Distribution and abundance of phytoplankton in Iranian shores of the Caspian Sea and its relationship with reduction of Rutilus frisii and Chelon auratus catch

The aim of this study was to investigate the changes in the structure of the plankton community in the southwestern Caspian Sea and its relationship with the reduction of Rutilus frisii and Chelon auratus catches. The current study was conducted seasonally in the coasts of Guilan provience from 2020 to 2021. A total number of 48 genera and species of phytoplanktons were identified. Tthe findings showed that the non-native species Pseudo-nitzschia seriata from chrophyta and the toxic species Nodularia spumigena from cyanobacteria formed the highest density. The increase of water temperature and nutrition and the decrease of wind speed were directly related to the bloom of phytoplanktons, especially cyanobacteria in the Caspian Sea, which caused the third bloom of N. spumigena in Anzali shores in July 2021. The results showed an increase in the abundance of phytoplankton by more than 7-folds compared to 1996. The highest abundance of phytoplankton was observed in Anzali shores in 5 m depth. The pattern of phytoplankton abundance and the abundance of R. frisii and C. auratus juveniles were similar to each other.  The food eaten by C. auratus and R. frisii juveniles showed that these fish species fed on phytoplankton detritus. Probably the reduction of Cerastoderma glucaum biomass, illegal fishing with gillnet, the quality and quantity of R. frisii release by Iranian fisheries, and the lack of restoration of the rivers where the natural reproduction occurs are the factors affecting the reduction of the R. frisii catch in recent years.