The freshwater blenny, Salaria fluviatilis, and the marine peacock blenny,S. pavo, are closely related and can survive in both freshwater and seawater. Salaria fluviatilis is widespread in rivers and lakes in the vicinity of the Mediterranean basin, including Lake Kinneret (Sea of Galilee), and S. pavo is distributed in rocky parts of the Mediterranean coast. This study investigated the effect of salinity acclimation on routine activity level. Fish were acclimated for three months in 0%, 40%, and 100% seawater mixed with 100%,60%, and 0% dechlorinated tap water (<2, 14.4, and 36 ppt sea salt, respectively). The fish were then placed in aquaria equipped with IR beam projectors and detectors set up to count their activity level in the different salinities under a photoperiod of 14:10 L:D. S. pavo demonstrated a simple daily rhythm of activity, being active during the light phase, whereas S. fluviatilis was also active in the light phase, but demonstrated a bimodal activity pattern. In both species, activity decreased during the acclimation period. The bimodal daily activity pattern of S. fluviatilis might be a result of the daily wind regime on Lake Kinneret in summer (May–October). The activity reduction in lower or higher salinities (in relation to natural habitat) is suggested to be a tertiary stress response.