Detailed characterisation of the spawning distribution of Blue Whiting in the Northeast Atlantic

Supervisor: Mark Payne (National Institute of Aquatic Resources (DTU Aqua), Denmark)
Blue whiting (Micromesistius poutassou) is a pelagic gadoid found in the northeast Atlantic, spawning preferentially in the region west of the British Isles from February to May. The species is subject to an economically important fishery. However, the abundance of blue whiting has undergone strong variations for the past 15 years: a large increase after 1995 until 2005 followed by collapse in the following years. The mechanisms involved in the population dynamics and more particularly the spawning of blue whiting remains still poorly understood. The variations of the spawning distribution are partially associated with the change of strength of the north Atlantic sub-polar gyre. Furthermore, the population structure is also unclear. Although many studies (genetic analysis, otolith study and drift modelling) point towards the existence of two stocks, management of this fishery is based on a single stock hypothesis. The reproduction and the stock composition of blue whiting are still poorly understood and actually represent the main obstacle for an appropriate management. « Traditional» data sources (e.g. catches, surveys) mostly cover a short period of time (from 1980) and are too sparse in the studied region. To attempt to answer many of these questions, and around the limitations of other data sources, we used data from the Continuous Plankton Recorder which extend over a longer period of time (1948-2005) and cover the spawning area more thoroughly. This study allows a better visualisation of the spawning variability and similarly offers new insights about the blue whiting dynamic. We demonstrated the existence of a separation line between two populations of blue whiting in the northeast Atlantic between 52°N and 54°N. We showed that there are two sub-populations of blue whiting: one component located in the vicinity of the Rockall Trough and another component near the Porcupine Seabight. The study also reveals that the southern population spawns in general one month earlier than the northern population. Previous reports relating the effect of the north Atlantic sub-polar gyre to the spawning of blue whiting are confirmed and extended in time. However, the inter-annual variability appeared to be minor. In conclusion, we presented here a detailed characterisation of the spawning distribution of blue whiting and environmental correlates in the northeast Atlantic. This provided a better understanding of the structure and the processes involved into the dynamics of this population.