a reference site. Brown trout (Salmo trutta s.l.) survived in the LA stream already at 0.7 km from the lake. A stable trout population, including young-of-the-year fish increasing in numbers, was found in the LA stream between 2005 and 2010 despite the spring short-term acid episodes. Any recovery of the CT stream is unlikely as the outflow continued to be acidic at 1.5 km from the lake and Ali concentrations were still by one order of magnitude higher than the limit acceptable by trout.