Fecundity of the mayfly Ephoron virgo (Olivier, 1791) (Ephemeroptera: Polymitarcyidae): A long-term study in the River Rhine

The fecundity of the mayfly Ephoron virgo in the River Rhine was studied over a period of twelve years (1992-2003) and compared to four European rivers in 1998. The field data were complemented by mass-rearing in flumes with fresh running river water (1999-2001). The dry mass of a single egg averaged to 4.15 ± 0.25 μg, without any significant spatial or temporal differences. Total egg mass per female was found as a reliable measurement of fecundity in E. virgo that significantly increased with increasing weight of a female. Within a sample, individual females differed tremendously in egg numbers. This high variability remained even under more homogenous conditions in a densely populated experimental flume. In 1998, fecundity of E. virgo in the Rhine tributaries of Main and Neckar was significantly lower than in the rivers Rhine and Lahn, averaging between about 2500 and 4100 eggs per female, respectively. Fecundity did not differ throughout the flight period at the Rhine. Though differences in fecundity between 1992 and 2003 were observed in the Rhine, no clear temporal tendency could be detected. Under extremely high population densities of E. virgo in experimental flumes, fecundity was slightly lower compared to parallel field samples. Based on these fecundity data, the decline of the population of E. virgo in the Lower Rhine is discussed with respect to some potential environmental factors, giving clear hints that it is not a simple effect of food limitation.