The obtention of time series in the structure and biodiversity of marine populations is of great interest in spite of increasing anthropic perturbations of direct type (e.g. fishing or littering) or of more indirect nature as the climate change. However, the lack of reliable and long-lasting sampling/observational technology deeply constrains our understanding of ecosystem functioning and its response to any perturbation. For the first time, we continuously monitored the seasonal variation in individuals for different fish species within a Western Mediterranean marine protected area by means of the cabled video-observatory OBSEA (20 m depth). During 1 year (from June 2009 to July 2010), frames for fish visual counting were acquired by a digital still camera portraying two different reef and water locations, at each daylight hour within the two central weeks of each month. In images we reported the total of counted fishes, the total of fishes per species, the number of species, the Shannon Diversity and Mean Trophic Level. A comprehensive faunistic list including not only fishes but also diving seabirds was obtained. Significantly higher number of species and their abundances were observed in images including the artificial reef location versus those of the water column. A seasonal variability was observed in Diplodus vulgaris, Chromis chromis, Diplodus sargus, Diplodus annularis, Dentex dentex, Spondyliosoma cantharus, Symphodus melanocercus, having this phenomenon an effect on monthly Shannon Diversity Index estimates. The seasonal variation of these species was not linked to their Trophic Level. Moreover, a weak intra-day (i.e. morning versus afternoon) variation in fish counts was also reported, being not significant. Present results indicate that cabled video-observatories are a reliable and powerful method to monitor seasonal changes in species composition and biodiversity, potentially providing important data for the modelling of marine ecosystem functioning.