Cyclic fluctuations of blue whiting (Micromesistius poutassou) linked to open-sea convection processes in the northwestern Mediterranean

In the Mediterranean, blue whiting, Micromesistius poutassou, constitutes a traditional fisheries resource. Over several decades, blue whiting landings in the Catalan coast (northwestern Mediterranean) have displayed cyclical variations, of c. 6yrs, slightly decreased to five in the last two decades, as shown through wavelet analysis. These fluctuations have persisted under very different levels of fishing effort. This study evaluates the hypothesis that deep-water formation in the adjacent Gulf of Lions, and the enhanced primary productivity related to it, determines recruitment strength in blue whiting that results ultimately in the observed periodicity of the blue whiting landings. The link between landings and environmental drivers was explored using lagged cross-correlations, with 0- and 1-yr lag. The variables considered included large-scale indices [North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO)], Mediterranean climate indices [MO and Western Mediterranean Oscillation (WeMO)], and variables defining the local environmental conditions in the northwestern Mediterranean (sea-air heat flux, winter air temperature anomaly and Rhone river runoff). Significant correlations were only found between landings (1961-2011) and sea-air heat flux, which is generally taken as an indicator of processes of deep water convection, at 0 and 1-yr lag. These results suggest that the observed fluctuations in blue whiting landings respond to oceanographic processes taking place in the Gulf of Lions.