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CS 3: Cod and sprat in the Central Baltic Sea

Foto and Copyright©: Christopher Zimmermann, Rostock (Germany)

Objectives

Cod (Gadus morhua) and sprat (Sprattus sprattus) are the two dominant, interacting species in the ecosystem of the Central Baltic Sea. Although both species spawn in similar locations, i.e. the deep Central Baltic basins, changes in environmental conditions since the 1980s have had opposite effects on the reproductive success of both species. Pronounced oxygen depletion caused regularly high cod egg mortality and associated declining salinities negatively impacted zooplankton availability for early cod larvae. In contrast prevailing warm winters and associated relatively high water temperatures favoured sprat egg survival and increased the production of zooplankton prey suitable for sprat larvae (Köster et al., 2003). In combination with high fishing pressure the reduced reproductive success resulted in a strong decline of the cod stock throughout the 1980's and early 1990's. A release of sprat from predation pressure by cod and concurrent high reproductive success resulted in a drastic increase in the Baltic sprat stock. Despite the existing long-term management and recovery plans, the cod stock has not yet recovered to safe biological limits.

In order to understand the mechanisms which support or counteract recovery of the Baltic cod stocks, the objectives of this case study are to:

  • characterize the contrasting states of the ecosystem which allow high cod or sprat production;
  • understand and model the biological/ecological processes and interactions with fishery activities leading to contrasting reproductive output and recruitment success under varying environmental scenarios;
  • understand and model the effect of social, economic and governance influences on the success of the recovery and management strategies;
  • develop and test strategies which allow a recovery of eastern Baltic cod and assure a healthy Baltic sprat stock.

Hypotheses

In this case study, four major hypotheses will be investigated:

  1. Stock structure, reproductive potential and recruitment dynamics of the target species as well as trophic interactions change depending on environmental conditions and target specific fishing intensity, which define carrying capacity and recovery potential of a stock;
  2. Recovery plans accommodating the state of the environment and thus the ecosystem state-dependent recovery potential are able to give more realistic prognoses of recovery rates of the depleted cod stock;
  3. Considering environmental effects on sprat stock dynamics in long-term management plans reduces the risk of a stock depletion even after rebuilding the cod stock;
  4. Social, economic and governance issues are important factors to be considered in designing recovery plans to ensure acceptance and compliance.

 

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Description of work

Background information to this case study area

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