Temporal Overlap Between Regime Shifts in Cod Productivity
and Putative Drivers
Regime shifts were detected in multiple hypothesized drivers of cod
productivity. However, not all of these had obvious, or at least
immediate, consequences for cod. That is, regime shifts in NAO,
zooplankton abundance, and water temperature were not always followed
within 5 years by regime shifts in cod productivity (Fig. 6a).
The 1961 NAO regime shift (a decline from 0.1 to −2.0) had no
discernable impact, although the cod regime shift that began in 1974 was
preceded by regime-shift increases in both NAO (−2.0 to 1.5) and fishing
mortality (1.10 to 1.52 Flim ). The initial
~75% reduction in copepod abundance in 1981 followed,
rather than preceded, the 1974 shift in cod by 6-7 years (Fig. 6a).
Although the further reduction (~90% decline) inC. finmarchicus in 1997 preceded the 1999 regime-shift decline of
cod (Fig. 6b), cod did not respond when zooplankton increased in 2008
back to its level in the 1981-1996 period. The nearly 50% reduction in
average cod abundance that began in 1974 was not preceded by a regime
shift in water temperature in any month (Fig. 6a). However, the 1999 cod
regime shift either coincided with (October) or was preceded 2-5 years
earlier (July-September) by regime-shift increases in water temperature
of 10 to 20C (Fig. 6b), reaching a
maximum monthly mean of almost 190 C in August.