This study investigated the effect of hydrological stress (streamflow reduction) on the reproductive and nesting behaviour of freshwater blenny (Salaria fluviatilis) males in a Mediterranean-type stream in NE Spain (a tributary of the Ebro River). The investigation included two study periods: (i) before stream flow reduction (sampling in 2004) and (ii) afterwards (study year 2011). Nesting males in 2004 (N = 31) and 2011 (N = 11) were measured, weighed and photographed in the field. The size of the total egg cluster (male reproductive success) was measured and photographed for each nest found (N = 137 in 2004 and N = 28 in 2011). Nesting area was measured to determine nest density for each sampling period. The degree of secondary sexual traits (SSTs) development was measured later through the photographs. The age of males was assigned according to the length-intervals established by Vinyoles and De Sostoa (2007) for this species in the same study area. After flow reduction in 2011, the flooded area of the river bed was reduced by more than 80%. A great proportion of small males (1 year old) with developed SSTs (cephalic crest and anal glands) were found to defend a nest. This is the first time that parental care is found for the male of the freshwater blenny at a young age. Total cluster size (mean SE) is halved under the low flow conditions (from 45.4 2.8 cm2 to 22.9 2.7 cm2), but an increase in the number of partial clusters per nest was found. This study highlights the sexual plasticity of freshwater blenny males depending on environmental conditions and the vulnerability of this endangered species to the hydrological changes of anthropogenic origin in Spain.