Temporary Increase in Sea Salt Deposition Accelerates Recovery of Brown Trout (Salmo Trutta) Populations in Very Dilute and Acidified Mountain Lakes

Abstract

We studied the recovery of brown trout populations from 1970 to 2010 in acidified mountain lakes with low ionic content in southwestern Norway. A total of 181 test fishing surveys with gill net series were performed in 59 lakes. We found that the most significant recovery occurred during the 1980s and early 1990s. In this period, only limited improvement in the water chemistry related to acidification, i.e., pH, was observed. However, due to a temporary increase in sea salt deposition, water conductivity almost doubled during this period. In many of the mountain lakes in the study area, the brown trout populations are restricted by ion deficit. Moreover, greater ionic strength ameliorates the effects of acidification by increasing the tolerance to H+. Well-established relationships between conductivity and the relative abundance of brown trout (CPUE) explain the observed recovery. We conclude that the dynamics of the sea salt ion contribution should be taken into consideration wherever biological recovery in very diluted water qualities is being evaluated.