Effects of Acid Stress on the Distribution and Reproductive Success of Freshwater Fish in Dutch Soft Waters

The distribution and reproductive success of fish were examined in Dutch soft waters of varying pH. In total, 20 fish species were encoun­ tered. The lowest pH-values at which fish species were observed varied remarkably. Umbra pygmaea and Tinea tinea appeared to be extremely acid tolerant and occurred in waters with pH-values below 4.0. Abramis brarna, Cyprinus carpio, E’sox iucius, Misgumus fossidisj Perea fluviatilis and Scardinius erythrophthalmus tolerated moderately acidified water with pHvalues between 4.0 and 5.0. The remaining species were only recorded in waters with pH-values above 5.0. Only 17 species showed successful repro­ duction in Dutch softwater ecosystems. For Abramis brama, Blicca bjoerkna, Cyprinus carpio, Esox lucius and Scardinius erythrophthalmus especially the reproductive phase seemed to be very sensitive to low pH. Waters with nonlethal pH-values showed ageing populations of these fishes. The reproduc­ tive failure of these species at sublethal pH may be due to impaired gona­ dal maturation, spawning, fertilization, embryonic development and hatching. In extremely acid waters (plf<4.0) only Umbra pygmaea and Tinea tinea exhibited successful reproduction. The reproductive cycle of Umbra pygmaea was studied more in detail. Normal gonadal maturation of Umbra pygmaea and fertilization, development and hatching of its eggs were observed among a wide pH-range (3.5<pH<8.0).