Assessment of the state of the Baltic Sea fish community - Understanding the impacts of environmental change to support effective management

Abstract

Climate change is rapidly advancing worldwide, causing a multitude of detrimental impacts and changes to ocean ecosystems. Among the many consequences for marine fish, shifts in species distributions and changes in community composition are highly prominent. These often result in fundamental alterations of ecosystem-wide structure and functioning, making the management of natural resources more difficult. Fish communities that are stable in time and space offer a sense of predictability on which foundations of successful industries, especially fisheries, can be built. When this stability is compromised, management must mitigate negative consequences, sometimes jointly across multiple countries, which can further complicate the situation. Adjusting even to positive developments can take time and require significant financial resources. It is therefore not surprising that management efforts are often static, reactive, and burdened by implementation lags. To alleviate these issues, an in-depth understanding of fish community dynamics, species and stock distributions and ecosystem functioning is advantageous. Examining possible shifts under future environmental change can be particularly helpful for establishing proactive and dynamic management plans. Against this global background, the Baltic Sea represents a highly affected region. As a brackish, semi-enclosed sea and one of the fastest warming large marine ecosystems (LMEs), it is experiencing rapid environmental change. Warming temperatures, expanding hypoxic zones, high eutrophication and intensive fishing have all contributed to major shifts in its ecosystem structure and functioning. Several commercially exploited fish stocks have declined substantially, most notably the two Atlantic cod stocks (Gadus morhua) and Atlantic herring (Clupea harengus). While cod distributions appear to have contracted, European flounder (Platichthys flesus) has expanded its range. Continued environmental change is expected to trigger further distributional shifts, potentially altering spatial overlap among species, with consequences for food web dynamics and ecosystem functioning. Understanding these processes is therefore crucial for anticipating future developments. To investigate how individual species and stocks respond to changing environmental conditions, advanced species distribution models (SDMs) were developed for eight species (including two cod stocks) for the period between 2001 and 2020 (Chapter 1). These models revealed clear spatiotemporal shifts in several taxa, namely European flounder, European plaice, and Eastern Baltic cod, and identified the drivers behind them. They also allowed us to evaluate how spatial overlap among species pairs has changed. We found six cases of significant overlap and five cases of significant separation. Overlap occurred mainly among species and stocks concentrated in a relatively small region (ICES Division IIIa and subdivisions 22-24), whereas separations were more common among those distributed across the wider Baltic Sea. Additionally, eight species pairs showed significant long-term trends in spatial overlap. Building on this analysis, the same modelling framework was applied to explore how species distributions may develop under future climate (RCP4.5 and RCP8.5) and eutrophication (BSAP and REF) scenarios until 2098 (Chapter 2). The projections indicate significant shifts across all scenarios. European plaice, Eastern Baltic cod, and European sprat are likely to benefit from future changes, while common dab, European flounder, Western Baltic cod and whiting are projected to experience negative impacts. Responses of Atlantic herring remained uncertain due to weak model performance. The three-spined stickleback shows mixed responses across scenarios and between time periods (middle and end of the century). It appears to benefit particularly from the environmental conditions projected under RCP8.5. Spatial overlap among species also changes markedly over time under all scenarios, highlighting the need for carefully planned management efforts under future environmental change. Understanding community-level responses requires going beyond species-level modelling. Therefore, structural and functional ecological indicators were analyzed to assess the spatiotemporal state of the Baltic Sea fish community between 2001 and 2020 (Chapter 3). A suite of 10 indicators, including 6 structural and 4 functional indicators, was selected, modeled and spatially predicted, revealing temperature as the dominant driver across indicators. Derived spatial indices were examined using Multiple Factor Analysis (MFA) to disentangle the contributions of structural versus functional properties and different spatial dimensions. Converting predictions into cumulative Z-scores enabled the identification of areas with the strongest changes in community state. These areas showed limited overlap with HELCOM Marine Protected Areas (MPAs) and a fisheries closure area, and an expansion of wind farms is expected in several of them. These results underscore the importance of integrating spatial and functional dimensions into ecosystem state assessments. To complement these modelling approaches, the vulnerability of 22 fish species to rising temperatures was evaluated for the Western Baltic Sea using an expert-based assessment (Chapter 4). Traditional target species such as cod and herring, as well as species with complex life histories, were identified as particularly sensitive to climate change. Conversely, invasive and better adaptable species may be better positioned to thrive under future warming scenarios. In summary, this thesis demonstrates that environmental pressures have reshaped the Baltic Sea fish community and are likely to continue doing so. These changes affect both individual species distributions and the structural and functional characteristics of the community. Incorporating these insights into adaptive and forward-looking management strategies will be essential to safeguard the health and productivity of the Baltic Sea ecosystem.