Why Do Anglers Target Dawn and Dusk?
― When the Sun Sets, the Largest Migration on Earth Begins in the Sea
Anglers have a saying: "Fish bite at dawn and dusk mazume." It is not superstition. When the sky darkens, small sea creatures rise to shallow water all at once. And the dim hours are also a time that favours the fish that hunt.
It is 5 a.m. and still dark. Several anglers stand in a row on a sea wall. The spot gave almost nothing by day. But as soon as the sky turns pale, rods begin to bend all along the wall.
Once the sun is high, the excitement dies away as if it never happened. In the evening, the same thing happens again.
Experienced anglers call these hours "mazume" and plan their trips around them. Why do fish get active at dawn and dusk, as if they were carrying a clock?
There are two main reasons
Tiny plankton hide in the dark depths by day and rise to shallow water at night. The "road" they travel passes through the shallow layer just at dawn and dusk.
When the light changes fast, the eyes and group defences of hunted small fish are thought to struggle to keep up. For big fish, it is the perfect time to hunt.
Because these two overlap, both the food and the fish that chase it are most active in shallow water. That is exactly where an angler's rod can reach.
A huge migration happens in the sea every day
The sea is full of countless tiny shrimp-like animals and water fleas, too small to see easily. They are called zooplankton. During the day they stay sunk at depths of tens to hundreds of metres. In bright shallow water, fish that hunt by sight would spot them and eat them at once.
But when the sun sets, they all rise together. The shallows hold plenty of plant plankton to eat. As dawn approaches, they head back down into the deep. This daily up-and-down trip is said to be the largest animal migration on Earth.
So what tells the plankton when to move? The main clue is thought to be brightness. Light in the sea fades with depth. Plankton try to stay at a depth with a darkness that feels safe to them. Look at Figure 1. When the sky darkens, that "same-darkness depth" moves upward.
When the food rises, the small fish that eat it rise too. The big fish that hunt the small fish follow. At dusk, the whole line of "eaters and eaten" is lifted into shallow water. At dawn, the line passes through the shallow layer once more on its way back to the deep.
In the 1940s, ships' sonar began to show a "false seabed" in mid-water that bounced sound back. The layer rose towards the surface at night and sank again in the morning. It turned out to be a vast crowd of small animals moving up and down.
Why does dim light favour the hunter?
The second reason lies in the eyes of living things. A fish's eye has two kinds of vision: one for bright places and one for dark places. Dusk and dawn, when brightness changes fast, are thought to be times when the switch between the two struggles to keep up.
By day, small fish form large schools and protect themselves by watching their neighbours move. As the light dims, the school's movements become harder to keep in step. Meanwhile, many large predators are thought to have eyes that spot prey easily in dim light. And seen from below, a small fish stands out as a dark shadow against the still-bright surface.
A study that watched coral-reef fish from evening into night reported that attacks by predators were concentrated at dusk and dawn. In the "shift change", when day fish return to their shelters and night fish come out, gaps seem to open in the defences. The saying anglers learned from experience, "fish bite at mazume", means they are casting bait right into this hunting time.
Experienced anglers also say, "On cloudy days, mazume lasts longer." The reason is thought to be that under a dark sky, the "same-darkness depth" does not sink so deep even by day. It is also known from observation that on bright full-moon nights, plankton do not rise all the way.
Summary
Fish bite well at dawn and dusk because swarms of food in the sea move up and down in a mass migration as the sky's brightness changes. In addition, the dim hours are thought to loosen the defences of hunted small fish and favour the hunters. The anglers' rule of thumb had pinned down the sea's twice-daily migration and its hunting time.
Sea creatures move by watching darkness, not a clock.
Mazume is the time when the edge of that darkness rises to the depth a rod can reach.
The wisdom of fishers who read fish from flocks of seabirds is explained in "How Do Fishers Know Where the Fish Are Just by Looking at Seabird Flocks?", and why big waves arrive in groups is explained in "Why Do Master Rock Anglers Watch the Sea for a While Before Fishing?". See also "Why Is the Sea Blue?" for why sea light stays blue.
- Prepare a clear glass and a sheet of white paper with black writing. Place the paper under the base of the glass.
- Add milk to the water one drop at a time, stirring, and look at the writing from directly above. Compare how hard the writing is to see when you double the depth of the water.
- If you can get to the sea or a pond in the evening, watch the water surface for about 30 minutes before and after sunset. Count how the number of small fish jumping changes over time.
When you double the depth, the light that reaches does not simply halve. For example, at twice the depth where half the light arrives, only half of a half (a quarter) arrives. This is what it means for light to fade "by a fixed proportion" with depth. Do your observations at the waterside from a safe spot, and finish before it gets dark.
For those who want to know more ― Terms, formulas and links to textbooksShows which level each topic belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school biology and physics
- High school+Advanced high school material, or a textbook sidebar
- UniversityUniversity specialist subjects (marine ecology, underwater optics) not taught in high school
- ResearchNot yet taught even at university as settled fact; being studied by researchers now
Junior highTerms: This phenomenon has a name
- Diel vertical migration: Plankton and other animals moving between deep and shallow water within a day. Most are "shallow at night, deep by day".
- Mazume: An angling term for the dim hours around dawn and around sunset.
- Food chain: The "eat and be eaten" links: plant plankton → zooplankton → small fish → big fish.
Junior highHigh schoolCheck with a formula: How many metres does the swarm rise when the sky darkens?
Light in the sea fades by the "same proportion" for every metre it travels. Written as a formula, this lets us work out how far the "same-darkness depth" rises when the sky darkens.
| In symbols | I ＝ I₀ × e−K z , so Δz ＝ ln( I₀ ÷ I₀′ ) ÷ K |
| In words | Distance the swarm rises ＝ natural log of "how many times dimmer the sky became" ÷ how easily light fades |
| Where it comes from | The law that water absorbs and scatters the same proportion of light per metre (the Beer–Lambert law). If we assume plankton "always stay at the same darkness depth", we get the formula above. |
| I | Brightness at depth z |
| I₀ | Brightness just below the surface (proportional to sky brightness) |
| K | How easily light fades (extinction coefficient). Unit: 1/m |
| z, Δz | Depth, and its change. Unit: m |
| How easily light fades (a guide for fairly clear coastal sea) | K ＝ about 0.1 /m |
| Change in brightness from sunset until the sky is dark enough | said to fall to about one hundredth or less |
| Natural log of 100 | about 4.6 |
| Depth of the swarm by day (an assumption in this calculation) | 70 m |
| Distance the swarm rises | 4.6 ÷ 0.1 ＝ 46 m |
| Swarm depth after dusk | 70 − 46 ＝ 24 m |
| Light left after 1 m (0.1 as an exponent) | about 0.9 (fades about 10% each time) |
| If the sky dims a further tenfold (one thousandth in total), extra distance risen | 2.3 ÷ 0.1 ＝ 23 m |
Just by dimming the sky to one hundredth, the "same-darkness depth" rises by about 46 m. Brightness changes by tens of times, yet depth changes only by addition. That is the feature of this formula. So in the tens of minutes around sunset, the swarm surges up almost to the shallows. The slider in Figure 1 runs on this same formula.
High schoolHigh school+Why bother to move every day?
High schoolPlant plankton photosynthesise, so they gather in the shallow, sunlit layer. For zooplankton, the shallow layer is full of food, but by day it is also a dangerous place where fish that hunt by sight can find them. Feeding in the shallows "only at night" is explained as a way to get both food and safety.
High school+A fish's retina has cones, which work in bright light, and rods, which work in dim light. In many fish, the switch happens as the cells in the retina physically shift position with the light, and this is said to take tens of minutes. The sudden change in brightness at dusk is a race against this switch.
UniversityUnderwater optics and "surfaces of equal brightness"
In oceanography, the fading of light is described by the diffuse attenuation coefficient and handled in the form of the Beer–Lambert law. The surface joining depths of equal brightness is called an isolume, and researchers have studied how closely the movement of zooplankton follows it. The meaning of the migration (the predator-avoidance hypothesis) and its contribution to the "biological pump", which carries ocean carbon into the deep, are summarised in reviews of diel vertical migration. The moving layer seen by sonar is called the deep scattering layer.
📖 Derivation of the formula and further reading: Beer–Lambert law (Wikipedia, in Japanese) ／ Review of zooplankton diel vertical migration (Hays 2003)
ResearchWhat is still not clear
- Do they really watch only brightness? It has been suggested that the speed of change in brightness, the body clock, water temperature and food supply may also act as cues.
- Migration continues even in the Arctic winter, when almost no light reaches. There are reports that even in the season when the sun does not rise, animals move in step with moonlight and faint twilight, and researchers are studying how weak a light they can sense.
- The breakdown of why fish bite at mazume. How much the movement of food and how much the hunters' sharper eyes each matter is thought to differ by species and place, and has not yet been sorted out.
So the content of this article too is "an explanation within what is known now". The anglers' rule of thumb may have known part of the answer before science did.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: links between living things (food chain) | The whole line rising together as it chases food |
| High school | Biology: ecosystems and photosynthesis, animal behaviour | Why plant plankton are plentiful in the shallow layer |
| High school+ | Biology: photoreceptor cells (cones and rods) | Why hunters have the edge in dim light |
| University | Marine ecology, underwater optics | The Beer–Lambert law and isolumes |
| Research | Zooplankton behaviour research | Cues for migration, and migration in polar night |
| ― | Links to daily life | Choosing fishing times, catches on cloudy days |
- Hays, G. C. (2003) A review of the adaptive significance and ecosystem consequences of zooplankton diel vertical migrations. Hydrobiologia 503
- Hobson, E. S. (1972) Activity of Hawaiian reef fishes during the evening and morning transitions between daylight and darkness. Fishery Bulletin 70
- Kirk, J. T. O. Light and Photosynthesis in Aquatic Ecosystems. Cambridge University Press
- Wikipedia, "Beer–Lambert law" (ランベルト・ベールの法則)
* This article is a general-audience science explainer. The figures given are guides and rough estimates to help you understand the mechanism. How easily light fades and the depth of the swarm vary greatly with sea area, season and species. When fishing or observing at the waterside, watch the weather and your footing, and do not enter restricted areas.