Shun is the window when a fish carries the most fat. Fish feed heavily and store fat before spawning and before winter, and the timing of both is set by water temperature — which differs from sea to sea. So the same species peaks at different times in different places. That single fact is why regional sushi exists.
Every English-language guide to seasonal sushi is a list: spring is sea bream, autumn is saury, winter is yellowtail. The lists are accurate and they explain nothing. They cannot tell you why the same yellowtail is a winter fish in Toyama and an ordinary fish two hundred kilometers away, or why one species of tuna arrives twice a year as two different foods, or why a fish whose written name contains the character for “spring” is a winter delicacy in Osaka.
The answer is the same in every case, and it is not complicated: follow the fat.
Why fish have a season at all
The spawning clock
A fish preparing to spawn is a fish preparing to spend an enormous amount of energy at a moment when it will barely eat. The solution is to eat ferociously beforehand and bank the surplus as fat. In the weeks leading up to spawning, the fat content of the flesh climbs steeply, and with it the concentration of the fatty acids — DHA and EPA among them — that give oily fish both their nutritional reputation and their particular richness on the tongue.
Then the fish spawns, and the account is emptied. The same species, caught a few weeks later, can be thin, watery, and unremarkable. Nothing about the fish has changed except its calendar.
Cold water thickens the line
The second driver is winter. Fish in cold seas lay down fat before the cold arrives, both as insulation and as reserve, which is why so many of Japan’s great winter delicacies come out of the Sea of Japan and the northern Pacific. Cold water also firms the flesh. Warm-water fat and cold-water fat are not quite the same material: the cold-season version tends to be denser and to melt more slowly, which is precisely the sensation a sushi chef is aiming at when a slice of winter yellowtail sits on warm rice.
And because water temperature is what sets both clocks — spawning and wintering — the season moves with the sea. A current that runs two degrees warmer shifts a fish’s peak by weeks. The calendar is written by the ocean, not by the country.
A note on technique
It is worth adding that Japanese sushi technique is not only a celebration of peak seasons; some of it grew from their opposite. The classic Edo-mae methods — marinating in soy (zuke), curing with salt and vinegar (shime), simmering and glazing — are generally understood to have arisen from the practical needs of an age without refrigeration, and they also do useful work on fish that are not at their fattest. A lean fish handled well can be superb. Seasonality is a preference, not a rule with no exceptions.
Case study: buri, and a round trip of Japan
The clearest demonstration of all this is a single fish making a single journey.
The route
Japanese amberjack — buri, yellowtail — spawns in spring in the seas around Kyushu, in the southwest. The young fish are carried north by the Tsushima Current, the warm flow that runs up the Sea of Japan side of the archipelago, and they keep going: over the following seasons they travel the length of the country, reaching the rich, cold feeding grounds off Hokkaido, where summer and autumn are spent eating.
Then winter comes, and the fish turn around. Heading south to spawn, the schools run down the Japanese coast — and a portion of them, at the point where the coastline bends, drive straight into Toyama Bay. They arrive at Himi having eaten all summer, having fattened all autumn, and having not yet spawned away a gram of it.
That is kanburi — cold yellowtail — and its season runs roughly from late November to February. Which means the fish sold at a fine counter in Tokyo in January is the same animal, from the same spawning grounds, as a rather ordinary fish caught off Kyushu in May. Where you intercept it on the loop decides everything.
Seasons that are declared, not assumed
Himi does something with this that is, as far as culinary bureaucracy goes, quietly magnificent. The town’s fish branding council issues an official “Himi Kanburi Declaration.” Weight, fat condition, and the outlook for stable supply are assessed together, and only when the criteria are met does the council announce that the season has opened. Fish landed after the declaration can be sold under the Himi kanburi name; the announcement makes the local news.
Consider what that means. In Himi, the arrival of winter’s best fish is not a guess, a tradition, or a marketing slogan attached to a date on a calendar. It is verified, and then declared. Somewhere in Toyama, every year, a committee decides that the season has officially begun — which is either the most bureaucratic thing in Japanese food culture or the most romantic, depending on how you look at it.
Case study: katsuo, one fish and two seasons
Skipjack tuna makes the second point: a species can have two seasons a year, and be two different foods in them.
Katsuo migrate north along the Pacific coast in spring, following warm water, and back south in autumn. Caught on the way up, they are hatsugatsuo, “first bonito”: lean, clean, bright, faintly metallic in the best way — the flavor of a fish that has been swimming, not feeding. Caught on the way back down, after a summer of eating in northern waters, they are modorigatsuo, “returning bonito”: heavy with fat, soft, almost sweet, closer in character to fatty tuna.
Same species, same year, opposite ends of the flavor spectrum. Ask for katsuo in April and October and you are ordering two different dishes.
The cultural weight sits on the spring version. Edo had a mania for hatsumono — the first of anything in its season — and the first bonito of the year was the crown of that obsession. It is said that Edo townspeople paid extravagant, faintly ridiculous sums for the earliest fish, and the boast survives in a famous line of verse about a first bonito being worth pawning one’s wife for. The prices are folklore; the appetite behind them is not. In a city with no refrigeration, the first arrival of a fish was the most reliable news of the season anyone had.
Case study: sawara, the same fish in opposite seasons
The third case is the strangest, because the fish does not move much at all — the definition does.
Spanish mackerel, sawara, is a spring fish in the Kanto and Tokai regions, at its best roughly March to May, when it comes into coastal waters. In Kansai and around the Seto Inland Sea, the prized season is winter, December to February, when the fish carries far more fat. Same species, same country, opposite seasons — and both regions are right, because they are measuring different things: one region prizes the fish’s arrival, the other prizes its fat.
The written name records the argument. Sawara is written 鰆 — the character for “fish” beside the character for “spring.” The name reflects the regions where spring was taken as the fish’s season, which makes the kanji a small monument to a disagreement that was never settled and never needed to be.
A map of the sushi year
With the mechanism in hand, a list finally becomes useful — not as a rulebook, but as a set of expectations to test against whatever the counter actually has.
Spring (March-May) — sea bream (madai, at its celebrated “cherry blossom” peak), sawara, the first bonito, firefly squid, short-neck clam, rockfish
Summer (June-August) — horse mackerel, sardine, sea bass, octopus, conger eel, and freshwater eel — the last of which has an entire midsummer ritual attached to it, the Day of the Ox, which we cover in our guide to unagi
Autumn (September-November) — Pacific saury, returning bonito, mackerel, salmon, scallop
Winter (December-February) — yellowtail at its kanburi peak, cod, flounder, snow crab
Two habits make this list work harder. First, treat the boundaries as soft: a season is a curve, not a switch, and a fish two weeks early or a warm year can move everything. Second, remember that the same list would look different if written in Hokkaido, or in Kyushu, or four hundred years ago.
Using this at the counter
Here is the practical payoff, and it is a single sentence: ask what is in season today. Kyō no osusume wa? — “what do you recommend today?” — or simply, “what’s good right now?” No question does more work in a sushi restaurant. It signals that you are interested in the fish rather than a list of names you memorized, and it gets you the best thing in the building, which by definition is the thing that arrived this morning at its peak.
This is also, structurally, what omakase is. Leaving the selection to the chef is not a leap of faith or an upsell; it is a delivery mechanism for seasonality. The chef bought what was best this morning, so the chef decides the order. That is the entire logic of the format.
Menus tend to help too. Look for the character 旬 itself, for 本日 (“today”), or for seasonal words like 春 / 夏 / 秋 / 冬 attached to a dish. And if a shop lists a prefecture next to a fish — Toyama beside the yellowtail, Aomori beside the tuna — that is a claim about the exact loop-point where the fish was intercepted, which is now something you can read. For the rest of the counter etiquette, we have a guide to eating sushi properly.
One more pairing worth knowing: seasonal fish and seasonal sake work on the same principle. Lean spring fish tend to want light, dry, cool sake; fatty winter fish stand up to warmer, richer pours. The reasoning is covered in our guide to sake.
Two technologies, one food
There is a neat symmetry in all of this, and it is worth naming.
Sushi began as a way of defeating time. The original form was fermented, packed in rice and weighted for months or years so that a summer catch could be eaten in February — a technology built to make the calendar irrelevant, which we traced in our history of fermented sushi and where the word comes from. It was a brilliant answer to the problem of a fish that will not keep.
Shun is the opposite technology. It does not fight the calendar; it obeys it, completely — waits for the exact weeks when a particular fish in a particular sea is at its richest, and eats it then, unpreserved, as close to the boat as possible.
Preservation is the art of holding time off. Seasonality is the art of arriving on time. Japanese food spent a thousand years developing both, and modern sushi is what happens when a culture that mastered the first decided it could finally afford the second.
FAQ
What does shun mean? Shun (旬) is the peak season of an ingredient — the period when it is most abundant, most flavorful, and, for fish, at its fattest. For seafood it is set by the spawning cycle and by water temperature, so it varies by species and by region.
Why is the same fish in season at different times in different places? Because fish store fat before spawning and before winter, and both are triggered by water temperature, which varies from sea to sea. Migratory fish add another layer: they are fattest at a specific point along their route, so the season depends on where the fish is intercepted. Sawara is a spring fish in Kanto and a winter one around the Seto Inland Sea.
What is kanburi? Kanburi is winter yellowtail, at peak fat after a summer of feeding in northern waters and before it spawns. Toyama Bay’s Himi is the most famous source, and the local fish branding council issues an official declaration each year when the season’s fish meet the standard. The season runs roughly from late November to February.
What sushi should I order right now? Rather than memorizing a list, ask the chef what is in season today — it is the single most useful question at a sushi counter. As a rough guide: spring for sea bream and first bonito, summer for horse mackerel and conger eel, autumn for saury and returning bonito, winter for yellowtail and flounder.
