comparison of human pressures and fish assemblages in the salmonid and cyprinid streams of the southern caspian sea basin

various human pressures are affecting the aquatic biocoenosis in running waters of iran - similar to many other areas of the world. overall 190 sites, 88 at salmonid (44 reference/impacted respectively) and 102 at cyprinid (50 reference/52 impacted) streams of the southern caspian sea basin were investigated in terms of human pressures present and the related impact on fish assemblages. in total, 30 potential human pressure variables acting on biological communities at four different spatial scales and associated with seven pressure types (land use, connectivity disruption, morphological alteration, hydrological alteration, water quality deterioration, biological (i.e. alien species, overfishing etc.) and non-predicted pressures) were identified. further, a regional pressure index was derived, taking additive effects of multiple pressures into account. our analysis showed that the most dominating human pressure in both zones was land use (i.e. urbanisation in salmonid streams and agriculture land use in cyprinid streams) followed by water quality and morphological pressures especially in salmonid streams and hydrological and morphological pressures in cyprinid streams. most sites of the salmonid zone were affected by double and triple pressures, whilst multiple pressures were dominating in the cyprinid zone. after analysing the fish assemblages and structural and functional traits, eight fish metrics remained in the salmonid zone and 25 metrics in the cyprinid zone – both for reference sites. the response of fish metrics on human impacts was tested with mann–whitney u test and regression analysis. some fish metrics (e.g. total species, herbivorous species) showed significant positive reactions to the human pressures in the salmonid zone but significant negative reactions in the cyprinid zone. furthermore, some metrics (e.g. number of alien species or eurytopic species) responded similarly in both zones. however, some fish metrics of the cyprinid zone (e.g. planktivorous species or phytophilic species) did not show any significant reaction.