Working Group on Mackerel and Horse Mackerel Egg Surveys (WGMEGS)

The Working Group on Mackerel and Horse Mackerel Egg Surveys (WGMEGS) is responsible for the planning, data collection, and data analysis of the ICES triennial mackerel and horse mackerel egg surveys. 2025 is the year of the actual survey and the working group (WG) performs most of its duties by correspondence. However, similar to 2022, a post-survey meeting was held in Copenhagen, Denmark, on 25th and 26th August 2025 chaired by Brendan O’ Hea to review the preliminary results of the 2025 survey. The meeting was a hybrid meeting with eight people attending in Copenhagen, and a further nine attending online. As such, this report focuses heavily on the ToRs that are directly involved with the successful execution of the mackerel and horse mackerel egg survey (MEGS) in 2025.

The results of the two online Workshops on Mackerel, Horse Mackerel and Hake Eggs Identification and Staging (WKMACHIS2, 21-25 October 2024) and on Adult Egg Production Methods Parameters estimation in Mackerel and Horse Mackerel (WKAEPM2, 11-15 November 2024) were discussed, with the subsequent enhancement and recommendations proposed during these workshops outlined in the workshops reports (ICES, 2022a, b) and also incorporated into both of the WGMEGS manuals (ICES 2019a, b).

The 2024 WGMEGS meeting drew up a survey plan for 2025, dividing the survey into six time periods and allocated countries to these periods. After the meeting both Portugal and Norway announced that, due to national considerations, they would be unable to participate on the survey in 2025. Portugal was, however, in a position to collect adult samples from markets. Norway was also able to participate in the screening and analysis of the histology samples.

In 2025, Spain (IEO and AZTI), Ireland, UK/Scotland, UK/England, the Netherlands, Germany, the Faroe Islands, and Denmark participated in the egg survey.

In 2025 the survey was split into six sampling periods. Due to the withdrawal of the Portuguese survey no sampling took place in ICES area 9a. The first targeted survey commenced at the start of March and the final period ended in late July. The Cantabrian Sea was sampled in periods 3 – 5 while Biscay was sampled in periods 3 to 6. The Celtic Sea and waters west of the British Isles were sampled during periods 3 to 7, with sampling taking place in the North Sea during period 6.

Mackerel daily egg production was highest in period 5, (May), for the western component, while for the southern component the maximum spawning intensity was observed in period 3 (March). Total mackerel egg pro-duction (provisional, southern and western component combined) was 1.162 * 1015, a reduction of 41% on the 2022 figure. This comprised a reduction of 36% in the Western area and 76% in the Southern. The combined estimate is the lowest in the time series.

For calculating the spawning stock biomass, SSB, WGMEGS used an averaged realised fecundity figure based on the last three surveys, 2016, 2019 and 2022. The provisional SSB estimate was 2.26 * 106 tonnes, a decrease of 36% from 2022. The final egg and SSB estimate will be provided in 2026 once all the fecundity samples have been analysed.

For the western stock of horse mackerel, highest mean daily egg production was estimated during June, period 6. Spawning was higher in April and May compared with that found during 2022, with an obvious peak occur-ring in period 6. Total annual egg production for western Horse mackerel was 5.68 * 1014, a 10% increase on 2022.