Robust, ecologically-economic multispecies management of the fishery resources of the central Baltic Sea

The Baltic Sea is one of the largest brackish water bodies in the world with a centuries-long history of fisheries. Environmental conditions have changed rapidly in recent decades, including a temperature increase of more than 2 °C in the last 50 years. It is also characterized by low biodiversity and a small number of key species for fishing: Cod, Herring and Sprat. Fisheries in the Baltic Sea are in distress. In the central Baltic, fisheries management is challenged by reduced productivity of cod stocks and changing interactions between species.

The authors of the study, published in the ICES Journal of Marine Science, Rudi Voss (Center for Ocean and Society, Christian-Albrechts-Universität zu Kiel; German Center for Integrative Biodiversity Research (iDiv) Leipzig), Martin Quaas (German Center for Integrative Biodiversity Research (iDiv) Leipzig) and Stefan Neuenfeld (National Institute of Aquatic Resources, DTU Aqua, Lyngby) use an age-structured, multispecies ecological-economic model for their study that incorporates the latest biological and economic knowledge to improve understanding of optimal fisheries management and the associated trade-offs among user groups under these changing conditions.

In this study, the authors combine new ecological information with state-of-the-art economic cost functions in an age-structured multispecies ecological-economic optimization model to determine optimal management of the three-species system (cod, herring, and sprat). In doing so, they advance the scientific debate in three main areas: first, they investigated whether declining productivity of the cod stock combined with (possibly) increasing mussel production will lead to a "brave new Baltic" dominated by small pelagic species in both ecological and economic terms? For the first time, long-term outcomes (i.e., reference points for SSB, catch, and fishing mortality) of optimal management were measured for old (early decade, i.e., 2013) and current (2019) stock productivity and food web interactions.

Second, the study quantifies the economic consequences and emerging trade-offs among different management objectives. In doing so, the authors take a more holistic view and include consumer benefits in their analysis in addition to fishery gains. The "consumer surplus" is quantified in euros and can be directly compared to fishery profits so that trade-offs can be calculated. It is a measure of the additional benefit consumers receive because the price they pay for fish in the market is lower than what they were willing to pay. It depends on prices and therefore on supply and ultimately on management.

The article contributes to the scientific debate by showing that the economic importance and optimal stock size of cod is largely declining under prevailing conditions, while the importance of clupeids, such as herring and sprat, is increasing. In addition, the current maximum sustainable yield (MSY) management objective in a multispecies environment (MMSY) is questioned by the authors and it is suggested that a multispecies economic management objective (MMEY) may be more appropriate for setting future management tariffs. New tradeoffs and synergies were identified by incorporating the consumer perspective:

There is a win-win situation for ecological conservation and fisheries profits, while fisheries management faces trade-offs between these two aspects on the one hand and consumer surplus on the other. Finally, the authors propose an easily implemented new management approach, called robust management, that is able to better deal with fluctuations and temporal trends in recruitment, as seen in cod, in order to protect the fishery resources of the central Baltic Sea.

The original publication can be found here:

https://academic.oup.com/icesjms/article-abstract/79/1/169/6479779