Embryonic mortality and morbidity were seen in an indigenous population of lake trout (Salvelinus namaycush) inhabiting a lake experimentally acidified over a period of 3 yr to a mean summer epilimnion pH of 5.84. The pH of the hypolimnetic zone where trout spend the summer and undergo gametogenesis was 6.2. Although fertilization appeared to be successful, 59% of eggs incubated in the acidified lake had died or failed to gastrulate by 15 d, and 60% of the surviving embryos displayed gross anatomical malformations. Prior to hatching only 6% of the eggs contained embryos and all were anomalous. In a control lake, 92.9% of eggs contained normal embryos prior to hatching. Both the volume and dry weight of eggs of fish inhabiting the acidified lake were significantly less than eggs from the control lake. When gametogenesis and fertilization occurred in the acid lake but embryogenesis occurred in untreated water in the laboratory, there was a 50% reduction in the viability of eggs of one fish by day 31, compared to controls.Key words: lake acidification, teratogenesis, lake trout
