Changes in fish health and local fish communities in eelgrass meadows following a bunker oil spill in the Baltic Sea.

Accidental oil spills can have detrimental impacts on marine ecosystems, but the impacts can be difficult to quantify due to lack of knowledge of pre-accident status and suitable controls. In a relatively rare BACI-study (Before-After-Control-Impact), here we report on fish abundances in eelgrass meadows from the Baltic Sea before (summer 2022) and after (summers 2024-2025) the grounding of the ferry Marco Polo in October 2023, which caused a 50-90 m3 spill of heavy bunker oil. In summer 2024, we also collected fish samples for histopathological analyses and polycyclic aromatic hydrocarbons (PAH) in eelpout (Zoarces viviparus) and three-spined sticklebacks (Gasterosteus aculeatus; PAH only) across three sites with varying levels of oil contamination to assess individual fish health. Our results indicate lowest PAH-concentration in fish from the least oil contaminated site, but PAH-levels were generally low, < 23 μg/kg wet weight. All examined eelpouts exhibited only minor gill and liver tissue alterations, including those from the reference site, suggesting that the observed alterations were not a result of oil exposure. At the fish community level, there was a clear shift from eelpout to sticklebacks in oil contaminated sites in 2024, although eelpout abundance increased again in 2025. In conclusion, while the oil spill may have resulted in somewhat elevated PAH-concentration in fish, this had no marked histological impact on fish health. Still, there was a clear shift in fish community composition in the summer following the oil spill, but abundances were back to 2022 levels within two years after the accident.