An investigation on the effects of fish farming in marine cages on abundance and structure of Mnemiopsis leidyi and Beroe ovata (Ctenophora: Lobata) in the southwestern Caspian Sea during 2018-2020

The purpose of this  study was to investigate changes in abundance of combs and non-biological parameters around fish cage in southwestern Caspian Sea. This study was conducted with 3 stations near the cage fish farming site and 3 reference stations far from the cage during 2018-2020. Total number of Mnemiopsis leidyi in the stations near to fish farming site (13200 ind.m-3) increased by 42% compared to far stations from the fish farming site (9500 ind.m-3) in 2018-2020. M. leidyi had the highest monthly abundance at the station near the cage fish farming site with 3011 ind.m-3 in February 2019. The lowest abundance of M. leidyi was observed at the far station from cage fish farming site almost 4.0 ind.m-3 in February 2020. The length group of less than 5 mm dominated M. leidyi populations by more than 96% and was the predominant M. leidyi population at the cage fish farming site. The findings showed, the abundance of B. ovata fluctuated between 27 and 47 ind.m-3 at near the stations cage, notably B. ovate was not observed at stations far from the cage fish farming site. CCA analysis confirmed that there was a strong relationship between M. leidyi abundance and nutrients levels (r=0.99). Increasing the amount of nutrients generated from feed and excretion of fish farming in cages is one of the main reasons for increase in the abundance of M. leidyi and B. ovata at stations near the fish farming site. Therefore due to the closed environment of Caspian Sea, without investigating cage culture effects on native fauna and flora, development of fish farming sites in sea cages is not sustainable.