Marine Research Findings of the VECTORS Project

This website provides access to the research results of the VECTORS project, which can be used to support marine management decisions, policies and governance as well as future research and investment. VECTORS was a large scale project that brought together more than 200 expert researchers from 16 different countries. It examined the significant changes taking place in European seas, their causes, and the impacts they will have on society.

Changes in fish distributions in the Baltic Sea call for spatial management measures

Since 2007, the eastern Baltic cod abundance has increased, partly due to effective management measures. Unexpectedly, and despite the biomass still being relatively low compared to historical levels, recent observations show that the cod has become lean, which indicates food limitation. To elucidate possible reasons as well as ecological consequences of these changes for ecosystem functioning, we investigated area-specific population dynamics of cod along with that of its major fish prey (sprat and herring), including spatially explicit species interactions through predation. Further, we analysed the consequences of the increase recruitment production of the eastern Baltic cod population for the adjacent western Baltic management unit to provide input to fisheries management advice of the two stocks, taking into account changing fish distributions.

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High cod abundance in the southern Baltic Sea, where biomass of sprat and herring is low, results in locally high predation mortality on prey species and cannibalism of cod. In line with low prey availability, nutritional condition of cod declined. Accounting for spatial overlap between species is crucial for Ecosystem Based Fisheries Management (EBFM).

We applied a spatially resolved multispecies model to elucidate area-specific population dynamics of cod and its major fish prey (sprat and herring), including spatially explicit predation mortalities by cod. The analyses showed cod densities in Bornholm Basin close to the highest level in the time-series since the 1970s. Since 2007, several strong year-classes have increased the abundance of cod in this area, whereas in the northeastern Baltic Sea, the abundance of cod has been relatively low since the 1990s. Thus, most of the adult cod is currently concentrated in Bornholm Basin. In contrast, only about 10–15% of the herring and sprat biomass is distributed in this area. The high density of cod in Bornholm Basin is estimated to result in pronounced cannibalism in this area, compared with much lower or negligible predation mortalities of young cod estimated for the other areas. Similarly, predation pressure on sprat and herring is highest in Bornholm Basin.

Fisheries in the Baltic Sea are targeting all three species in the entire distribution area of the stocks. Sprat and herring in Bornholm Basin are harvested at a similar rate as the stock components distributed further northeast. Consequently, the small stock components of sprat and herring overlapping with cod are exposed to a high pressure both from exploitation and predation. In contrast, the abundant resources of sprat and herring in more northerly areas are less utilised.

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Increased cod abundance in the eastern Baltic Sea has resulted in spill-over to the adjacent western management unit. This can mask the poor state of the biologically western cod population. To reduce the risk of local depletion, the stock structure within a management area should be taken into account.

We investigated links between the adult cod dynamics across the borders of the eastern and western management units. This is done by (i) developing time series of adult cod abundance and biomass by subareas within the western Baltic Sea and comparing these with the dynamics in the bordering area to the eastern Baltic Sea, (ii) investigating differences in body weight and nutritional condition of cod by subareas; and (iii) genetic analyses. Within the western Baltic, cod abundance in Belt Sea has generally declined in later years, while it has substantially increased in Arkona Basin, following trends in the eastern Baltic Sea. Genetic assignment of the fish sampled from Arkona Basin indicated a mixture of fish from different populations with the majority (88%) of the cod assigning to the eastern Baltic cod population and 10% to the western population. Despite the record high stock size in Arkona, recruitment to the entire western Baltic Sea has been low since the mid-2000s.

Periods of frequent stronger year-classes have generally coincided with periods of relatively high cod stock in the Belt Sea. Currently the harvest rate in the Belt Sea is exceeding that in Arkona Basin, which is a matter for concern given the poorer stock status in SD 22. Further, fishing mortality estimates for the western management unit, used as basis for fisheries management, maybe biased by mixing of biological populations. Continued research activities towards an appropriate long-term solution to account for mixing of fish across the borders of management units are needed.

Lead Author:

Margit Eero
(mee@aqua.dtnospamu.dk)
Danmarks Tekniske Universitet (DTU-Aqua)
Date of research: March 2014

Related articles:

Population dynamics of sprat in the Baltic Sea 

Changes in herring larvae and environment 1957-2010

Early life stage survival of Baltic cod

Functional responses of herring and sprat to prey 

Vital rates of fish larvae 

Baltic cod recruitment and predation

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This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 266445
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