VN Thurzov č. 1 a 2

• Water hardnessLevel of evidence: Hypothesis

  ≈ 6.4 °dH (1.2 mmol/l of calcium and magnesium), ŠGÚDŠ atlas

  Moderately soft water: productivity is average.

  What we know about it

  Water with calcium and magnesium is more productive and buffers acidity swings better: trout production was higher in calcareous hard waters (Almodóvar et al. 2006), and an index based on dissolved solids and depth predicts fish yield from lakes (Ryder et al. 1974). In Irish streams, however, geology did not change trout density (Lehane et al. 2004), and Downing et al. (1990) showed that production is related rather to primary production. Hence only ±1 point. Hardness comes from the ŠGÚDŠ atlas, not from a measurement in the fishing district; it replaces the separately assessed bedrock of the basin.

  Sources: Almodóvar et al. 2006; Lehane et al. 2004; Ryder et al. 1974

  69
• Water acidity (pH)Level of evidence: Partially supported

  pH 7.8, ŠGÚDŠ atlas

  The pH is within the range in which fish commonly thrive.

  What we know about it

  Below about pH 6 fish suffer, and trout and grayling are more sensitive than cyprinids (Alabaster and Lloyd 1982); in acidic soft water, aluminum is released from the soil and damages gills (Gensemer and Playle 1999). The pH from the atlas is a model estimate, so the reduction is only −1 or −2, and only where we have no pH measurement (which is assessed by a separate factor).

  Sources: Alabaster and Lloyd 1982; Gensemer and Playle 1999

  100
• MineralizationLevel of evidence: Hypothesis

  ≈ 178 mg/l, ŠGÚDŠ atlas

  Medium mineralization corresponds to productive streams and reservoirs.

  What we know about it

  Dissolved solids (mineralization) are related to the fertility of the water, which is why Ryder used them to estimate fish production in lakes (Ryder 1965, Ryder et al. 1974). Downing et al. (1990) showed, however, that primary production is a better predictor of production. It does not enter the score, only the suitability index.

  Sources: Ryder 1965; Ryder et al. 1974; Downing et al. 1990

  85
• Metals in sedimentLevel of evidence: Partially supported

  10 elements among the highest 10% in Slovakia (ŠGÚDŠ atlas 1999)

  Metals in sediment can accumulate in benthic animals and fish; high values are often natural in ore areas.

  What we know about it

  The toxicity of metals to fish decreases with water hardness, because calcium competes with the metal for sites on the gills (Pagenkopf 1983). The ŠGÚDŠ atlas gives only the rank among streams, not a concentration comparable with a limit (MacDonald et al. 2000). In hard water we therefore rate metals more leniently, in soft water more strictly.

  Sources: Pagenkopf 1983; Gensemer and Playle 1999; MacDonald et al. 2000

  30

The index 0 to 100 compares the properties of the water with the requirements of the species (60 is the neutral value, 100 the ideal). The verbal rating comes from comparison with other fishing districts of the same type, because most Slovak waters have good values for their species. The index does not affect whether fish really occur in the fishing district (that is decided by stocking and the manager). Hardness, pH and mineralization from the atlases are historical and model estimates. Only a small part of this feeds into the conditions score and fishing district quality (hardness ±1, atlas acidity −2 to 0, stream zone match ±1 to quality).