The Bothnian Sea is under increasing pressure from human activities. Increases in fishing and phosphorus have led to a decline in the health of the open sea food web – which represents a complex network of interactions between species. The deteriorating trend is bad news for the Baltic Sea, as healthy food webs are vital to supporting ecosystem resilience as well as water quality, recreational opportunities, and sustainable fish production.
The results are presented in a recent study from SLU Aqua and are based on a comprehensive analysis of 42 years (1979-2021) of data, including phytoplankton, zooplankton, benthic invertebrates, fish, and seals.
“This long-term data set enabled us to understand how the food web has evolved over time in relation to changes in a variety of environmental changes, such as salinity and climate change, and human-induced factors like fishing and nutrient concentrations” says Carolyn Faithfull.
The study highlights that herring, a central fish species in the Bothnian Sea food web, has been increasingly affected by overfishing. Over time this has led to smaller herring size and less herring biomass. Herring is crucial for small-scale coastal fisheries and a key target species for large-scale pelagic offshore fisheries, which have more than doubled since the early 2000s.
A key food source for herring in the Bothnian Sea, the bottom-dwelling glacial relict Monoporeia affinis has also declined, compared to the beginning of the time series. This was mainly attributed to rising water temperatures and elevated phosphorus concentrations in the analyses.
Furthermore, the study shows rising phosphorus levels over the past 15 years despite efforts to reduce nutrient runoff from land, which are associated with an increase in cyanobacteria.
“Urgent actions are needed to reduce fishing pressure and phosphorus concentrations, to avoid further deterioration of the Bothnian Sea’s food web.” says Carolyn Faithfull.
This study is based on a range of analytical methods, including multivariate analysis and non-linear modeling and contributes to the development of methods to assess food web status for the EU Marine Strategy Framework Directive and the HELCOM Baltic Sea Action Plan. These findings will contribute to shaping future policies aimed at improving the status of the Baltic Sea environment.
Read the article Temporal changes in the Bothnian Sea food web reveal a deterioration linked to fishing pressure and recent eutrophication in ICES Journal of Marine Science.
Carolyn Faithfull, researcher
Department of Aquatic Resources, Institute ov Coastal Research , SLU
carolyn.faithfull@slu.se, +46(0)10-478 41 74
Lena Bergström, Researcher
Department of Aquatic Resources, Institute of Coastal Research, SLU
lena.bergstrom@slu.se, +46 10 478 41 16