Good point. Did the USSR leaders know about the positive health effects of fish (vitamin D, fatty acids etc.) though, and if they did, why didn't they use that as an argument to convince people?
Come to think of it, they probably didn't.
But a quick online search seems to show that the 70s was around when the world understood Vitamin D a lot more, so could be.
We live far north in Norway, sunlight is relatively scarce through the year, which means Vitamin D deficiency is a concern.
But what is also a good source of Vitamin D? Fish!