Intersex in wild roach (Rutilus rutilus) from Danish sewage effluent-receiving streams

Roach (Rutilus rutilus) from Danish streams that receive discharges of domestic sewage effluent were examined for the presence of alterations to gonadal development. In male roach, intersex was observed at a prevalence of 4.5-5% at reference sites and 6.7-6.5% at sewage effluent-impacted sites. A positive correlation was found between sewage effluent load and intersex frequency among male roach. The highest frequency of intersex (26.5%) was found in the stream Kristrup Landkanal, which had the highest percentage and load of sewage effluent (87,578 population equivalents). Further, a tendency to an average higher severity of the phenomenon (calculated as an intersex index value) was seen in males from streams impacted by sewage effluent compared to males from reference sites. However, this was significant only in male fish from Egaa Brook. Among roach from the Kristrup Landkanal 5.8% of male and 0.8% of female roach were infected with the parasite Pleistophora mirandellae, causing degenerative changes in testes and ovaries. No correlation was seen between the intersex condition and the presence of P. mirandellae in the gonads of roach.