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Oithona similis is an abundant but poorly studied cyclopoid copepod in the brackish Central Baltic Sea. We describe the spatio-temporal distribution of O. similis in a Central Baltic deep basin (Bornholm Basin) during spring and summer 1999. Using vertically resolving sampling in parallel with hydrographic measurements, we found the copepod to dwell in the permanent halocline characteristic of a Central Baltic deep basin. The habitat of O. similis is thus limited from above by low salinity and from below by low oxygen conditions, both characteristic for the area. Horizontally resolving sampling yielded abundance surfaces which were compared by analysis of variance showing similar patterns among sampling dates. Comparison with flow fields from a three-dimensional hydrodynamic model suggests that the horizontal distribution is primarily the result of circulation in the dwelling depth. The study shows how the physical environment in the area determines the spatial distribution which might affect abundance and production of this copepod. [ABSTRACT FROM AUTHOR]

Marine fisheries are increasingly impacted by climate change, affecting species distribution and productivity, and necessitating urgent adaptation efforts. Climate vulnerability assessments (CVA), integrating expert knowledge, are vital for identifying species that could thrive or suffer under changing environmental conditions. This study presents a first CVA for the Western Baltic Sea's fish community, a crucial fishing area for Denmark and Germany. Characterized by a unique mix of marine, brackish, and freshwater species, this coastal ecosystem faces significant changes due to the combined effects of overfishing, eutrophication and climate change. Our CVA involved a qualitative expert scoring of 22 fish species, assessing their sensitivity and exposure to climate change. Our study revealed a dichotomy in climate change vulnerability within the fish community of the Western Baltic Sea because traditional fishing targets cod and herring as well as other species with complex life histories are considered to face increased risks, whereas invasive or better adaptable species might thrive under changing conditions. Our findings hence demonstrate the complex interplay between life-history traits and climate change vulnerability in marine fish communities. Eventually, our study provides critical knowledge for the urgent development of tailored adaptation efforts addressing existing but especially future effects of climate change on fish and fisheries in the Western Baltic Sea, to navigate this endangered fisheries systems into a sustainable future., (© 2024. The Author(s).)