INTERNATIONAL BLUE WHITING SPAWNING STOCK SURVEY (IBWSS) 2026
Ciaran O’Donnell -Fisheries Ecosystems Advisory Services (FEAS) – Marine Institute
In March 2026, the RV Celtic Explorer participated in the International Blue Whiting Acoustic Survey (IBWSS). This annual survey estimates the size of the spawning stock of blue whiting (Micromesistius poutassou) in western waters. The 2026 survey was conducted jointly by vessels from Ireland, the Faroe Islands, the Netherlands, Norway and Spain. The survey provides essential data for the annual stock assessment carried out by the International Council for the Exploration of the Sea (ICES). Ireland has contributed to this survey since 2004.
Survey Coverage
The IBWSS takes place during the blue whiting spawning season. Each year, the core distribution area along the continental shelf edge is consistently surveyed. The western boundary of the survey area depends on real time observations during the survey and can extend as far west as the Rockall Bank to ensure that the stock is fully contained by the survey grid.
In 2026, the RV Celtic Explorer commenced surveying on 16 March. Over the following 21 days, the 5 survey vessels jointly surveyed waters from the Porcupine Seabight to the Faroe–Shetland Ridge (Figure 1). Over 5,300 nautical miles (nmi) of survey transect, covering an area of more than 102,500 nmi² of was completed.
Each vessel followed a pre-assigned survey grid, progressing northward. The combined effort resulted in high-resolution transect spacing and effective multi-vessel sampling across the region.
Figure 1. IBWSS coverage and trawl stations by vessel / nation, 2026.
Stock Distribution
The spawning distribution of blue whiting is thought to be partially influenced by the North Atlantic Sub-Polar Gyre, a large, relatively cool and fresh body of northern-origin water. The strength and size of the gyre varies over periods extending across several years, altering the hydrographic conditions over the Rockall Bank and Rockall Trough.
Spawning blue whiting are sensitive to both temperature and salinity. A strong gyre contracts the potential spawning area to the shelf edge due to the narrow northward-flowing shelf-edge current (leading to an eastward-contracted distribution). In contrast, a weaker gyre allows Atlantic waters to extend westward, increasing potential spawning habitat into the Rockall Trough and across Rockall Bank (a westward-expanded distribution).
Between 2020 and 2024, a weakening gyre resulted in a broader spawning range, with blue whiting schools observed further west. However, from 2025, the distribution appears to be constricted towards the shelf edge (Figure 2).
In 2026, most of the stock was located within the Rockall Trough between 55°N and 59°N, along the shelf edge, accounting for 77% of the total survey biomass and 75% of total abundance. Survey scientist Ciaran O’Donnell said, “A more northern distribution of the stock this year would suggest that peak spawning occurred earlier, as in 2025. Most fish in the samples were spent and migrating northward to summer feeding grounds.”
Figure 2. Combined blue whiting acoustic density in 2025
Figure 3. Combined blue whiting acoustic density in 2026
Stock Composition
The age composition of the stock was determined from 35 trawl samples collected during the survey (Figure 1). In total, 7,845 fish were measured for length. Otoliths were taken from 2,000 specimens for age determination, with observed ages ranging from 1 to 10 years.
The observed stock was dominated by 3–5-year-old fish, which together accounted for 72% of total stock biomass (TSB) and 68% of total stock numbers (TSN). Five-year-olds (the 2021 year-class) were the single-most abundant (30%), followed by 4-year-olds (20%). Immature fish comprised just 3% of TSB and 5% of TSN, a small increase from the 2025 survey (2% and 3% respectively). Over 92% of 2-year-old fish were mature, similar to 2025 (96%) and an increase on 2024 (67%).
Stock Size
The total blue whiting stock estimate for 2026 showed a 32% increase in TSB and 34% increase in TSN compared to 2025. Spawning stock biomass and spawning stock numbers increased by 31% and 33%, respectively.
The age structure is now dominated by 3–5-year-old fish and the strong 2020-year class (6-year-old fish) is now declining.
As the IBWSS focuses on the spawning stock, it primarily captures data on mature fish. The survey does not accurately estimate immature fish abundance until these younger cohorts begin recruiting into the spawning population.
O’Donnell noted, “The previously dominant 2020-year class is now in decline due to a combination of natural mortality and fishing mortality. The potential of the 2025-year class will be assessed as returning mature 2-year-old fish during the 2027 survey”.
