a) Shown are near-surface (50 m) silicate concentrations averaged over the period 1960–2013. The thick red contour line emphasizes the diatom-limiting level of 1.5 µM. b) C. finmarchicus abundances (stages V and VI) from the near-surface continuous plankton recorder (CPR) survey are shown for the period 1958–2005. The general mackerel feeding migration is illustrated with grey arrows.