Biogeography and fish community structure in Irish estuaries

Abstract Estuaries represent important transitional waters where marine and freshwater ecosystems meet and mix. Estuaries are dynamic systems due to their tidal nature and their ecology is expected to shift with climate change and the arrival of new species. Spatially extensive descriptive studies that provide temporal baselines for species distribution and abundance, and associated environmental variables deliver a comprehensive reference point against which to monitor change. To provide this biogeographic context for Ireland, which is surrounded by temperate Atlantic waters, the biogeography and fish community structure of Irish estuaries was examined using a large dataset comprised of 208,313 individual fish, 80 different species sampled from 37 estuaries from 2008–2017. Species richness was strongly correlated with the area of shallow littoral and subtidal habitats. Estuaries at higher latitudes tended to have lower species richness in shallow littoral areas. Estuary mouth width and proportion of subtidal area were both positively related to species richness in subtidal habitats. The main driver of estuary clustering is the changing proportion of estuarine species versus marine migrants. The results show a dominant group of larger, more open estuaries where marine migrants consistently dominate the fish population. This study confirms the role of marine migrants in contributing to fish community composition in estuaries and highlights the vital nursery function that Irish estuaries perform for estuarine-dependent marine fish species. The associations between fish community structure and broad-scale environmental parameters provide a time stamped baseline for monitoring future climate change impacts on estuarine fish assemblages in Ireland.