You could acclimatize yourself to raw fish now.
I am not a big fan of fish and I do not find it hard to avoid fish products when I’m in Japan (although I suppose I could be consuming them unwittingly). In most large, commercial places the Japanese are quite accepting of foreigners’ strange requirements, although in the smaller restaurants and bars where English is not spoken you will find it difficult.
You could try this website for more advice: http://isitveganjapan.com/ and I am sure Google can furnish you with many others.
Credit:stackoverflow.com‘