Ikura in sushi restaurants always makes me think of ikra (икра), which is caviar in Russian. It can't be a coincidence but I don't know how it got there.
Hmm - the Google-translated version of that page says that Russian POWs in the Russo-Japanese war used salmon roe (i.e. ikura) as a substitute for caviar. Interesting—thanks!
It maybe makes a bit more sense when you consider 'икра' doesn't just mean caviar but alsо fish roe in general and is also applied even more broadly in entirely fishless things like 'икра баклажанная'