My main interests are:

• Aquatic food webs interactions and food web modelling
• Fish and fisheries
• Grey seal predation on fish populations in the Baltic Sea

Lundström K., Hjerne O., Alexandersson K. & Karlsson O. (2007) Estimation of grey seal (Halichoerus grypus) diet composition in the Baltic Sea. pp. 177-196 in Haug T., Hammill M., Ólafsdóttir D., (ed), NAMMCO Scientific Publications 6 NAMMCO Tromsø

Österblom H., Hansson S., Larsson U., Hjerne O., Wulff F., Elmgren & R. Folke C. (2007) Human-induced Trophic Cascades and Ecological Regime Shifts in the Baltic Sea. Ecosystems 10(6): 877-889

The ecosystems of coastal and enclosed seas are under increasing anthropogenic pressure worldwide, with Chesapeake Bay, the Gulf of Mexico and the Black and Baltic Seas as well known examples. We use an ecosystem model (Ecopath with Ecosim, EwE) to show that reduced top-down control (seal predation) and increased bottom-up forcing (eutrophication) can largely explain the historical dynamics of the main fish stocks (cod, herring and sprat) in the Baltic Sea between 1900 and 1980. Based on these results and the historical fish stock development we identify two major ecological transitions. A shift from seal to cod domination was caused by a virtual elimination of marine mammals followed by a shift from an oligotrophic to a eutrophic state. A third shift from cod to clupeid domination in the late 1980s has previously been explained by overfishing of cod and climatic changes. We propose that the shift from an oligotrophic to a eutrophic state represents a true regime shift with a stabilizing mechanism for a hysteresis phenomenon. There are also mechanisms that could stabilize the shift from a cod to clupeid dominated ecosystem, but there are no indications that the ecosystem has been pushed that far yet. We argue that the shifts in the Baltic Sea are a consequence of human impacts, although variations in climate may have influenced their timing, magnitude and persistence.

Hansson S., Hjerne O., Harvey C., Kitchell JF., Cox SP. & Essington TE. (2007) Managing Baltic Sea fisheries under contrasting production and predation regimes: ecosystem model analyses. Ambio 36 (2-3): 259-265

Based on an earlier published ecosystem model, we have explored possible effects of different management scenarios for the Baltic Sea. The scenarios include an oligotrophication of the system, a drastic increase in the number of seals, and changes in the fishery management. From these simulations we conclude that fisheries, seals, and eutrophication all have strong and interacting impacts on the ecosystem. These interactions call for integrated management. The modeling highlights the potential for conflicts among management mandates such as flourishing fisheries, rebuilt seal populations, and substantially reduced eutrophication. The results also suggest that fisheries management reference points have to be adjusted in response to changes in the presence of natural predators or ecosystem productivity.