Climate change and abundance of the Atlantic-Iberian sardine (Sardina pilchardus)

Climatic warming is affecting oceanic circulation patterns in coastal upwelling areas, but the impact of this climatic change on pelagic fish populations remains unclear. From juvenile landings collected over 38 years, the thresholds of environmental factors were determined that limited the optimal environmental window (OEW) for sardine (Sardina pilchardus recruitment success in the northwestern Iberian peninsula. The environmental factors considered were: water column stability in February, offshore water transport in March-April (Q(x)MA), upwelling intensity in the preceding year from May to August (Q(x)MJJA), and the winter North Atlantic Oscillation (NAO) index. From 1875 to the mid-1920s, the mean number of years within the OEW was relatively constant. However, since the mid-1920s, there have been oscillations and alternating decades with high and low number of years within the OEW, which were related to oscillations in sardine landings. From 1906 to 2000, there were four record, tow sardine catches in the 1920s, 1950s, 1970s and 1990s, related to a high number of successive years with prevailing conditions out of the OEW. From 1875 to the present, a high year-to-year variation of the NAO, Q(x)MJJA and water stability in February was observed, although with mean values usually within the OEW. The collapse in the 1950s was related, partly, to successive years with low Q(x)MJJA. Successive years with high NAO values may be related to the collapse of the sardine fishery in the 1990s. Q(x)MA has been the most significant factor controlling SRS in this area, being the factor related to the low catches observed in the 1920s, 1950s and 1970s. Water stability was not responsible for any of the collapses observed, but since the 1920s, there has been a significant trend toward decreasing water column stability before the onset of the spring bloom.