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The study of monitoring the environmental effects of fish cageculture in the southern of the Caspian Sea in the Nowshahr region was carried out from the fall of 2017 to the summer of 2018. Sampling of macrobenthic invertebrates in four directions north, east, south and west around the cages, at 3 stations including station 1 (cage establishment area), station 2 (distance 200 m from the cage) and station 3 (distance 1000 m from the cage) face accepted. Sampling is done during four periods, including before the start of breeding, the beginning of breeding activity, the middle of the breeding period and the end of the training period. Sediment sampling was performed using Grab sediment trap with 3 repetitions. Specimen of macrobenthose were identified in the laboratory after isolation. In total, 142363 samples were isolated with a mean (±standard error) density of 618.9± 51.38 N.m2 and an average (± standard error) of biomass was 0.936±0.175 g.m2. The identified macroinvertebrates belonged to 4 calass, 6 orders, 6 families, 7 genera and 7 species. macroinvertebrates identified include Streblospio gynobranchiata from the family Sionoidae, Hypaniolla kowalewskii from the Amphartidae, Hediste diversicolor from the Nereidae, Pseudocuma grasiloides and Schizorhincus edoreloides from the Pseudocumalani , Amphibalanus improvises from Balaniidae and Cerastoderma glaucum from Cardiidae. The results showed that all species were present in all stations except S. edoreloides which was not present only in the cage stablishment area. In general, the lowest and highest frequencies in terms of density were related to C. glaucum and H. kowalweskii with 2.6% and 21.7%, respectively, where 3 species including H. kowalweskii with 21.7%, S. gynobranchiata with 21.3% and H. diversicolor with 20.4% formed the dominent of macrobenthos populations. In total, the frequency of species was 32% in the, 35% in the 200 m of the cage and 33% in the 1000 m of the cage. In general, in the cage establishment area, the highest density and biomass with mean (±standard error) were 1131.52±208.07 N. m2 and 5.248±1.408 g.m2, respectively, belonging to A. improvises and S. gynobranschiata with average (±standard error) density and biomass were ranked 715.95±89.8 N.m2 and 0.106±0 0.026 g.m2, respectively. Due to the physiological characteristics of A. improvises, for adhering to rock and wood substrates and also for adhering to the hulls of ships, it is known as glued ship. In this study, due to the presence of cage structures, its frequency in the shadow of the cage was higher than other benthic organisms. At a distance of 200 m from the cage, the highest density and biomass with average (±standard error) were 1389.35±128.6N.m2 and 0.386±0.046g.m2, respectively, which belong to species H. Kowalewskii and S. gynobranschiata with average density and biomass were 1148.85±118.8N.m2 and 0.209±0.036 g .m2, respectively. At a distance of 1000 m from the cage, the highest density and mean biomass (±standard error) are 1405.10±136.6 N.m2 and 0.378±0.031 g.m2, respectively, which belong to species H. Kowalewskii and S. gynobranschiata with mean (±standard error) density and biomass were 909.27± 90.25 N.m2 and 0.209 ±0.026 g.m2,respectively. Throughout the period, a significant difference between density and biomass was found between different macrobenthic invertebrates (p <0.05) but there was no significant difference between the density of macrobenthic invertebrates at different distances (p>0.05) but this difference was found in terms of biomass between distances (p <0.05), which could be due to the large size of some benthic organisms, especially C. glaucum and A. improvises It has a shell and thick limestone shell. Given that some of macrobenthic invertebrates, especially H. diversicolor, H. kowalwskii, S. gynobranchiata and C. glaucum has an infauna life, so the deposition of food waste not consumed by the fish in the cage and the excretion of fish waste in the cage can cause bioturbation in the bed and cause changes. Create them in the habitat, so to get more accurate information, long-term research on marine cages is needed. Key words: Density, Biomass, Macrobenthose, cage, Nowshar, Caspian Sea
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