How exactly do cod and flatfish live and survive in the brackish water environment of the Baltic Sea? We equip fish with transmitters so that they can tell us about their lives - and we better understand how they behave under changing environmental conditions.
The cod stock (Gadus morhua) in the western Baltic Sea is at an all-time low. Environmental changes and high fishing pressure are the most likely causes for the observed decline. Despite a recent drastic reduction in removals, the stock shows no signs of recovery. Poor recruitment over many years and thinner cod (Receveur et al., 2022) suggest that unfavorable environmental conditions play an important role in the decline. In particular, the warm summer period appears to be a bottleneck period for cod in the relatively shallow western Baltic Sea (Krumme et al. 2020), as they tend to prefer cooler temperatures.  
Little is known about the individual movements and behaviors of western Baltic cod (Hüssy et al., 2020). Recent research has shown that the fish exhibit clear seasonal patterns in habitat use (Funk et al. 2020). For example, Belts Sea cod use deeper, saltier, and muddier basins only during winter spawning. After spawning, the distribution shifts to shallower areas on slopes to feed, where cod show a clear preference for areas with hard substrate. As the surface waters warm, cod are then forced to move to deeper areas on the slopes to spend the summer months. Once the shallow waters cool in October, cod regularly move from their daytime resting sites in deeper areas to their nighttime feeding sites in very shallow water (Burrows et al., 1994; Freitas et al., 2016). Slopes appear to play a key role for cod as a transitional area during their seasonal and diurnal movements.
To better understand cod life history, we investigate habitat use, movements, and behavior of adult western Baltic cod in a natural slope area in southern Mecklenburg Bay. Results from a nearby cod tagging study on an artificial reef (Nienhagen Reef) in the western Baltic Sea indicate high site fidelity of cod to reef structures. However, it is unclear whether this behavior is representative of cod behavior outside this artificial habitat (Dolk, 2015; McQueen et al., 2019). 
The primary focus of this study is on cod, but because flatfish are also an important part of the community and cod densities are currently relatively low, we are investigating the life history and movement patterns of cod and flatfish together in this area. Flatfish are currently the most important demersal resource in the Baltic fisheries and more detailed knowledge of their life history is of great importance for sustainable exploitation. Details on the movement patterns of flatfishes in the western Baltic Sea are poorly known and findings from studies in other marine areas are of limited transferability due to the specifics of the Baltic Sea.
The objective of this study is to use acoustic telemetry and data loggers to investigate how individual cod and flatfish in the western Baltic Sea use a slope area with a typical habitat mix of rocky areas, sandy areas, and seagrass and algal beds. Specifically, the research questions are: 
1) To what extent do cod and flatfish show site fidelity in a natural slope area in the western Baltic Sea?
2) How do cod and flatfish move and behave in this slope area over time (day, sunset, night, sunrise; season) and in interaction with fluctuating environmental conditions?