1. Why Ice Forms on a Lake's Surface, Not at the Bottom
Imagine a frozen lake on a cold morning. The water touches the freezing air at the surface. However, ice forms there while liquid water remains below because of a rare property of fresh water. As water cools, it becomes denser and heavier. But instead of reaching maximum density at 0°C (32°F), fresh water is densest at about 4°C (about 39°F) [1]. Water colder than 4°C is lighter than water at 4°C, so it floats on top and freezes from the surface down. Additionally, ice is less dense than liquid water, so the ice that forms does not sink but stays afloat [1].
This creates a lid of ice with liquid water underneath. The temperature under the ice is usually reported to be around 3–4°C (about 37–39°F) [2]. For fish, this means there is still a place where they do not freeze. Note that the rule about water being heaviest at 4°C applies to fresh water; salt changes this behavior [1]. This article focuses on freshwater lakes that freeze over. So, what are fish doing in this cold water?
Try it yourself. Put water in a glass and gently add one ice cube. Observe how the ice floats and see how much of it sticks out above the surface. Do not touch it for too long because it is very cold.
2. How Fish Stay Still and Save Energy Under the Ice in Winter
The basic survival strategy for fish in winter is to stay still and use little energy [2]. It is said that even a difference of just a few degrees in water temperature changes how active fish are and how much they eat [2].
Surprisingly, some fish do not eat at all during winter. Fish like bluegill manage by living off fat stored in autumn. Northern pike and bluegill may lose weight during the winter months, which is thought to be because they are using up their stored fat [2].
However, this depends on the species. You cannot say that "all fish stop eating under the ice." It is also better to avoid claiming they are in a state exactly like hibernation. This explanation is based mainly on information from the Center for Limnology at the University of Wisconsin–Madison. Whether the same things have been confirmed for fish in Japanese lakes is not clear within the scope of this article [2]. They stay still, do not eat, and wait. The biggest challenge then becomes oxygen, which is needed for breathing.
3. How Fish Get Enough Oxygen When Ice Covers a Lake
When ice covers a lake, very little oxygen from the air can dissolve into the water. It is explained that the bottom of lakes is especially likely to have low oxygen levels [3].
Snow plays a role here too. If snow piles up on top of the ice, light becomes weaker, which can make it harder for water plants and algae to produce oxygen. Oxygen is also used up when dead plants decompose [4]. Shallower lakes are more prone to oxygen shortages, which can lead to many fish dying in winter (called winterkill). The severity of the damage depends on the depth of the lake and the amount of snow, so specific numbers are not given here.
Lake Biwa's North Basin provides a contrasting example. This lake usually does not freeze completely. From spring to autumn, water layers separate, causing oxygen at the bottom to decrease, reaching its lowest point in late autumn. When the surface cools in winter, the whole lake mixes (full circulation), returning oxygen to the bottom. This is said to be confirmed typically from early to mid-February [5]. Lakes that freeze and lakes that mix to restore oxygen have different winter mechanisms.
One fish that spends the winter under these harsh oxygen conditions is the smelt (wakasagi). The next section looks at life under the ice and how people interact with it.
4. How Smelt Live Under the Ice of Lake Abashiri and Are Caught
In Lake Abashiri in Hokkaido, holes are made in the frozen surface, and nets are pulled under the ice. This is called under-ice net fishing, and the season runs from January to March [6]. Smelt are fish that have a wide range of adaptation to water temperature and salinity, and they are also strong against cloudy water; they are found in Hokkaido [6].
How do smelt spend their time under the ice? A study using fish finders at the Gunma Prefectural Fisheries Experimental Station found that smelt move in schools during the day but break up and scatter at night. Schools often swim in layers slightly below where plankton is found, but how they choose these routes is said to be largely unknown [7].
It is important to note that this observation is not written as being limited only to winter. Details about which temperature layer or depth smelt live in under the ice cannot be stated from these materials. There are still many things we do not know about smelt.
"Ice fishing for smelt," where people drill holes in frozen lakes to fish, is introduced as a winter tradition in Hokkaido [6]. People fish for smelt on frozen lakes in winter.
5. Experiments You Can Try Next: Watching Ice Float in Water
You can check this at home by watching how an ice cube floats in a glass. Using ice and water from your freezer, you can observe how ice floats on water. If you are near a pond or lake, try observing the surface from the shore in winter. Do not step onto the ice.
If you want to look up information online, the Hokkaido Government's fisheries web encyclopedia has details on smelt fishing and ice fishing. The Gunma Prefectural Fisheries Experimental Station page has information on smelt school behavior. The University of Wisconsin–Madison blog is detailed about oxygen under ice and fish behavior, but it is in English, so you might want to read it with an adult if you are interested.