Distribution and Diversity of Coastal Fish in Fresh and Oligohaline Waters: An Example of the Largest Baltic Sea Estuary of the Neva River

ABSTRACT Understanding how low salinity gradients (0‰–5‰) influence fish life cycles is critical for clarifying principles of fish assemblage formation in estuarine ecosystems. This study is intended to provide a foundation for predicting changes in fish populations amidst ongoing surface salinity decreases in the Baltic Sea. In the Neva River Estuary, differences in fish diversity and distribution between fish assemblages in freshwater and oligohaline areas were assessed using beach seine and multi‐mesh gillnet surveys. The use of multiple sampling gears greatly enhanced understanding of fish distribution across salinity gradients. Fish assemblages were analysed depending on the different areas of the estuary, seasons of sampling, salinity and confounding environmental variables such as water temperature and depth. Salinity was the major influencing factor for the surf zone fish assemblages and a season of sampling was the main factor for coastal assemblages of larger fish studied with the gillnets. Fish assemblages in the surf zone exhibited significant but gradual changes in freshwater and marine‐estuarine fish occurrence and abundance along the critical oligohaline barrier ‘δ‐horohalinicum’ (0.5‰–1.9‰). Although freshwater species showed high occurrence and abundance throughout the estuary, their juveniles were less abundant in the oligohaline surf zone with salinity ≥ 2‰, where small‐size marine‐estuarine and euryhaline species predominated. Coastal multi‐mesh gillnet catches showed remarkable similarity between assemblages of larger fish in freshwater and oligohaline areas. Differences in species richness and diversity indices were not pronounced throughout the salinity gradient; the decline in freshwater species was offset by an increase in marine‐estuarine and euryhaline species.