Advances in ECOLOGICAL RESEARCH VOLUME 7 This Page Intentionally Left Blank Advances in ECOLOGICAL RESEARCH Edited by J. B. CRAGG Environmental Sciences centre (Kananaskis), University of Calgary, Calgary, Alberta, Canada VOLUME 7 1971 ACADEMIC PRESS, INC. (Harcourt BraWiLs st1b~tikilti~lly less than the ~)erioclfix t,tw food poj)iilatioii to recover, but it does illustrate the range of possibilities open. Sucli considerations of the food supply also suggest that a stock/rccruitmcnt curve with a maximum is quite feasil)lc evcn when cannibalism docs not occur. D. YEAR-CLASS FLUCTUATIONS The application of thc Rickcr or Beverton and Holt or other model to a particular fishery is made difficult, by thc commonly very great fluctuations in the number of' recruits, which are clearly not related in any simple way to adult stock. In tempcrate and sub-arctic waters the fluctuations in year-class strength can bc very great. The single 1904 year-chss of Noiwrgian herring was about three times as large as that of the nest largest year-class between 1899 and 1920, and perhaps twenty times thc rtveriigo of thr rest. More recently thc year-classes of haddock on 1902 and l ! W on Georgcs Bank, snd 1962 in the North Sea \rwe all several tinws ttic average, and a t lcast 100 timerj the strength of some recent weak year-classcs (Jones, 1 !MA; (iraharn, 1067). The haddock catch on Crorgcs Bank row to 1.55 000 tons in 1965, coniptired with catch undcr hcst managemen t from year-classcs of arcragc strength of 45 000 tons; in the North Sea the haddock catch rosc from under 60 000 tons in I963 to 260 000 tons in 1966. The size of the spawning stocks from which these outstanding year-classes originated was by no means abnormal, and very similar to the size of the parent, stock from which very weak year-classes originated a t a separation of' only a few years. Subsequent year-classes on Georges Bank have. up to 1968, all been very weak, but those in the North Sea have been good, one even better than 1962. These year-class fluctuations produce a great scat 0.05) between temperature and precipitation with a decline from 102 t o 103% of yearly precipitation in the warmer phase from AD 1150-1300 t o 92.6-96*5% in the cooler phase from 1550 to 1750.This relationship reflects the correlation of + 0.42 found between annual rainfall and annual mean temperature since 1740 (Lamb, 1985). Summer precipitation however, showed an inverse trend t o temperature ( p = -0.27, P > 0.05), with the highest summer precipitation occurring during the 1550-1750 temperature minimum. This pattern also applies t o the mean high summer wetness/dryness index for Britain, Germany and Russia near 50"N (Table 111) which is correlated ( p = -0.26, P > 0.05) with the winter mildness/severity index for the same area. There seems t o be a tendency in these areas for colder periods t o be drier on a yearly basis, but wetter during the summers, and for warmer periods to be wetter as a whole, but drier during the summers. RECENT CLIMATIC CHANGE 187 The positive correlation in England between total yearly precipitation and prevailing annual and winter temperatures was explained by Lamb et al. (1966) as being related to the prevalence of south-westerly winds. Mild epochs had a high prevalence of south-westerly winds and therefore high sea temperatures. This rguulted in more moisture input into the atmosphere and moister winds blowing across the country. Colder epochs were associated with blocking anticyclones from Greenland or northern Europe and therefore less rain-produced upward motion and drier winds. Lamb et al. considered these relationships were probably characteristic of the western half of Eurasia in the latitudes where the westerly winds prevail and that similar conditions might be expec