Pollutants acting as oestrogens are responsible for this phenomenon which, among other changes, is causing ovocytes -- immature ova -- to appear in male fish, biologists claimed.
The acquisition of feminine features by male fish has been detected, to a greater or lesser extent in all the estuaries -- not only in the characteristics of the gonads of the specimens analysed but also in various molecular markers.
"The results show that endocrine disruption is a phenomenon that has spread all over our estuaries, which means that, as has been detected in other countries, we have a problem with pollutants," explained Miren P. Cajaraville, director of the group of Cell Biology in Environmental Toxicology at the University of the Basque Country.
The team has conducted research using thick-lipped grey mullet and has analysed specimens in seven zones in coastal Spain -- Arriluze, Gernika, Santurtzi, Plentzia, Ondarroa, Deba and Pasaia.
The results of the research were published in the journal Science of the Total Environment and Marine Environmental Research.