YEAR CLASS STRENGTHS OF ZOOPLANKTON IN THE NORTH-SEA AND THEIR RELATION TO COD AND HERRING ABUNDANCE

Time series of North Sea cod, herring and zooplankton indicated a peak in biological production in the northern North Sea in the late 1960s and early 1970s. An increased recruitment of cod corresponded with hig h and increasing rates of fishing mortality. During the same time, the Buchan herring increased in abundance under stable fishing mortality. A synthetic analysis of CPR Area C2 'total Calanus' and 'Paracalanus/ Pseudocalanus (PP)' reflected a declining abundance in the early 1970s . An analytic analysis of time series enabled the identification of la rge and small year classes of Calanus and PP. Large year classes of co d appeared to require large year classes of either PP or Calanus, but not both. Large year classes of cod appeared to be more associated wit h large year classes of PP rather than large year classes of Calanus. Simultaneous large year classes of PP and Calanus rarely occurred. A l arge year class of either Calanus or PP appeared to be required to pro duce a large year class of cod. However, the presence of a large year class of Calanus or PP did not guarantee a large year class of cod. Th e biological changes in the North Sea appeared to be coupled with a di fficult to define climate signal involving changes in the windfield, d eep convection in the north Atlantic, the great salinity anomaly, and changes in the vertical structure of the ocean off Bermuda.