Assemblage structure of small sized fishes and reflections from differences in abiotic conditions

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The small-sized fish assemblages of 11 different shallow, structurally simple sandy beaches were compared on the SW coast of the Gulf of Finland. Sampling with small beach seine was carried out throughout the growth season in 2005. The differences in species richness, diversity and abundance among beaches were assessed and the significance of environmental factors, as exposure to wind, depth slope, surface area of shallow water, coarseness of sediment, temperature and turbidity for fish assemblage composition were investigated. A total of 21 species belonging to 12 taxa were recorded; Gobiidae, Gasterosteidae and Ammodytidae were the most representative during the study period. Major changes in the fish assemblages were observed on temporal scale: the hatching of fish larvae multiplied the abundance of fishes. The community composition of larvae and older fishes differed significantly. On the whole, shallow sandy beaches are not ideal reproductive areas for most of the species except for Gobiidae. The study shows that temporal and spatial variations in the small-sized fish assemblages are related to environmental conditions and biological dynamics. The extremity of temperature and lack of shelter for fishes are factors affecting the community composition and defining the species that live succesfully in harsh conditions. The results suggest that abiotic features together with biotic parameters regulate the small-sized fish assemblage at nearshore shallow environments.