The Norwegian coastal waters are the most important spawning ground of the herring populations of the North Atlantic, and the hatching occurs in March. The eggs float to the surface and are washed off the coast by the northward current. Whereas a small herring population remains in the fjords and along the northern Norwegian coast, the majority spends the summer in the Barents Sea, where it feeds on the rich plankton. Upon reaching puberty, herring returns to the Norwegian Sea. The herring stock varies greatly between years. It increased in the 1920s owing to the milder climate and then collapsed in the following decades until 1970; the decrease was, however, at least partly caused by overfishing. The biomass of young hatched herring declined from 11 million tonnes in 1956 to almost zero in 1970; that affected the ecosystem not only of the Norwegian Sea but also of the Barents Sea.