Changes in population depth distribution and oxygen stratification explain the current low condition of the Eastern Baltic Sea cod ( Gadus morhua )

Abstract. During the past twenty years, hypoxic areas have expanded exponentially in the Baltic Sea, which has become one of the largest marine dead zones in the world. At the same time, the most important commercial fish population of the region, the Eastern Baltic cod, has experienced a drastic reduction in mean body condition, but the processes relating hypoxia to condition remain elusive. Here we use extensive long-term monitoring data on cod biology and distribution as well as on hydrological variations, to investigate the processes that relate deoxygenation and cod condition during the autumn season. Our results show that the depth distribution of cod has increased during the past four decades at the same time of the expansion, and shallowing, of the waters with an oxygen concentration known to be detrimental for cod performance. This has resulted in a spatial overlap between the cod population and low-oxygenated waters after the mid-1990s, which relates with the observed decline in cod mean body condition. Complementary analyses on fish otolith microchemistry also revealed that since the mid-1990s, cod individuals with low condition were indeed exposed to low-oxygen waters during their life. This study helps to shed light on the processes that have led to a decline of the Eastern Baltic cod body condition, which can aid the management of this population currently in distress. Further studies should focus on understanding why the cod population has moved to deeper waters in autumn and on analysing the overlap with low-oxygen waters in other seasons to quantify the potential effects of the variations in physical properties on cod biology throughout the year.