| Summary: | The effects of climatic variations are felt most clearly in the case of the pelagic fish populations, those of the herring, anchovy, pilchard, bonito, tuna, menhaden and mackerel, the most marked periodicity being noted in the Atlanto‑Scandian and Bohuslän herring populations. Some regular fluctuation may also be detected in stocks of the Baltic herring, in which spring and autumn‑spawning populations exist side by side, but in varying proportions, with the spring‑spawing variety more numerous during periods characterized by severe winters, e.g. 1926‑30, 1940‑50, and the autumn‑spawning variety during periods of milder winters, e.g. 1931‑39, 1951‑57. The former variety is dominant in more northerly waters, e.g. the Barents Sea, the Norwegian Sea, the White Sea and the northern Baltic, while the latter is most frequently encountered further south, in the North Sea and the southern Baltic. Only under exceptional conditions does the autumn‑spawning herring form a commercially exploitable population in the northern Baltic. The reasons for the variations in the abundance of spring and autumn‑spawning herring lie in the differing capacities of the larvae to survive the cold winters and to exploit the nutrient resources of the sea. The production of zooplankton is more pronounced in the summers following severe winters than in those following mild winters, while a marked peak in production following a mild winter occurs at the major feeding period of the breeding individuals of the autumn‑spawning herring and the critical feeding time for its larvae i.e. early in the spring, while the corresponding peak following a severe winter falls towards the middle or end of the summer, the feeding time for the spring‑spawning herring and for its larvae. Climatologically, an alternation may be detected between periods of severe and mild winters, the forecasts arising from those patters suggesting in turn that a proliferation of autumn‑spawing Baltic herring on the coast of Finland could not be expected before the late 1980's at the earliest.
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