Recovery of young brown trout (Salmo trutta) in acidified streams: What are the critical values for acid-neutralizing capacity?
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The recovery of young allopatric brown trout (Salmo trutta) grouped into YoY (age 0þ) and older parr (age 1þ) fish, was studied in acid-sensitive streams in a Norwegian watershed during a 24-year-period (1987e2010). Their abundance was assessed by electrofishing. Most sites typically had 5.0e5.5 in pH, 0.4 e0.7 mg L 1 Ca, 10e20 mg L 1 inorganic toxic aluminum (Ali) and acid-neutralizing capacity adjusted for organic acids (ANCOAA) of - 15 to þ25 meq L 1. Densities of both YoY and older parr increased significantly during the study period. Water quality also improved in recent years with respect to pH (5.8e6.0), Ali (5 e15 mg L 1) and ANCOAA (10e20 meq L 1). However, some negative trends in both fish density and water chemistry were found during both the first (1987e1993) and last years (2004e2008) of the study. Initially, YoY densities remained at about 16e20 specimens 100 m 2 (1987e1990), declined to 10e15 specimens 100 m 2 in the early/mid 1990s, and rosed to 30e50 specimens 100 m 2 in recent years (1997 e2010). Their densities correlated significantly with ANCOAA, and at least three stages in the recovery process were recognised: (i) Low density with 10e20 specimens 100 m 2 at 18 to 5 meq L 1, (ii) medium and unstable density with 20e30 specimens 100 m 2 at 5 to 10 meq L 1, and (iii) increasing density to 40e50 specimens 100 m 2 at 10e25 meq L 1. The decline in brown trout density in the earlymid 1990s coincided with high sea salt depositions, which caused increased acidification. Component 1 in a PCA explained 51% of the variation in fish densities, including conductivity, Mg, Ca, Na, alkalinity and TOC. Component 2 explained an additional 31% of the variation, including pH, Ali and ANCOAA. Multiple regression analysis coefficients showed that the two components explained 41% of the variance in total fish density. Young brown trout suffered a high mortality during the initial phase of the study in spite of relative low levels of Ali. This is probabaly because the study streams have very diluted water. The densities of young brown trout have levelled off in recent years, indicating a development towards reaching carrying capacity and hence full recovery. However, still some annual fluctuations in density are recorded, which may be related to an unstable water chemistry.