An acoustic survey targeting blue whiting (Micromesistius poutassou) spawning and post spawning aggregations in the north east Atlantic in 2004. This survey took place in March"April on board the RV Celtic Explorer. The purpose of this cruise was to survey blue whiting spawning stock using acoustic techniques in coordination with vessel from other participating vessels. Biological sampling of echotraces was carried out to determine species composition, age, spawning state and age profiles of blue whiting. The acoustic survey programme is carried out to estimate the abundance and distribution of target species in the seas around Ireland for fishery management purposes.