The Caspian goby, Neogobius caspius, is an endemic species of Gobiidae family in the Caspian Sea and it has ecological importance (as food for Caspian commercial fish such as sturgeons). The main aims of this study are, studying distribution, length, weight and age structure, sex ratio, diet, reproduction biology and morphological characters of this species in Guilan coast at waters of the Caspian sea and the sampling has been done monthly from Oct. 2005 to Sep. 2006 in 4 station from Astara to Chaboksar, with bottom trawl which had 12.5 meter length, 4.7 meter gape width and 2 mm mesh size in code end. The results showed N. caspius abundance is 9.82±11.93% of Gobiids and 7.92±10.10% of total fish numbers sampled in the study area (13824 specimens belong to 16 fish species) and there were significant differences among stations and seasons. CPUA of N.Caspius was estimated 52.5±105.9 ind/hec and 412.7±770.7 g/hec and there were significant differences among stations and seasons. Maximum body weight, total length and age were measured 65.10 g, 176 mm and 6 years old with an average 8.74±9.9 g, 84.83±28.4 mm and 1.83 years old, respectively and maximum age of males and females was 6 and 5 years old respectively and there were significant differences among stations and seasons and between sexes. It was estimated algometric growth model from length-weight regression, coefficient of K Von Bertalanffy growth model was estimated 0.42 and 0.68 in male and female, respectively. Sex ratio was 1.27 female to 1 male. Coefficient vacuity, relative gut length and intensity of 169 fullness were 8.72%, 0.76±0.17 and 267.7±263.3 for, respectively and Caspian goby fed on 30 different prey consist Zooplanktons (8 types), Benthic animals (15 types) and fishes (7 types) and young and yearlings have fed mainly mixed food (zooplankton and benthic animals) and adults on zoobenthose. Gammarids, Bivalvia, Cumaceae and Vormes constituted 44.7, 38.9, 24.8 and 19.7 % of prescence in full-gutted samples respectively, they were mainly preys (number and weight), hence this species is considered as euryphagus, carnivorous, benthphagus species. It was determined, Caspian goby spawns from March until last June, males and females take part in spawning in 2.8g and 65 mm and 1.3 g and 52 mm, respectively. Length at 50% maturity (LM50) was 80.2 and 77.5 mm in males and females, respectively. Maturity took place at 2 years old for both sexes, too. The absolute fecundity was estimated 109-1350 (451.6±216.3) eggs and relative fecundity 18-80 (36.8±9.2) eggs per one grams of body weight and diameter of ripe eggs was measured 1.31-2.60 (2.18±0.19) mm. Morphologically, there were 15 morphologically significant statistical differences between males and females and 15 differences among stations, sexual dimorphism was observed, too.
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