The Collapse of Atlantic Cod in the Oslofjord

Atlantic cod in the Oslofjord have been declining for years, leading the Norwegian government to close large parts of the fjord to fishing from 2026. However, few quantitative estimates of how much the population had fallen have been available. A recent study (Langangen et al. 2026) from the Department of Biosciences at UiO addresses that gap.

plot of the negative trend in the population of cod in the oslofjord

Figure 1. shows the total landings (right) and catch pr boat trip (proxy for cod biomass). Black lines are point estimates and grey areas indicate the uncertainty. The hatched black area are trawl data while the hatched grey area are gillnet data. Red arrows indicate the introduction of sorting grids, while the blue dotted lines indicate seasonal closures.

By Øystein Langangen

Published Sep. 11, 2026

Unlike more offshore cod stocks, which are assessed annually by ICES, the Oslofjord cod has no formal population assessment. The long and extensive beach seine survey (since 1919) mostly tracks newly hatched juvenile cod. Adult surveys exist but are often geographically limited and relatively short. The researchers asked to what extent can commercial fishing records fill this gap?

The team used landing data from the Norwegian Directorate of Fisheries covering 2000–2024 over a million records, including more than 188,000 records involving cod, mostly caught as bycatch in shrimp trawls and gillnets. From this, they calculated catch per unit effort (CPUE): the average amount of cod landed per fishing trip. CPUE serves as a proxy for biomass, since it partly corrects for changes in fishing effort over time. To ensure reliability, they applied bootstrapping (repeated resampling of the data) to estimate uncertainty, and adjusted for known regulatory changes — a 2015 rule requiring bycatch-reduction grids on shrimp trawl and 2019 seasonal fishing restrictions.

Total cod landings peaked around 250,000 kg in 2004, fell to about 84,000 kg by 2010, stayed relatively stable through 2015, then dropped sharply to under 5,000 kg by 2024 (Figure 1). CPUE followed a similar pattern: relatively stable at 20–30 kg per trip before 2015, then falling to around 6 kg by 2018 and roughly 3 kg after 2020 (Figure 1). By 2018, the biomass proxy had dropped below 20% of its 2004 peak. This decline held across different gear types and matches an independent survey showing a comparable drop in cod abundance since 2011.

This pattern isn't unique to the Oslofjord. Cod stocks in the southern North Sea, the Skagerrak coast, and the Kattegat show similar declines over the same period (the Öresund strait has been a notable exception, trending upward). The study doesn't pinpoint a single cause — likely contributors include fishing pressure, warming waters (cod here are near the southern edge of their range), pollution, shifts in prey availability, and oxygen depletion linked to eutrophication, a known issue for cod in the region. Genetic studies also show Oslofjord cod include both a fjord-resident ecotype and a more offshore North Sea ecotype, with some mixing from neighboring populations — meaning the fjord's cod are ecologically connected to a wider, also-struggling regional picture. The study's main contribution is a quantitative, uncertainty-aware baseline — something the Oslofjord has lacked.

References

Langangen Ø, Kaartvedt S, Hylland K, Durant J.M. (2026) The collapse of Atlantic cod in the Oslofjord. Front Mar Sci:13:1746235.