Atlantic mackerel spawning in the Northeast Atlantic fell sharply in 2025, with egg production and spawning stock biomass dropping to the lowest levels on record, according to preliminary results from ICES’ Working Group on Mackerel and Horse Mackerel Egg Surveys (WGMEGS).
The total annual egg production for mackerel in the western and southern areas combined declined by 41% relative to 2022. The provisional spawning stock biomass is estimated at 2.3 million tonnes, down 41% from the previous survey and the lowest estimate in the time series dating back to 1992.
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Sharp fall in mackerel egg production
The 2025 survey shows a marked decline in both central components of the stock. In the western region, total annual egg production declined by 36% relative to 2022.
In the southern region, centred on the Cantabrian Sea and the south Bay of Biscay, egg production dropped by around three quarters, making it the lowest value ever recorded for that component.
ICES scientists found that spawning was more restricted than in recent surveys. Egg distribution was primarily confined to the shelf edge, close to the 200-metre depth contour, with little of the westward offshore expansion seen in earlier years. Peak spawning in the western area occurred in May, while the southern component peaked earlier, in March.
Overall egg production across all periods was weaker, flatter, and more compressed in space than in previous surveys, despite sea temperatures being generally higher than in earlier years.
Biomass estimate hits record low
Using the provisional egg production figures and an average realised fecundity from the last three surveys, WGMEGS estimated a combined spawning stock biomass of roughly 2.3 million tonnes. This compares with almost 3.9 million tonnes in 2022.
The decline was driven mainly by the western component, which remains the dominant part of the stock. The southern component contributed only a small share of total biomass in 2025.
ICES stressed that these figures are preliminary. Final estimates will be published in 2026, once all fecundity and histology samples have been thoroughly analysed. Even so, the working group notes that both egg production and biomass indicators point clearly downward.
Mixed picture for horse mackerel and the North Sea
The picture was different for the western horse mackerel. Its total annual egg production increased by nearly 10% relative to 2022, with peak spawning observed in June. However, egg densities remained low across much of the survey area, with spawning concentrated near the shelf edge.
In the North Sea, a separate coordinated survey by England and Denmark recorded a 17% increase in daily mackerel egg production compared with 2022.
Egg numbers were more evenly spread, and low levels of spawning were also observed in the Skagerrak for the first time. These results have not yet been used to calculate a North Sea biomass estimate.
WGMEGS emphasised that changes in spawning timing, distribution, and intensity introduce uncertainty into stock assessments and underscore the need for continued, coordinated surveys across the Northeast Atlantic.