Spatial dynamics of exploited fish populations in the Baltic Sea: applications for fisheries and ecosystem-based management

Marine ecosystems are severely impacted by external pressures, such as fishing, eutrophication and climate change, and the Baltic Sea is a clear example of this multiple external impact. In the fields of fishery science and conservation biology, the developing concept of an ecosystem-based management requires detailed knowledge on, and integration of, both the temporal and spatial processes leading to the observed ecosystem changes. In particular, with increasing use of marine environment, spatial aspects in marine management are getting increasingly important including conservation issues which are becoming an integral part of the overall spatial planning. However, spatial considerations have started to gain attention in fisheries management only recently.

Particularly for the Baltic Sea, the importance of studying the changes in spatial distribution of the exploited fish populations has recently been emphasized by ICES (International Council for the Exploration of the Sea), HELCOM (Helsinki Commission) and the European Commission. Moreover, in a multi-species context, when different species are strongly interacting in a food-web (ex. predator and prey), it is of paramount importance to understand the spatial dynamics of the different species and the causes of their distribution changes.

Although the Baltic Sea ecosystem is well studied and fairly understood, several crucial gaps for the understanding of its dynamics still exist. Most of the investigations present in literature dealing with the Baltic food-web have focused on specific locations, seasons and time periods. On the other hand, trophic interactions may vary temporally and along spatial gradients. A new combined approach incorporating wide-scale temporal and spatial aspects in the food-web dynamics is lacking, albeit necessary to better comprehend ecosystem structure, functioning and resilience to external pressures. My current and future activities are mainly addressed to fill this gap.

My presentation focuses on the current knowledge of the large-scale spatial dynamics of the commercially and ecologically key fish species of the open Baltic Sea. I present the results of recent analyses on the changes in the spatial distribution of these species in the open waters and the biological and ecological effects of these changes in the Central Baltic Sea and other adjacent areas. Some considerations about the causes of the changes in the fish spatial distribution are provided, and ideas for new investigations aimed to better understand spatio-temporal population dynamics are put forward. I finally explain how the knowledge on spatio-temporal dynamics could be applied into fisheries management and into ecosystem-based management as specified by the EU Marine Strategy Framework Directive.

/Michele Casini