I've seen it mentioned multiple places that Japan was reliant on the mainland for their food supply, and that Allied forces especially US submarines had been seriously damaging resupply efforts to the Japanese islands.
Usually, this is just a footnote or something along with talk of the bombings and surrender, but I was curious what the actual situation was in terms of severity.
This answer will be incomplete because I am missing the best source for this-Bruce Johnston's seminal work on "Japanese Food Management during World War II." If you have access to it I suggest reading it for more clarity.
While Japan certainly imported a substantial amount of agricultural products, both from its colonies in Korea and Taiwan as well as abroad, the state of Japanese agriculture in of itself was not so dire on a basic level. However, several aspects of the war dealt major blows to the state of Japan's agriculture and food supply chains that resulted in a society that was on the brink of food riots by the time of the Japanese surrender.
The Japanese military essentially had total control of the Japanese economy at this time. They had, among other things, made military procurement of goods the absolute priority, set price controls on many items, including basic food supplies, and had begun mobilizing the populace for war. However, these sorts of policies all impacted the food supply chain. Military procurement naturally gave food priority to the military and soldiers (many of whom were already starving, but that's another story), but more importantly other industrial products and resources, such as energy and petrol for use in mechanized agriculture and transport of food products (by ship or rail, for example) were impeded. This was both due to the military prioritization as it was to the fact that Japan had originally imported much of these resources. While Japan did not benefit from mechanized agriculture as much as the US did, partly due to the fact that for the most part Japan's farms were essentially tenant based (with one landlord and a bunch of farmers working the land), this nonetheless created downward stress in yield.
Normally, this could be countered by an increased application of human labor. However, with the Japanese military drafting more and more men, as well as redirecting other workers into military related activities (such as digging trenches and forts, etc.) there were less and less hands to do work that required more and more human effort. This led to some interesting situations, such as in Rabaul, where a large Japanese military force trapped on the island without resupply was able to sustain themselves until the end of the war by farming, due to the fact that many of the soldiers had previously been farmers or farm hands. This was while people on Japan itself were becoming more and more desperate.
Fish, historically an important part of the Japanese diet, was also more difficult to get, as fishing vessels were less able to operate due to a lack of oil, aside from the fact that there was a good chance they would be attacked and sunk, as with any other Japanese merchant shipping. This, combined with the price controls on food (thus reducing supply), meant that the black market became a main source of food for much of the Japanese population. As a side point, Japanese merchant shipping was already overtaxed at the start of the war as the Japanese economy relied on foreign shipping (as much as 50% of domestic shipping capacity) that stopped upon the commencement of hostilities. Thus, imports of any kind-abroad or from colonies-became extremely difficult by 1945.
Allied strategic bombing was primarily enacted to stop Japan's war capabilities, such as by destroying aircraft and armament factories. However, the Japanese were able to rebuild many such factories as they were nowhere near as sophisticated as the ones in the US. It took the destruction of small-scale "mom and pop" industry via massed firebombings to cripple the Japanese warmaking ability. This had the obvious side effect of rendering construction of things like agricultural tools and spare parts for machines difficult, not to mention the destruction of distribution centers, logistical channels, warehouses, etc. All these stressed the Japanese economy as a whole, and food distribution in particular. While Japanese rice fields were typically not bombed (due to the lack of military value or cost-effectiveness) the ability to get that food into the cities became a major concern.
As mentioned earlier, the Japanese merchant marine was already taxed well even at the start of the war. Allied submarine warfare combined with the mass mining of Japanese ports essentially shutoff Japan's naval shipping even further. This not only stopped the import and export of colonies but also hampered internal transport, for instance historical areas of commerce like Kobe were essentially shut down due to losses to subs and mines. The end result was that the only method to move food around was basically on land routes, and the lack of motorized vehicles meant that more often than not food would be transported by horse and buggy, severely limiting the speed by which food could be transported.
These factors in tandem led to an unenviable situation for Japanese civillians, where food was becoming more and more difficult to acquire. In the immediate aftermath of the war, starvation was a major problem as the collapse of the militarists' rule led to a good deal of lawlessness in the immediate post-war period. Yet a liberal application of US and foreign aid helped to solve most of these problems, and a major land reform, combined with the economic boom Japan experienced as a result of the Korean War, helped erase the damage caused by the war and the militarist mismanagement.
Hello there. I'm a first time contributor on this subreddit, so please anyone correct me if I'm wrong. John Dower in Embracing Defeat, speaks towards your question. Japan has long had issues with providing enough calories for its citizens. in fact, according to Dower:
Food Shortages had begun to appear in some parts of the country even before Pearl Harbor...
If they started the war from such a desperate place, it's completely unsurprising that the war made it considerably worse. He goes on to say that:
...by 1944 theft of produce still in the fields led police to speak of a new class of "vegetable thieves" and the new crime of "field vandalizing".
The shortages were so bad that, apparently, criminal enterprises were set up to 'procure' food for cities from the countryside. These kinds of shortages and starvation had a direct effect on the war effort (emphasis mine):
In a typical case that August [1944], well before the systematic destruction of urban centers by air raids, 30 percent of the work force at the Mitsubishi glass factory in Tsurumi was found to be suffering from beriberi [a malnourishment disease]. By 1945, food shortages were disrupting the war effort and rending the social fabric. Factory absenteeism rose nation-wide, in large part because workers took time off to bargain and barter for food in the countryside. By July [1945], absentee rates in major cities stood at 40 percent or more, with the food problem being cited as a major contributing factor.
As illustrated here, before the US even began seriously interdicting Japanese merchant vessels, there were still major starvation problems. When the US finally began widespread bombing and submarine operations, the problems got considerably worse. A 40% reduction in workforce efficacy is a major blow to the war-efforts of Japan. He goes on to illustrate the key point your question raises (emphasis mine):
The Allied policy of "economic strangulation" had sent most of the navy and merchant marine to the bottom of the ocean by mid-1945, choking off supplies to the home front as well as the war front. In the Southeast Asian and Pacific theaters, starvation became a major cause of death among fighting men. The home islands were heavily dependent on Korea, Formosa, and China for basic foodstuffs. Before Pearl Harbor, imports from those areas accounted for 31 percent of Japan's rice consumption, 92 percent of its sugar, 58 percent of its soy beans, and 45 percent of its salt.
Clearly the loss of these resources severely impacted the civilian populace of Japan, not to mention the manufacturing capability of its factories or the efficacy of its fighting men. We can see that in some of the shocking statistics recorded by the government:
The average calorie intake per person had by this time [late 1945] declined to far less than deemed necessary even for an individual engaged in light work.
So, in conclusion, it's clear that the Japanese people were well on their way to mass starvation. Crop failures compounded by the complete destruction of the merchant marine pushed the country to the brink of open famine. The surrender on Japan on September 2nd, 1945, was incredibly important for the average person. The US nearly immediately began massive food imports in order to address the clear and well documented caloric deficiencies of the populace. While hunger remained a serious issue in Japan for several years following WWII, it never again was as potentially disastrous as it was in late 1944 leading into the summer of 1945. As always, please let me know if I can clarify anything.