Is there more fish on a full moon (spring-tide) day?
No difference
Anglers have a stubborn belief — "solunar theory" — that the moon's position governs when fish feed, so they bite, and are caught, around the full and new moon (the spring tides). We test it against how much fish reaches the market: the daily wholesale volume of seafood at Tokyo's central wholesale market.
Total wholesale volume (all seafood)
- Average (whole period)
- 1,758,310 kg/day
- Baseline (same weekday & month average = 100)
- 100.0
- Full moon days (±24h)
- 98.6
- New moon days (±24h)
- 99.6
- Verdict for full moon days
- No difference
Fresh fish
- Average (whole period)
- 653,098 kg/day
- Baseline (same weekday & month average = 100)
- 100.0
- Full moon days (±24h)
- 97.8
- New moon days (±24h)
- 100.3
- Verdict for full moon days
- No difference
Frozen (a control the moon can't touch)
- Average (whole period)
- 298,438 kg/day
- Baseline (same weekday & month average = 100)
- 100.0
- Full moon days (±24h)
- 96.8
- New moon days (±24h)
- 98.1
- Verdict for full moon days
- No difference
Data: 2004–2026, Tokyo Metropolitan Central Wholesale Market daily report (seafood, all markets), 5,970 trading days
Act II: If not the moon, what moves the fish?
Came looking for a difference? Here is a real one.
−57%
The biggest month is December, about 1.1x the yearly average — set not by the lunar phase but by the season and the year-end table. By weekday too, the busiest Monday runs about Sundayx the quietest 1.1787573316661861: the market's opening rhythm matters far more than the full moon.
And the real change isn't the moon — it's the era. From 2004 to 2026, daily volume fell about 57%. While the phase cycled every 28 days, the fish on our tables quietly thinned out.
The full moon itself doesn't move the catch. "They bite on the spring tide" is the tide's doing, not the moon's — and the tide is something the moon genuinely does move.
The "solunar theory" anglers swear by
Fishing has its own old moon lore. The positions of the moon and sun are said to govern when fish are active, and the bite is supposedly best around the full and new moon — the spring tides. This is "solunar theory," proposed by the American John Alden Knight in the 1930s and built into fishing-forecast apps ever since. Plenty of anglers still cast their lines because "the tide is big today."
If fish were really caught more then, that fish would travel from port to market and across the auction floor. So if the full moon moved the fish, the volume reaching the market should rise around it. We test that against ~22 years of daily wholesale volume at Tokyo's central wholesale market — Japan's largest fish market.
How the verdict is computed
- Data is the daily wholesale seafood volume from the Tokyo Metropolitan Central Wholesale Market's daily report (2004 onward; every trading day's Shift-JIS CSV aggregated). Beyond the total we split out fresh fish, where a fishing effect would show first, and frozen, which the moon cannot possibly move — a built-in control
- A fish market has a strong weekday rhythm (closed Sundays, some Wednesdays) and seasonality (the season's catch, year-end demand). So we use the "same weekday × same month" average as the expected value and compare actual ÷ expected — the same adjustment as the births and accidents topics
- We mark "full moon days" (instant ±24h) and "new moon days" from the lunar phase at noon Japan time, then compare each group's mean index against normal (100)
See the methodology for the full criteria.
Reading the data with care
Market arrivals are a proxy for supply, not the catch itself. Landed fish take roughly a day to reach the floor, and deep-sea frozen tuna flows year-round, almost detached from moon or season. So this data reflects the rhythm of the coastal fresh catch to a degree, but it is not a complete record of how much was caught. Even so, if the full moon swayed fishing meaningfully, its shadow should fall across thousands of trading days.
The real culprits: tide, season, and the era
There may be a grain of truth in "the bite is better on the spring tide." But that is the tide the moon raises, not the moon itself — and the tide is something the moon genuinely moves; on this site too, tides came out clearly "higher" (see tides). What moves the market's volume, on top of that, is the season, the year-end table, and the market's opening rhythm. The lunar phase is none of them.
Sources
- Tokyo Metropolitan Central Wholesale Market daily report (daily wholesale seafood volume, all markets; source: Tokyo Metropolitan Central Wholesale Market)
- Lunar age and the instant of each phase are computed in-house from the algorithms in Jean Meeus, *Astronomical Algorithms* (UTC basis)
Last updated: June 14, 2026 01:17 UTC (rebuilt daily)