Growth patterns and the shift from predominantly female to balanced sex ratios in Carassius gibelio (Bloch, 1782): a comparative study of lotic and lentic populations in Serbia

Since its introduction into Serbian watercourses, the invasive Carassius gibelio (Bloch, 1782) (Prussian carp) has spread rapidly. This paper provides an up-to-date overview of the sex ratio, age structure, and growth parameters in Serbian populations of this fish and describes how growth varies in two contrasting ecosystems: lotic and lentic. A total of 133 individuals were sampled. The individuals from lentic habitats were longer on average, with no significant differences in average weight or age between the two ecosystem types. There was a difference in the sex ratio between the ecosystems, as indicated by a higher proportion of males in lentic waters (43.42%), than males in lotic waters (28.07%). Positive allometric growth was observed in both ecosystem types (b = 3.1693 and 3.0316). The individuals from lotic habitats had better growth performance and greater potential for reaching larger sizes (L∞ = 59.50 cm, K = 0.09, t0 = −0.07), whereas those in lentic waters grew faster and more efficiently (L∞ = 43.02 cm, K = 0.12, t0 = −0.30). The proportion of males was higher than anticipated for species known for gynogenesis in both ecosystem types, especially in lentic habitats. This suggests that Prussian carp has undergone naturalization events in Serbian watercourses over nearly 7 decades—a strategy that could increase genetic diversity and promote long-term population stability.