Determinants of fish distribution pattern and habitat preference in protected Karaj River, Iran

Lack of information about ichthyofauna makes it difficult to predict human effects on the communities. In this study, the effects of the environmental and human factors were examined on altitudinal patterns of fish habitat selection, faunal changes, species richness and fish abundance from upstream to downstream in the southern part of the central Alborz Protected Area during 2016 and 2017. 366 fish individuals of five species in three families were identified and brown trout was the dominant species. The river was divided into three zones based on habitat structure, species composition and influence of physical and chemical variables. The results indicate that, in some situations, including small changes in habitat structure indices, physical and chemical variables along the altitudinal gradient are more effective in habitat preference, describing the composition, and abundance. Chemical variables can help to have a better understanding of the conventional patterns, shaped by the habitat structure and physical variables. Habitat suitability index model for brown trout in three zones is completely different over the two years and this species could overcome these conditions. However, upstream areas were more appropriate. Dam, with decreases of outlet water temperature, also creates homogeneous and simple habitats leading to changes in expected fish distribution patterns with decreasing altitude. These conditions may have increased habitat suitability index for the brown trout in downstream, while despite favorable ecological conditions in the middle zone, habitat suitability index and alternative habitats available in this zone are likely to decline due to density and interspecific competition. The natural pattern of fish distribution and habitat selection varies along the altitudinal gradient due to various factors and human disturbances. Optimal management of protected areas can control the effects of some of these factors at least temporarily.