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.