Abiotic and biotic drivers of flatfish abundance within distinct demersal fish assemblages in a brackish ecosystem (western Baltic Sea)

The study explores the fine scale spatial and temporal distribution of the entire demersal fish and flatfish assemblages in a brackish ecosystem with a special focus on the abiotic and biotic drivers influencing the abundance of the three commercially and ecologically important flatfish species, namely flounder (Platichthys flesus), plaice (Pleuronectes platessa) and dab (Limanda limand). Using a 12 year time series obtained by a summer survey in the western Baltic Sea, at least three distinct demersal fish species assemblages were identified. Their spatial distribution was relatively stable over time and could be linked to environmental gradients. The demersal fish assemblages were dominated by few species and differed primarily in the percentage of common species. Both, the demersal and the flatfish assemblage were structured according to different habitat preferences. While there were species-specific differences, boosted regression tree modelling (BRT) showed that interannual fluctuations explained a large percentage of the variance in flatfish abundance whereby salinity, water temperature and sediment type were identified as the most important abiotic drivers. Dab was mainly influenced by sediment type and high salinity, while for flounder the main driver was water temperature. Plaice was also impacted by salinity, but was primarily influenced by biotic variables. The availability of benthic prey organisms in the area was verified as biotic driver for flatfish, especially for plaice. The results underline the importance of accounting for fine scale spatial heterogeneity in marine spatial planning in the Baltic Sea and comparable brackish water ecosystems and highlight the importance of considering benthic prey availability and diet analyses in demersal fish monitoring programs.