Introduction: One of the species that is abundant in the Caspian Sea and is not exploited is the Atherina caspia species of the Atherinidae family. Due to its high abundance, this species plays an important role in the formation of ecological networks in aquatic ecosystems and provides connections between different trophic levels. Therefore, considering the significant reserves of this fish in the Caspian Sea and the Gulf of Gorgan, its biological and ecological studies are of particular importance and lead to a better understanding of this species. This fish was transferred from the Black Sea to the Caspian Sea by Russia between 1953 and 1954 and is present in all parts of the coastal waters of Iran, the Caspian Sea and its rivers and forms a large population (Esmaeili et al., 2014). Considering the wide distribution of this species in Eurasia, it has a high population diversity. In this study, this variability was investigated on a small geographical scale (southeast Caspian Sea region) to determine the possibility of diversity at the population level of a region. Therefore, the main goal of this study is to prove the existence of seasonal and habitat-based variability of some growth parameters.

Material and Methods: In order to conduct the relevant studies, sampling was carried out monthly from April to March 2019 for one year using a gillnet with a mesh diameter of 2.5 mm, a length of 20 m, and a height of 1.5 m. The total number of samples examined in this study was 14,791 fish. The samples caught on site were fixed with 5% formalin and after being transferred to the Gonbad-Kavoos University laboratory, they were biometrically measured. The total length was measured using a biosafety board with an accuracy of 1 mm, and the total weight of the samples was measured using a scale with an accuracy of 0.001 g.

Results and Discussion: The maximum length and total weight were 13.5 cm and 14.20 g for females and 12.2 cm and 9.92 g for males, respectively. The length-weight relationship in the population of male and female Caspian eel had a high correlation coefficient. The slope values ​​of the length-weight regression line (b) for this species varied between different seasons and showed a significant difference with the isometric growth coefficient (3). The value of this allometric coefficient showed the highest value in autumn. Comparison of this coefficient among different habitats also showed that there is a statistically significant difference in this coefficient among habitats (p<0.05). The condition coefficient of this species showed both seasonal and habitat diversity. In general, the results showed that the two basic population factors of this species, including growth pattern and condition coefficient, had high variability, which indicates the diversity of populations of this species. The results of the present study in Gorgan Bay showed that the growth pattern for both males and females was positive allometric, but the results of the study by Harak-Ata et al. (2018) in Gorgan Bay showed that the growth pattern for both males and females was negative allometric. The growth pattern of the studied population compared to studies conducted in the Sefid Rud River estuary area, the growth coefficient (b) was smaller than 3, indicating negative allometric growth for the population of A. caspia (Zahedi and Rahimibashar, 2015), which is contrary to the growth pattern in the present study. This study showed that the growth parameters of the Caspian Sea aster have high variability. 

Conclusion: This variability at the population level can guarantee the survival and presence of this species in the changing ecosystem of the Caspian Sea. This finding is of great importance. Species with low variability may suffer severe population decline or even loss of populations in the region under new ecological conditions.