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The Development of Fish Stocks and Fisheries in the Baltic Sea Since the Last Glaciation  

Henrik Svedäng

The fish fauna of the Baltic Sea reflects its 9 KY history of Arctic and temperate conditions and is a mixture of species that have invaded from the Atlantic and the continental watersheds. In spite of the challenging environmental conditions, such as low salinity in the entire Baltic Sea and varying temperature conditions, limiting the possibilities for successful reproduction, the number of species is comparably high. Except for the Baltic Ice Lake and certain stages of the cold Yoldia Sea and freshwater Ancylus Lake, the fish fauna of the Baltic Sea, as recorded by archaeological and historical notes, has to a large extent remained unchanged. Some freshwater and cold-water species such as Arctic char may have disappeared while others, such as fourhorned sculpin and eelpout, have adapted and persist as “ice age relicts.” There are few viable introductions of novel species; the round goby may be the most conspicuous invasive species. The extinction rate is still low; the loss of sturgeon and the common skate within the HELCOM (the Helsinki Commission) area is due to fishing, and, for the riverine sturgeon, due to damming. Since the formation of the Baltic Sea, fishing has played an essential role in supplying coastal settlements and their hinterlands and in trading. The archaeological and historical records have indicated fishing conducted with varying intensity using different methods. Herring fishing has been a significant economic driver from the Middle Ages onward. Recent archaeological findings indicate that organized fishing was established at the outer archipelagos along the present Swedish east coast on predominately herring and cod archipelagos for self-sufficiency shortly before or during the Viking Age, and later to engage in barter. The fact that the cod abundance has sometimes been sufficient for letting cod fishing be the most important fishery in the northern Baltic Proper alongside the fishery on herring may indicate that the eastern cod stock had relatively high productivity even when the Baltic Sea was significantly less eutrophic than it has been since the mid-20th century. This preindustrial variability in cod abundance suggests that climatic changes leading to changes in inflows of oceanic water may have affected salinity levels in the Baltic Sea. Fisheries show substantial variability, especially over the last century. Fish production may have increased due to nutrient enrichment of the Baltic Sea. Higher yields have also been obtained due to higher fishing intensity and technological changes. Fishing has, therefore, become a major driver in shaping fish stocks. The eutrophication of the Baltic Sea, leading to higher primary productivity and increasing water temperature and reductions in ice cover, may have led to changes in ecosystem structure and productivity. It should be underscored that such changes may also be amplified by the increasing fishing pressure on the cornerstone species such as herring, leading to significant disruptions in the food web.


Effects of Climate Change and Fisheries on the Marine Ecosystem of the Baltic Sea  

Christian Möllmann

Climate change and fisheries have significantly changed the Baltic Sea ecosystem, with the demise of Eastern Baltic cod (Gadus morhua callarias) being the signature development. Cod in the Central Baltic Sea collapsed in the late 1980s as a result of low reproductive success and overfishing. Low recruitment and hence small year-classes were not able to compensate for fishing pressures far above sustainable levels. Recruitment failure can be mainly related to the absence of North Sea water inflows to the Central Baltic deep basins. These major Baltic inflows (MBIs) occurred regularly until the 1980s, when their frequency decreased to a decadal pattern, a development attributed to changes in atmospheric circulation patterns. MBIs are needed for ventilation of otherwise stagnating Baltic deep waters, and their absence caused reduced oxygen and salinity levels in cod-spawning habitats, limiting egg and larval survival. Climate change, on the other hand, has promoted a warmer environment richer in zooplanktonic food for larval Baltic sprat (Sprattus sprattus). Resulting large year-classes and low predation by the collapsed cod stock caused an outburst of the sprat stock that cascaded down to the zoo- and phytoplankton trophic levels. Furthermore, a large sprat population controlled cod recruitment and hence hindered a recovery of the stock by predation on cod eggs, limiting cod larval food supply. The change in ecosystem structure and function caused by the collapse of the cod stock was a major part and driver of an ecosystem regime shift in the Central Baltic Sea during the period 1988 to 1993. This reorganization of ecosystem structure involved all trophic levels from piscivorous and planktivorous fish to zoo- and phytoplankton. The observed large-scale ecosystem changes displayed the characteristics of a discontinuous regime shift, initiated by climate-induced changes in the abiotic environment and stabilized by feedback loops in the food web. Discontinuous changes such as regime shifts are characteristically difficult to reverse, and the Baltic ecosystem recently rather shows signs of increasing ecological novelty for which the failed recovery of the cod stock despite a reduction in fishing pressure is a clear symptom. Unusually widespread deficient oxygen conditions in major cod-spawning areas have altered the overall productivity of the population by negatively affecting growth and recruitment. Eutrophication as a consequence of intensive agriculture is the main driver for anoxia in the Baltic Sea amplified by the effects on continuing climate change and stabilized by self-enforcing feedbacks. Developing ecological novelty in the Baltic Sea hence requires true cross-sectoral ecosystem-based management approaches that truly integrate eutrophication combatment, species conservation, and living resources management.


Post-Glacial Baltic Sea Ecosystems  

Ilppo Vuorinen

Post-glacial aquatic ecosystems in Eurasia and North America, such as the Baltic Sea, evolved in the freshwater, brackish, and marine environments that fringed the melting glaciers. Warming of the climate initiated sea level and land rise and subsequent changes in aquatic ecosystems. Seminal ideas on ancient developing ecosystems were based on findings in Swedish large lakes of species that had arrived there from adjacent glacial freshwater or marine environments and established populations which have survived up to the present day. An ecosystem of the first freshwater stage, the Baltic Ice Lake initially consisted of ice-associated biota. Subsequent aquatic environments, the Yoldia Sea, the Ancylus Lake, the Litorina Sea, and the Mya Sea, are all named after mollusc trace fossils. These often convey information on the geologic period in question and indicate some physical and chemical characteristics of their environment. The ecosystems of various Baltic Sea stages are regulated primarily by temperature and freshwater runoff (which affects directly and indirectly both salinity and nutrient concentrations). Key ecological environmental factors, such as temperature, salinity, and nutrient levels, not only change seasonally but are also subject to long-term changes (due to astronomical factors) and shorter disturbances, for example, a warm period that essentially formed the Yoldia Sea, and more recently the “Little Ice Age” (which terminated the Viking settlement in Iceland). There is no direct way to study the post-Holocene Baltic Sea stages, but findings in geological samples of ecological keystone species (which may form a physical environment for other species to dwell in and/or largely determine the function of an ecosystem) can indicate ancient large-scale ecosystem features and changes. Such changes have included, for example, development of an initially turbid glacial meltwater to clearer water with increasing primary production (enhanced also by warmer temperatures), eventually leading to self-shading and other consequences of anthropogenic eutrophication (nutrient-rich conditions). Furthermore, the development in the last century from oligotrophic (nutrient-poor) to eutrophic conditions also included shifts between the grazing chain (which include large predators, e.g., piscivorous fish, mammals, and birds at the top of the food chain) and the microbial loop (filtering top predators such as jellyfish). Another large-scale change has been a succession from low (freshwater glacier lake) biodiversity to increased (brackish and marine) biodiversity. The present-day Baltic Sea ecosystem is a direct descendant of the more marine Litorina Sea, which marks the beginning of the transition from a primeval ecosystem to one regulated by humans. The recent Baltic Sea is characterized by high concentrations of pollutants and nutrients, a shift from perennial to annual macrophytes (and more rapid nutrient cycling), and an increasing rate of invasion by non-native species. Thus, an increasing pace of anthropogenic ecological change has been a prominent trend in the Baltic Sea ecosystem since the Ancylus Lake. Future development is in the first place dependent on regional factors, such as salinity, which is regulated by sea and land level changes and the climate, and runoff, which controls both salinity and the leaching of nutrients to the sea. However, uncertainties abound, for example the future development of the Gulf Stream and its associated westerly winds, which support the sub-boreal ecosystems, both terrestrial and aquatic, in the Baltic Sea area. Thus, extensive sophisticated, cross-disciplinary modeling is needed to foresee whether the Baltic Sea will develop toward a freshwater or marine ecosystem, set in a sub-boreal, boreal, or arctic climate.


Projected Oceanographical Changes in the Baltic Sea until 2100  

H.E. Markus Meier and Sofia Saraiva

In this article, the concepts and background of regional climate modeling of the future Baltic Sea are summarized and state-of-the-art projections, climate change impact studies, and challenges are discussed. The focus is on projected oceanographic changes in future climate. However, as these changes may have a significant impact on biogeochemical cycling, nutrient load scenario simulations in future climates are briefly discussed as well. The Baltic Sea is special compared to other coastal seas as it is a tideless, semi-enclosed sea with large freshwater and nutrient supply from a partly heavily populated catchment area and a long response time of about 30 years, and as it is, in the early 21st century, warming faster than any other coastal sea in the world. Hence, policymakers request the development of nutrient load abatement strategies in future climate. For this purpose, large ensembles of coupled climate–environmental scenario simulations based upon high-resolution circulation models were developed to estimate changes in water temperature, salinity, sea-ice cover, sea level, oxygen, nutrient, and phytoplankton concentrations, and water transparency, together with uncertainty ranges. Uncertainties in scenario simulations of the Baltic Sea are considerable. Sources of uncertainties are global and regional climate model biases, natural variability, and unknown greenhouse gas emission and nutrient load scenarios. Unknown early 21st-century and future bioavailable nutrient loads from land and atmosphere and the experimental setup of the dynamical downscaling technique are perhaps the largest sources of uncertainties for marine biogeochemistry projections. The high uncertainties might potentially be reducible through investments in new multi-model ensemble simulations that are built on better experimental setups, improved models, and more plausible nutrient loads. The development of community models for the Baltic Sea region with improved performance and common coordinated experiments of scenario simulations is recommended.