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CS 3: Baltic Sea

Background

Cod and sprat are presently the most important commercial fish species in the Central Baltic Sea. Sprat is the main food source of immature and mature cod (Uzars and Plikshs 2000), but sprat is also an important predator on cod eggs (Köster and Möllmann 2000). Although both species, spawn in the same area at a similar time their reproductive success has shown opposite tendencies since the late 1980s (Köster et al. 2003).

The climatic conditions during the 1990s, with mainly high values of the NAO, resulted in above- average water temperatures in the Baltic Sea (e.g. Matthäus and Nausch 2003). In addition, increased runoff and precipitation reduced the probability that inflows of highly saline and oxygenated water from the North Sea could be advected to, and re-oxygenate, the deep water of the Baltic basins (Matthäus and Nausch 2003). Low salinities and oxygen contents in the deep basins (MacKenzie et al. 1996), substantial egg predation by sprat (Köster and Möllmann 2000) and low availability of the copepod, Pseudocalanus sp. for larvae (Möllmann et al. 2003) resulted generally in low cod recruitment during most of the 1990s (Hinrichsen et al. 2002). In contrast, the prevailing warm water temperatures caused high sprat egg survival and optimal food supply for larvae which eventually resulted in a series of large sprat year-classes (MacKenzie and Köster 2004).

The differing recruitment success for the two species in combination with the high fishing pressure on cod and low predation pressure on sprat caused a “regime shift??? in the second half of the 1990s from a cod- to a sprat-dominated system (Köster et al. 2003). Clearly the ecosystem of the Central Baltic Sea changed from a state of high productivity for the cod stock, characterized by high salinity/oxygen conditions and low temperatures, to a state of high productivity for the sprat stock, characterized by low salinity/oxygen conditions and high temperatures. This shows that the carrying capacity of the system for both species changes depending on the environmental state, implying also changing potentials for recovery and changing long-term sustainable yield for both species.

Baltic fisheries are up to now managed by the IBSFC (International Baltic Sea Fisheries Commission). Eastern Baltic cod is regulated by gear restrictions, minimum landing sizes, and closed areas. In 1999 IBSFC adopted a long-term management strategy which identified target fishing mortalities and defined decision rules in relation to annual TACs dependent on SSB (IBSFC 1999, Resolution X). Further the introduction of technical measures was stipulated. Despite the long-term management strategy, the state of the stock worsened and a first recovery plan was adopted in 2001 which included detailed measures to recover the eastern Baltic cod stock (IBSFC 2001, Resolution XVII). The measures include a summer ban on cod fishing, closed areas, gear design and size restrictions, minimum mesh and landing sizes. However, the selectivity of the existing measures proved to be less effective than expected, fostering an urgent need for a review. In 2003 the existing recovery plan was updated (IBSFC 2003, Resolution XX) and additional emergency measures were taken to protect an incoming strong year class as a unique opportunity to accelerate cod recovery (ICES 2003). Also for sprat, IBSFC adopted a long-term management strategy which shall ensure a rational exploitation pattern and provide for stable and high yields. The plan includes target mortality and decisions rules for setting the annual TAC (IBSFC 1999, Resolution XIII).

While the sprat stock is presently within safe biological limits despite a large-scale industrial fishery since mid 1990’s, the eastern Baltic cod stock remains outside safe biological limits and ICES (2004) has recommended that there should be no fishing on the eastern Baltic cod stock in 2005. The critical status of the stock suggests that despite attempts to implement recovery plans, the present management regime is incapable of increasing the stock size. Thus, there is an urgent need for considering the available knowledge underlying the stock dynamics of cod and sprat in the Baltic under differing environmental regimes for designing successful rebuilding strategies. Further there is a clear need for investigating the effectiveness of recovery plans with regard to social, economic and governance influences, as acceptance of and compliance to management measures is low. A better understanding of environmental and fisheries effects on stock cod dynamics will also lead to an improvement of the long-term management strategies for the sprat stock.

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