River water with high iron concentration and low pH causes mortality of lamprey roe and newly hatched larvae

The effects of prevailing water quality of Perhonjoki River in Western Finland on the hatchability of lamprey roe and survival of newly hatched larvae were investigated. The river flows through sulfide-rich soils, and is characterized by high average total iron (1.5-3 mg liter-1) and aluminum (0.45-0.6 mg liter-1) concentrations and relatively low pH commonly 5.5 to 6.0, with acidification peaks below pH 5. The hatchability of lamprey roe was markedly reduced under these conditions as compared with ground water with low iron (0.002 mg liter-1) and aluminum (0.11 mg liter-1) concentrations but at the same pH. Furthermore, the hatchability of roe and survival of newly hatched larvae was especially affected by an increase in total iron concentration to values that may be observed after heavy rains and during the snow-melt period (4-6 mg liter-1). The results suggest that increases in total iron concentration may play a role in the reproductive failures of lampreys observed in Finnish rivers.