Seasonal changes in population of the amphipod Gammarus aequicauda in Miankaleh gulf-Iran

Seasonal changes in population of the Gammarus aequicauda were studied in Miankaleh Gulf, located in the south eastern of Caspian sea margin, during one year. The purpose of this research was to study the seasonal fluctuations in population structure and biology of G. aequicauda. Four stations (Galoogah,Ashooradeh,Bandar Gaz, Khaje Nafas) were considered for collecting the specimens. G. aequicauda were found in all stations. There was not significant difference in population frequency of each station in various season(P≤0.05). But comparison of population frequency between stations showed that there was significant difference between stations (P≤0.05). This difference was considerable between Galoogah and other stations; highest frequency was observed at Galoogah station in summer (1263ind./m²).The mean value of the sex ratio was 0.9. Mean value of male and female body length was 9.49 mm and 8.36 mm. Immatures exist throughout the year. Gravid females were found in all seasons. Immatures to matures ratio was maximum in summer(1.02%).Breeding activity that was assessed by estimating the percentage of gravid females (number of gravid females divided by total number of females ×100) was minimum in winter(27%) and maximum in spring (59%). There was significant relationship between the number of eggs in marsupium of female and its body length. Considering that the result of this study,it seems G. aequicauda would be favorite species in fisheries activities as a food of aquaculture.