The use of Saprobi and Hilsenhof indicators in assessment the water quality downstream of Tajan River in order to release fish fry

The purpose of this study is to identify the Zooplankton and Macrobenthic communities and to determine the water quality using the Saproby and Hilsenhoff indices in the downstream of Tajan River with the aim of investigating the place of release fish fry. In this research, 7 stations were selected along the Tajan River from the downstream of Tajan Bridge to the mouth of the river and sampling was done monthly from May to October 2022. In Zooplankton sampling, 100 liters of water were filtered by a plankton cone net. Sampling of Macrobenthoz was done by Grab device (225 cm2) and Sorber sampler (1600 cm2) with 3 repetitions in each station. A total of 25 Zooplankton species were identified during one-year sampling in different stations from Holoplankton and Zoobenthoz groups. During this survey, 8 families including Chironomidae, Simuliidae, Baetidae, Hydropsychidae, Gammaridae, Lumbriculidae, Spionidae and Erpobdellidae were identified. The results showed that all the stations are on average in the β-mesosaprobity quality class with relatively polluted water. Based on Hilsenhoff biological index, the degree of organic pollution and water quality in different stations range from the high quality class (no pollution) in stations 6 and 7 (at a distance of 1 km from the coastline and the estuary) to the medium class with relatively significant pollution in station 1 and 2 (after Tajan Bridge and 15 km below Tajan Bridge). According to the results, it can be said that these indicators are suitable for evaluating the water quality of Tajan River and more suitable indicators should be used for stations 6 and 7. Station 4 at a distance of 7 km from the sea coast (Soteh) had the best water quality conditions with little pollution, thus it seems that the best place to release fish fry in terms of water quality conditions is in Soteh. Due to the proximity of the water quality distance in the study area, habitat parameters should be taken into account to determine the exact location of the release of fish fry.