Recruitment mechanisms of Atlantic cod ( <i>Gadus morhua</i> ) living in an extreme low-salinity environment

Abstract A better understanding of recruitment mechanisms in fishes is a high priority for operational management, as it can improve both stock assessment models and impact assessments, given that early life stages are particularly sensitive to changes in the environment. The impacts from anthropogenic pressures, including climate change, can be especially severe in extreme environments, where fish are already living close to their physiological limits. In the brackish Baltic Sea, Atlantic cod (Gadus morhua L.) exhibits unique adaptations to the low salinity, but suffers from long-term overexploitation and a loss of spawning areas due to climate change and eutrophication. At the northern edge of its distribution range, in the Åland Sea (ICES subdivision (SD) 29N), a healthy stock component continues to thrive, despite living in an extreme low-salinity environment. Here, we study recruitment mechanisms of this stock component, to understand where cod in the area reproduces. Cod collected by commercial gillnet fishers at 220-250 m depth in May-June 2019 had a mean weight of 2.6 kg and were all in spawning condition. Laboratory experiments showed that fertilisation and egg development was not successful at a salinity corresponding to the Åland Sea (7.5 psu), but to some extent at 9 and 10 psu. Hydrographical modelling suggests that, in addition to known spawning areas further south, conditions in the northern Baltic Proper (SD 27, 29S and 32) may occasionally be suitable for reproduction. Larval drift models show that these areas are highly connected to the Åland Sea and may be important for recruitment, although larvae can also drift in from known spawning areas in the Gdansk and Gotland Basins (SD 26 and 28). To fully understand the recruitment mechanisms, further studies on the migratory behaviour, genetic population structure, and occurrence of eggs and larvae are needed.