Representation of the long-term changes in the expected abundance of (A) <i>C. finmarchicus</i> and (B) <i>C. helgolandicus</i> as a function of months and years for the period 1958–2008 (matrix 12×51).

<p>(A) Dashed black lines represent the months selected (from April to October) to calculate <i>C. finmarchicus</i>‘s expected abundance. For <i>C. helgolandicus</i>, all months were included (from January to December). Both observed (in black) and expected (in red) annual mean abundance of (C) <i>C. finmarchicus</i> and (D) <i>C. helgolandicus</i> are compared using a Pearson correlation coefficient. The period 1982–1988 characterising the abrupt ecosystem shift in the North Sea is represented by a shaded strip.