The Osprey population associated with Darwin Nature Reserve and the Rybinsk Reservoir increased from only a few pairs prior to the creation of the reservoir in the late 1940s , to about 45-50 pairs in 1994. Productivity rates were excellent in 1988 and 1989 (1.38 young/occupied nest), but extremely low in 1987 (0.47 young/occupied nest). A chemical spill into the Volga River in early 1987 resulted in a massive fish kill, which was believed responsible for low production that year. With the exception of the year of the chemical spill and 1992 (the year an egg was collected from 10 of 11 nests studied), production was comparable to rates observed in stable or increasing populations in Byelorussia, Finland, and Sweden. The p.p'-DDE (DDE), polychlorinated biphenyl (PCB), and mercury concentrations from eggs collected in 1992 were below known effect levels and eggshell thickness showed only 6.3 % thinning - an amount not associated with reproductive problems.