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Fish-community Shifts Matched Changes in PCDD/F Decline in Baltic Murre Eggs

Common murres at the Stora Karlsö breeding colony in Sweden
Research · Ecology

A study published on 3 May 2026 examined how long-term changes in the central Baltic Sea food web coincided with polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) concentrations in eggs of the common murre (Uria aalge) from Stora Karlsö in Sweden. PCDD/Fs are persistent, fat-soluble organic pollutants, produced largely as by-products of industrial and combustion processes, that can build up through food webs. The researchers combined contaminant measurements from eggs collected between 1968 and 2022 with fish-community and environmental data. In the monitoring programme, ten eggs were analysed each year, either individually or as pooled samples.

Three fish-community phases matched three contaminant trends

The fish community separated into three major phases. From 1976–1986, cod and herring were prominent; from 1987–2001, sprat dominated; and from 2002–2021, three-spined stickleback increased markedly while cod remained low. These phases coincided with very different trends in the toxic equivalents (TEQ) of PCDD/Fs in murre eggs: a decline of 6.4% per year in the first phase, a near-plateau with an estimated increase of 0.27% per year in the second, and a renewed decline of 3.8% per year in the third. Toxic equivalents combine the concentrations of different PCDD/F congeners into one toxicity-adjusted measure by weighting each compound according to its relative toxicity.

Graph of PCDD/F toxic-equivalent concentrations in common murre eggs from 1968 to 2022
PCDD/F toxic equivalents in common murre eggs from 1968 to 2022. The fitted segments use change points corresponding to the fish-community phases identified in the study. Figure 4 from Ammar et al. (2026), CC BY 4.0.

The fish-community shifts reflected broader ecosystem change

The authors also analysed environmental and human pressures associated with the changing fish community. Temperature, salinity, zooplankton size and fishing pressure all contributed to differences between phases. Cod spawning-stock biomass was the strongest variable separating the fish-community states, while the most recent phase was associated with warmer summer water, smaller zooplankton and lower cod abundance.

Diet shifts may have altered contaminant exposure

The link to the murres’ diet was especially important in the authors’ interpretation. During the sprat-dominated phase, high sprat abundance was accompanied by poorer growth and lower fat content in sprat and herring. Murres may therefore have had to consume more prey to obtain the same amount of energy, potentially increasing dietary PCDD/F exposure and helping to explain the 1990s plateau. In the most recent phase, three-spined sticklebacks may have become a more important prey. A recent diet study cited by the authors found that sticklebacks made up 27% of the diet of adult guillemots. Because sticklebacks are shorter-lived than sprat, they may accumulate less PCDD/F, so a dietary shift towards them could have contributed to the renewed decline. The authors stress that this stickleback pathway remains a hypothesis that needs direct testing.

One PCDD/F compound, 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), contributed especially strongly to the overall TEQ trend because it occurred at the highest concentration in the eggs, is highly bioaccumulative and receives a high toxicity weighting for birds in the TEQ calculation.

The researchers conclude that the long-term contaminant pattern in murre eggs cannot be understood from emission reductions alone. The authors could not assess the toxicological significance of the measured egg concentrations because thresholds for adverse effects of PCDD/Fs in birds remain uncertain.

Sources and image credit

  1. Ammar, Y., Olsson, J., Gorokhova, E., Sköld, M., Faxneld, S., Hentati-Sundberg, J., & Soerensen, A. L. (2026). The effect of shifts in fish community structure on PCDD/F temporal variability in common guillemot (Baltic Sea). Ambio. https://doi.org/10.1007/s13280-026-02387-x

Image: Sjunnesson — common murres (Uria aalge) at the Stora Karlsö breeding colony, Gotland, Sweden, photographed 2 June 2009; Wikimedia Commons; cropped to 16:9; representative colony photograph, not the sampled birds. · CC BY-SA 4.0 · Source

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