Juniper Publishers - Journal of Oceanography Abstract The European eel has been dwindling since the eighties of the previous century. Animals exhibiting temperature dependent sex determination (TSD), including eels, are perhaps most vulnerable to a warming of the Earth as highly skewed sex ratios can result, potentially leading to population extinction resulting from decreased male recruitment. Here we report based on time series dating from 1930 of annual catch of male and female silver eels from eight sites that are open for glass eel influx around Lake Ijssel, the Netherlands. We observed that the sex ratio was skewed gradually towards females. The significant female-skewness of the sex ratio validated that the population of European eels at these sites has declined to a very low level since the 1930s. From several field and laboratory studies we have indications that Environmental Sex Determination (ESD) is prevalent and that the temperature determines the ultimate sex of
