In order to implement the macro project "Biomass estimation of benthic resourcesin the Oman Sea and recording ecological parameters" by the Ferdows I research vessel in the coastal waters of Sistan and Baluchestan province in 2019 – 2020, the bloom of jellyfish (Crambionella Orsinifrom the family of Catostylidae) occurred in the region for the fourth time. The density of jellyfish in 2019 – 2020 has decreased dramatically compared to the previous years and has also covered a smaller area of the region. In the area covered by the aforementioned marine research patrol (from the Meydani region in the west to the Gwadar region in the east), the amount of catch per unit of swept area (CPUA) which represents the relative population abundance was 7491 kg per square mile for total aquatic organisms. The amount of catch per unit of swept area for the jellyfish (Crambionella Orsini) was 9.76 kg per
square mile which accounts for about 1.02 % of total aquatic organisms. Also, the amount of catch per unit of swept area was investigated in the five zones A, B, C, D, and E at depths of 10 to 20 m, 20 to 30 m, 30 to 50 m and 50 to 100 m, respectively. The bloom of jellyfish was not observed in the eastern areas of the province in the Gwadar and Beris regions, and its density in the zone C was 335 kg / m2, in the central and western regions of Chabahar (Gurdim, Rashedi, Pozm and Konarak), which showed the highest density as compared to those of the other four regions. Also, the highest amount of density was 145 kg per square mile which was observed at depths of 50 to 100 m. Based on the marine research patrols carried out to estimate the biomass of benthic resources in the region and according to the review of the history of bloom of jellyfish in the region, the bloom of jellyfish occurred for the first time in the coastal areas of Sistan and Baluchestan province in 2002 - 2003 and the jellyfish species was Crambionella Orsini which was similar to the jellyfish species in the bloom of jellyfish in 2019 – 2020. According to the project of biomass estimation of benthic
resources, the amount of catch per unit of swept area for this species in the study area was 26086 kg per square mile. It was reported that the abundance of jellyfish increased from the east to the west. The highest and lowest amounts of catch per unit of swept area were reported in the Gwadar region (42228 kg per square mile) and Meydani region (3573 kg per square mile), respectively. The bloom of jellyfish occurred for the second time in the coastal areas of the province in 2009 – 2010 and the family of jellyfish was Rhizostomatidae which was different from the family (Catostylidae) observed in the bloom of jellyfish in 2002 - 2003. The marine research patrols to estimate the biomass of benthic resources in 2009 – 2010 showed that the amount of catch per unit of swept area was 11297 kg per square mile. This index indicated that the abundance of jellyfish in 2009 – 2010 was lower than the abundance of jellyfish in 2002 – 2003. The abundance of jellyfish in 2009 – 2010 covered a large area of the region which was similar to what was observed in 2002 – 2003. The bloom of jellyfish occurred for the third time in the coastal areas of Sistan and Baluchestan province in 2015 - 2016 and the jellyfish species was not identified. According to the marine research patrols to estimate the biomass of benthic resources, the amount of catch per unit of swept area was 576 kg per square mile which was lower than the index of abundance in the past period (2009 - 2010).Looking
at the history of the bloom of jellyfish in the coastal areas of Sistan and Baluchestan province in 2002 – 2003, 2009 – 2010, 2015 - 2016 and 2019 – 2020, we can see that the bloom of jellyfish has increased over the course of four to seven years in the region, and their abundance has declined over time.There were certain periodic environmental conditions such as ocean currents, nutrients, sunlight, temperature, seasonal variations, predator decline and oxygen changes that made the jellyfish to form large masses and colonies. Ocean currents bring them together and this causes the jellyfish to form huge colonies which can lead to the bloom of jellyfish. In addition, the jellyfish can survive in nutrient – rich and low - oxygen water in competition with plankton. What is more important is the comparative study of the periodic bloom of jellyfish and the annual catch rate of the fish in the region. In this regard, the experience of 2002 – 2003 and 2009 – 2010 in Sistan and Baluchestan province indicated that the catch rate of some species of benthic organisms increased in the years following the bloom of jellyfish (2003 – 2004 and 2010 – 2011), which is likely to be related to the bloom of jellyfish and the issue of nutrients in the environment. In this regard, after careful consideration, the reasons can be cited.