Alternative reproductive tactics (ARTs) driven by environmental factors are common among fish. However, the flexibility of fish to adopt distinct tactics in response to the characteristics of their environment has received little attention. The aim of the present work was to study phenotypic plasticity in the adoption of dominant behaviour ("bourgeois tactic") by male freshwater blennies (Salaria fluviatilis (Asso, 1801)). For this purpose, two simultaneous experiments in aquaria were performed to examine the effect of social cues and nest abundance on the acquisition of secondary sexual characteristics (SSCs). Experiments were conducted with small (individuals without SSCs), medium-sized (1-year-old individuals), and large older dominant males (more than 2 years old), all collected in the wild. In experiment 1, the three sizes of males were combined to compare the development of SSCs depending on intrasexual context. In experiment 2, the effect of nest abundance (two nests vs. six nests) was tested for each size of male. Medium-sized males showed phenotypic plasticity in response to the environmental conditions simulated in the two experiments. The absence of larger dominant males was found to be the main factor enhancing SSCs and the onset of parental behaviour. Nest shortage also influenced the degree of cephalic crest development among medium-sized males. This knowledge helps to understand how the population of freshwater blennies still persists when it is reduced to young individuals during the summer droughts in Mediterranean streams.