Integrated assessment of environmental stressors improves marine management
The Baltic Sea ecosystem is subject to multiple pressures at the same time. This can lead to a loss of biodiversity and reduce the sea's ability to maintain important ecosystem functions. There is a strong need for improved assessments of these combined impacts in marine spatial planning and fisheries management, and now researchers in the BalticCAT project have made further progress in developing data and methods for such assessments.
The final report of the BalticCAT project has now been published, presenting the project results in Swedish for managers and policy makers. The project was based on the following research questions:
- How do environmental factors (e.g. temperature, salinity, oxygen depletion and eutrophication) and food web interactions (predation and fishing pressure) collectively affect the Baltic Sea ecosystem and fish species such as cod, herring and sprat?
- How will different management options (e.g. reduced nutrient discharges, changes in fishing intensity and area closures) affect the Baltic Sea under different climate scenarios?
- How can numerical food web modelling support cumulative environmental assessments for ecosystem-based management of the Baltic Sea?
The first question was addressed both with empirical studies focusing on Baltic cod and with a food web model for the whole Baltic Sea. The second and third questions were addressed using a food web model for the Baltic Proper.
Dire circumstances for cod
The situation of fish stocks in the Baltic Sea is a clear example of how the lack of knowledge of cumulative effects is a major problem for management. The project took a closer look at the disastrous development of the cod population.
" The cod population in the eastern part of the Baltic Sea has been in very bad shape since the 1990s," says Henrik Svedäng, leader of the cod studies in BalticCAT. "They are small and thin and die early. And we don't really understand what the problem is. Lack of oxygen, lack of food, parasites or vitamin deficiencies are some of the causes that have been suggested, but not yet confirmed".
In addition, the reproductive strategy of cod has changed dramatically; they now mature at smaller and smaller sizes.
" On average, they are half the length and one-eighth the weight that was normal in the mid-1990s.
" Our new studies show that there is plenty of food for the cod, at least as long as they feed on benthic animals, and that their protein intake has even increased," says Henrik Svedäng.
So it seems that starvation is not the problem.
Perhaps it is the deterioration in food quality that is causing the changes. There have been several profound changes in the food web during this time - from changes in plankton composition to changes in the ratio of herring to sprat - which makes it difficult to understand and predict what is happening.
An altered food web
To better understand the functioning of the Baltic Sea ecosystem and to identify future vulnerabilities and cumulative effects of different stressors, the project has further developed a food web model for the entire Baltic Sea. The food web is represented by a total of 43 functional groups, and the model has been adapted using a large number of time series from scientific surveys, stock estimates and reported catches.
" Food web modelling is a powerful tool for combining information from research and environmental monitoring to get an overview of how different types of impacts interact. This makes it a potentially good tool for ecosystem-based management," says Sofia Wikström, project leader of BalticCAT.
" The advantage of linking different time series is that we can better understand how changes in one species or trophic group can propagate through the food web and affect other parts of the ecosystem."
A challenge is that these models are extremely data and resource intensive to build and calibrate. The profound changes currently taking place in the Baltic Sea food web also make it difficult to predict changes based on historical data.
Scenarios provide insight
The project explored the combined environmental impacts of eutrophication, fisheries and global warming using six different scenarios representing different possible futures depending on global and national policy developments.
" The simulations showed that measures to reduce eutrophication and climate change are essential to maintain biodiversity and intact food webs in the Baltic Sea," says Sofia Wikström. The most obvious change was for adult cod, which increased significantly in the sustainably developing scenarios and decreased in the other scenarios. Other benthic species, such as flounder and small animals, also benefit from improved oxygen conditions and larger habitats.
Useful tools
The food web model can be one of several tools for planning fisheries and other measures to protect and restore the Baltic Sea ecosystem. However, it is important to remember that the results of the simulations are based on the assumption that historical relationships between environmental factors and ecological processes will not change in the future. Uncertainties in the models can therefore be large, and this needs to be made clear if they are to be used as a decision support tool.
" This also means that it is extremely important that we apply a greater margin of caution when deciding how much fishing should be allowed," concludes project manager Sofia Wikström.
" It is also important that we continue to monitor changes in the Baltic Sea environment through long-term environmental monitoring and that we adapt our use of the sea when conditions change."
Text: Annika Tidlund
Read more:
Report (in Swedish): Kunskap och metoder för att bedöma kumulativa miljöeffekter på Östersjöns ekosystem
Report references (in English)
Svedäng H., V. Thunell, A. Pålsson, S.A. Wikström and M.J. Whitehouse 2020: Compensatory Feeding in Eastern Baltic Cod (Gadus morhua): Recent Shifts in Otolith Growth and Nitrogen Content Suggest Unprecedented Metabolic Changes. Front. Mar. Sci. 7:565.
Svedäng H., Savchuk O., Villnäs A., Norkko A., Gustafsson B.G., Wikström S.A., Humborg C. 2022: Re-thinking the “ecological envelope” of Eastern Baltic cod (Gadus morhua): conditions for productivity, reproduction, and feeding over time. ICES Journal of Marine Science fsac017.
Svedäng, H., Savchuk, O., Villnäs, A.P.M., Norkko, A., Gustafsson, B., Wikström, S.A. & Humborg, C. 2022: On the decline of Eastern Baltic cod: we need to take more holistic views into account. Reply to Brander (2022) comment on Svedäng et al. (2022). ICES Journal of Marine Science, 79, 2157-2158.
Svedäng, H. 2023. The development of fish stocks and fisheries in the Baltic Sea since the last glaciation. Oxford Research Encyclopedia of Climate Science 24 May 2023.
Svedäng, H., Hornborg, S., Grimvall. A. Size at maturity of Eastern Baltic cod (Gadus morhua) over the last hundred years mirror conditions for growth. Ecology & Evolution 14 e70382.
Last updated: December 4, 2024
Source: Baltic Sea Centre