Significant changes in pelagic fish stocks of Clupeonella spp. in the south Caspian Sea

 The present study was carried out during ۲۰۰۶-۲۰۰۷ along the southern Caspian Sea. The species composition showed that the common kilka, Clupeonella cultriventris caspia, is dominant species with ۸۹% and then anchovy, C. engrauliformis, has shown only ۸.۷% while it was already dominant species in the past decades. The overall results indicated that common kilka was replaced with anchovy kilka as previous dominant kilka species due to invasion of Mnemiopsis leidyi to the southern Caspian Sea. Anchovy suffered greatly from M. leidyi so that the total catch dropped from ۹۵۰۰۰ mt in ۱۹۹۹ to less than ۲۰۰۰۰ mt in ۲۰۰۷. In our survey, ۱۴۱۵ fish specimens were collected randomly, including ۲۲۱ anchovy, ۲۸۱ bigeye and ۹۱۳ common kilka. The mean length of anchovy was ۱۱۷.۸ ± ۰.۲۷ mm, while it was about ۹۳.۰ mm during ۱۹۹۵ to ۱۹۹۸ before the invasion of M. leidyi. Analysis of age classes indicated the mean age of ۴.۵ ± ۰.۰۴ years for anchovy, ۳.۶ ± ۰.۰۳ for common kilka and ۴.۶ ± ۰.۰۸ for big eye C. grimmi. According to the age and the length structure of the specimens in this study, it could be concluded that in recent years, young fish population of anchovy has been decreased in comparison with the last decade. The results showed that some biological characteristics of kilka population were affected by M. leidyi. Recently fishing activities take place in shallow waters where schools of kilka are in suitable density for fishing. According to our results, ctenophore occupied the anchovy habitat where it was mainly more than ۵۰ m depth. The fishing has moved to areas with depths less than ۵۰ meters where it was the main reason to change in species composition. 

کلیدواژه ها:

Age structure ، Caspian Sea ، Clupeonella ، Species composition ، Mnemiopsis leidyi