A detailed characterisation the spawning distribution of Blue Whiting

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We provide here the strongest evidence to date supporting the existence of two independent Blue Whiting (Micromesistius poutassou Risso, 1827) stocks in the North Atlantic. In spite of extensive data collected in conjunction with the fishery and management of this species, the population structure of Blue Whiting is poorly understood. In particular, genetic, otolith and drift modelling studies point towards the existence of two stocks, but observations of adult distributions point towards a single stock. A paradox therefore arises in attempting to reconcile these two pieces of information. Here we analyse 1100 observations of Blue Whiting larvae from the Continuous Plankton Recorder (CPR) from 1948‐2005 using modern statistical techniques. We show a clear separation between a Northern spawning component, in the Rockall trough, and a Southern one, in the Porcupine Seabight. We further show a difference in the timing of spawning between these components of at least a month, and statistically significant differences in interannual variability. The results, taken with previous studies, therefore support the two‐stock hypothesis. Furthermore, we resolve the spawning paradox by showing that the acoustic observations cited in support of the single‐component model are not capable of resolving both stocks, as they occur too late in the year and do not extend sufficiently far south to cover the southern component.