How Norway Created Salmon Sushi
Another way of see this is that both, Japan and Norway have a big aquaculture industry and they compete.To assure the control of the salmon's market is an essential goal for Norway as nation; they produce 1 million of tons each year for the european market. When somebody tries to farm new salmon (in France or Spain or whatever) they typically flood the market with below cost salmon or trouts until the competence is distroyed by lack of rentability. This is probably the reason for not having salmon farms in the NW pacific that could be much more convenient for the Japanese market in the 80's. Only very big companies with lots of resources and money can dare to enter here.