Understanding the spatio-temporal dynamics of demersal fish species in the Baltic Sea

The Baltic Sea is one of the largest brackish areas in the world with mixed salt and fresh water. The particular hydrological characteristics of this sea, such as the temperature and salinity gradients from the Danish Straits to the northernmost part of the Gulf of Bothnia as well as the hypoxia of its deep waters, make it a unique ecosystem. During the last century the Baltic Sea has undergone major structural changes not only in its physical characteristics but also in the biological communities inhabiting it. Cod (Gadus morhua) and flounder (Platichthys flesus) are two key species of the Baltic Sea ecosystem. Cod in the Baltic has experienced huge changes in spatial distribution, variations in growth, mortality and abundance in the last 40 years, but still the environmental and the biological drivers that have caused these modifications are unclear. Much less is known for flounder even though it is predated by cod and could potentially compete with it for benthic resources. All these changes have caused and are causing problems to the fishing industry and the consequent management. The present situation of the Baltic Sea illustrates the importance of understanding on the processes causing spatial heterogeneity and changes in the fish population dynamics. Such knowledge is crucial in order to work out a better management of the resources. This essay has the aim of reviewing the state of the knowledge on the spatiotemporal dynamics of cod and flounder in the Baltic Sea and to point out the knowledge gaps that need to be filled