Occurrence and temporal variation in the size-frequency distribution of 2 bloom-forming jellyfishes, Catostylus perezi (L. Agassiz, 1862) and Rhizostoma pulmo (Cuvier, 1800), in the Indus Delta along the coast of Sindh, Pakistan


Abstract: Blooms of jellyfishes were studied at 3 sites, namely Bhanbore, Mirpur Sakro, and Keti Bunder. The sampling was carried out from April 2005 to March 2006. Only 2 species of jellyfish, Catostylus perezi and Rhizostoma pulmo, were found in samples. C. perezi was present at all 3 sampling sites, whereas R. pulmo was found only at Keti Bunder. Specimens of C. perezi appeared in the subsurface waters for a period of 6-8 months during April to August 2006 and February to March 2006. The smallest specimens of C. perezi appeared in January, and a gradual increase in size was observed in the population until August. The smallest specimens of R. pulmo were found in February, and their largest specimens occurred in July. The numeric abundance of medusae in zooplankton was also studied. They were more abundant at Mirpur Sakro, with a peak in December and January, while at Keti Bunder their highest number was noted in May. Their number at Bhanbore was comparatively lower than at the other 2 sites. No significant difference in physical oceanographic factors such as salinity, pH, dissolved oxygen, and suspended load of the sampling site was found, except for water temperature.

Keywords: Northern Arabian Sea, medusae, blooms

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