Effects of reduced liming on brown trout (Salmo trutta L.) in four small tributaries in eastern Norway

Acidification of surface waters due to acidic precipitation has led to a reduction in biological diversity. Through international conventions, emissions causing the acidic precipitation has decreased. The hallenges associated with acidification today is of a different character then it was earlier. Huge efforts have been done on a national scale in Norway to reduce acidification, mainly by adding lime to surface waters. Due to reduced acidic precipitation, lime treatment has been ceased in 800 locations, during the years 2004 - 2014. A monitoring of the water chemical and biological development in these locations adds knowledge about the consequences of ceased lime treatment. In this study brown trout populations and water chemistry is monitored in four tributaries draining into Lake Råsjøen, Nannestad municipality, Norway. The four tributaries are currently under four different lime treatment regimes. Letter codes have been given to the tributaries Elsjøbekken (A), Storrsjøbekken (B), Botnetjernsbekken (C) and Trasletjernsbekken (D). Tributaries C and D have ceased lime treatment, tributary B is treated with lime in parts of its catchment area, and tributary A is fully treated with lime in its catchment area. The chemical parameters pH and conductivity were measured in field and water samples were collected and sent to a laboratory. Water temperature and relative barometric pressure were registered once an hour by mounted loggers. Brown trout and minnow have been captured in the tributaries using an electrical fishing apparatus. Brown trout were then marked using PIT-tags, to monitor growth and movement. Stationary antennas were mounted at the inlets to Lake Råsjøen, a portable antenna was used in the tributaries, in addition to registration of previously marked brown trout upon recapture. The findings in this study indicates that the ceased lime treatment in tributary D were premature, based on the low abundance of brown trout compared to the other tributaries. Furthermore, tributary D has the lowest pH and ANC levels of the four tributaries. Tributary C does not show a low abundance of brown trout, but there are indications of a negative development as no female spawning brown trout are recaptured there and periodically low pH and ANC levels. Tributaries A and B are still treated with lime and show relatively high densities of brown trout and returning female spawning brown trout. The water chemistry of tributaries A and B are also more stable, compared