Fig. 16.20 Progressive shift in the relative abundance of Calanus finmarchicus and
Calanus helgolandicus (C. fin/total, %) in North Sea CPR surveys compared to
change in sea-surface temperature (line joining dots), 1958–2005.
(^) (After Beaugrand 2009.)
We understand these shifts in moderate detail. There is a theory, mostly due to the
Scottish worker Michael Heath, for this effect on Calanus. The main bulk of the
eastern Atlantic C. finmarchicus stock is in the Norwegian Sea. In periods of low
NAO, there is more deep overflow in winter from the Norwegian Sea southward
across the Iceland–Scotland ridge. This flow, driven by deep-water formation, travels
along a narrow, deep channel west of the Faroes Islands. It carries with it the resting
stage of C. finmarchicus. They mature in February–March and rise to the surface to
spawn. Some are then carried by surface flow into the northern North Sea, the CPR