Japan currently imports some 60% of its food; so they'd have to make it through a very bad famine at the start. By year 90 I expect that would have sorted itself out; though with a much smaller population than they currently have.
What the societal repurcussions would be of having such a huge chunk of the population starve to death, plus malnutrition effects on the children of year zero and those born in the generation after , is an interesting question (interesting from a safe distance, of course, especially the safe distance of a fictional context.)
IIRC, Iceland lost 2/3 of its population to famine. I think Japan is about as reliant on fish as Iceland is, too.
Age is really different, though - Iceland's median age is 36 with 1.7 births per woman, Japan's is 48 with 1.3 births per woman. Japan already has a big problem with it's ageing population and relatively low rate of childbearing.
It's likely that losses in Japan will be a larger percentage elderly compared to Iceland - which is still tragic but honestly it's far easier to swallow people over 70 dying than it is to swallow children dying.
If enough people are willing to focus resources on the young, then with a smaller percentage of young people then it's easier to avoid major long-term problems.
It's not fun to think about, but you can survive on vitamins and incredibly low calories. The vitamin supplement industry is pretty solid, I do feel confident in saying that at this moment the stores in most developed countries probably have enough supplements for everyone to have a one a day for at least a few months.
The newspapers in Sweden were half empty by day 9. So I'd say that within 2 weeks, the world had to know that this wasn't likely to blow over and long-term plans had to be made.
If they start rationing fairly quickly, then a few months of nutrition supplementing plus rations can let you put a lot in place.
Japan has a lot of medical infrastructure and while it depends on the supply chain, they could prioritize creating high nutrient supplements that keep you alive while working on bolstering farming. (And, again, this is something that will work better the younger and healthier you are)
Again this is assuming they shut down fast so aren't also grappling with the rash illness and trolls and are just grappling with supply limitations.
There would still definitely be losses to famine. The cities especially - Tokyo is MASSIVELY crowded compared to Reykjavik. Tokyo has 6k people per square kilometer, Reykjavik has 450. That won't be pretty.
Can you imagine if a blue whale or orca managed to grow limbs and climb ashore? They are giants naturally, and would be truly frightening as beasts.
I remember the big blobby thing from adv1 that started following them.
It seems like sea beasts are as vulnerable to drying out as their non-beast cousins. If that's true for whales then it's a "simple" matter of staying far enough from the shore that they simply can't make it without drying out.