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Causes of major changes in marine fish stocks in the Baltic Sea Ursachen grosser Bestandsveraenderungen bei Meeresfischen in der Ostsee

, : Causes of major changes in marine fish stocks in the Baltic Sea Ursachen grosser Bestandsveraenderungen bei Meeresfischen in der Ostsee. Meer und Museum, 17: 96-103

Among the fish species in the Baltic Sea only few have developed large stock biomasses. They are well studied as attractive fishery resources and characterized by conspicuous fluctuations: Herring and Sprat are dominating as planktivorous pelagic species and cod as carnivorous demersal species. During the last 2 decades the cod stock in the central Baltic developed from a historic maximum in the early 80s to a historic minimum in the early 90s and remained low since. The sprat stock developed in an opposite way. Herring shows an independent development with low fluctuations in total abundance and a general reduction in stock biomass; this species differs from the other two in reproductive biology and stock structure, which for comparison would require detailed regional assessments, not included here. Cod and sprat are more large scale in stock structure and show a substantial overlap in spawing time and place. The causes of the observed opposite stock development under comparable conditions are discussed in this contribution. In general, the reproductive strategy of these species, with high fecundity but small and delicate early live stages, allow a quick growth in stock size under favorable conditions; but on average, the early mortality is extremely high and year class success correspondingly uncertain, highly variable in most cases, and hardly related to the parent stock size; rather the parent stock size follows year class success. The regulating factors differ among the species, according to the following characteristics: Cod is very much bound to the permanent halocline in the deep basins of the central Baltic. The eggs can survive only in the saline deep water, and are thus affected by oxygen deficiencies during stagnation periods, when now major inflows of well oxygenated saline waters from the North Sea occur. The larvae ascend to upper water layers just below the seasonal thermocline; here they are depending on adequate by food supply and wind induced drift. Survival of juveniles is improved by wind conditions resulting in a quick drift to shallow, near shore areas, where they change from a pelagic to a demersal life, usually outside the reach of the adults. Cannibalism becomes important when the juvenile and/or adult stock is large with extending and more substantially overlapping areas of distribution. Sprat are more adapted to warmer conditions than cod. They spawn in the Baltic usually above the thermocline; the eggs sink down but have in general a higher buoyancy than cod eggs and remain distributed above the halocline. Thus, they are much less affected by oxygen deficiencies, but survival can be limited by low temperatures in the intermediate water layers. Strong winter cooling, which keeps intermediate water layers cool for an extended period of the year, is thus negative for sprat but may be favorable for cod as it is a prerequisite for a good oxygen supply of the deep water at inflow conditions. The larvae of sprat ascend far up into the warm surface layers and are drifted further inshore than cod where they keep there pelagic life. These differences could well be related to the observation that for cod, the egg to larval stage is the most critical period, whereas for sprat the period from larvae to juveniles is more critical in year class formation. In addition to the abiotic factors, biological interactions are important: Cod feed on herring and sprat. On the other hand, herring and sprat feed on cod eggs. Thus, there is a reciprocal limitation in stock development, stabilizing the dominance of either cod or sprat once established through abiotic factors. Herring is less important as egg predator and as prey for cod. Large stocks also limit themselves by cannibalism and competition for food. The quantitative importance of the predator-prey interactions is modified through hydrography again, influencing the overlap in distribution of both groups. Under improved oxygen conditions and in the last years due to late spawning of cod, the predation effect by sprat on cod eggs appears to be minor only. Fishery acts like an additional predator in this system, which however is hardly or not sufficiently regulated in "stock size" by the size of the prey population. Due to continuously high fishing pressure on cod despite low reproductive success, fishery acts in the same direction as the presently unfavorable natural conditions for cod and favors the dominance of the planktivorous species, resulting in a high grazing rate on zooplankton in the central Baltic. Quantitative estimates have shown that substantial reduction in fishing pressure would have a similar effect as an improvement of reproductive conditions for cod to about average levels. This would largely improve the stock size and the catch level for cod. It can be concluded that fluctuations in stock sizes and alternation of cod and sprat dominance is a natural process in the Baltic Sea, which cannot be avoided, but which is largely influe nced and could be substantially modulated by the fishery. The observed shift in stock size, the main regulating factors and feedback mechanisms and the required conditions for a hypothetical shift back to cod dominance are schematically summarized in Figure 5. The effect of sprat fishery on the cod stock cannot yet be quantitatively assessed. It appears to reduce the predation on cod eggs under certain conditions, but will reduce at the same time the food supply for cod, affecting its growth and spawning success. The cod stock and catch level could, however, well be improved by a substantial reduction of fishing pressure on cod, which would limit the sprat stock and the grazing level on zooplankton naturally by increased predation on sprat.

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