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In this Study, 186 pieces of Brown trout, salmo trutta fario were captured monthly from June to December 2005 with drift gill net from the Lar reservoir. The age of samples varied between 2 to 5 years with a mean of 3.0 ‌‌± 0.7. Average of fork length and weight respectively were 27.5 ± 6.0 cm and 235.1 ± 124.0 gr. Index of fullness average was 148.2 ± 85.9 that indicates an undesirable status of fish feeding. Average of relative length of gut was 0.47 ± 0.08 and condition factor were determined 0.91 ± 0.14. Gut contents of Brown trout in this lake included Daphniidae, Chironomidae and Cytheridae families with following percentages respectively 63/970, 17/981 and 13/034 which were the main preys of the fish. Although Tubificidae with 77.12% were the most frequent benthos organisms but their frequency in the digestive system of fish was 0.002% which showed fish reluctancy for consuming them. In addition to cannibalism, a wide range of macrobenthic invertebrates including aquatic insects, terrestrial organisms and mollusks of the Limnaeidae family were identified in fish diet. The results of this research indicated that Brown trout consumed a wide range of organisms belonging to the river ecosystem in addition to lake organisms in their natural diet that was due to frequent movement of fish between two ecosystems.

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Benthic Fauna study was conducted in four rivers (Dalichahi, Absefid, Elarm and Kamardasht) that leading to lake Reservoir behind the lar dam in 12 smpling larintervals and sampled monthly during June to October in the years 2009 and 2010. Samples were conducted using the Sorber (dimensions 40×40 cm square) in the four stations of the rivers with three replications. Samples collected and fixed by 4 percent formaldehydesolution then after isolation in the laboratory, the benthos were identified and counted. According to results 15 Order of Benthic were identified, among them anaquatic insect larvae had greatest diversity. species belonging to two orders Diptera and Ephemeroptera were the dominant organisms at all stations are. Mean total density and biomass were 13104 /m² and 70.62 g/m² respectively. The above Order had 85 genus and species. Order Diptera with a mean frequency of 10234 /m² (60/22 gr/m²) in the first rank, and then orders Ephemeroptera with a mean frequency of 1680 /m² (2.33 gr/m²) was the second highesity. The Most Macrobenthos order was Diptera with frequency in the month Jully total number of 2569. Based on Kruskal-Wallis test except the insects, in terms of abundance and biomass, has had a significant difference (P<0.05). Density at different depths within the sediments studied organic mater rates in sediments, a high correlation (R=20.92 - 0.98) was observed. The amount of organic matter in sediments at different depths in the summer with another season of the year showed a significant difference (P<0.05). there was no statistically significant difference in Total Macrobenthos biomass between 4 stations in the river, However, the abundance of organisms in the stations were significant different (P<0.05).