1. Where Do Rice Fish Live in Streams, Ponds, and Rice Paddies?
Rice fish inhabit waters near where people live, such as small streams, ponds, and rice paddies with their irrigation channels on plains. They are found throughout Japan, except in Hokkaido [1]. The Ministry of the Environment states that rice fish prefer places with no current or a gentle flow, where water plants grow thickly. This includes rice paddies, irrigation channels, and slow-moving parts of lakes and rivers in flat areas [2].
This preference matches their swimming ability. Materials from Fuji City explain that because their swimming power is weak, they are swept away if the current is fast [4]. In captivity, the Miyagi Prefectural Comprehensive Education Center notes that they are very vulnerable to stress from water flow, so the center advises against using air pumps. Instead, water plants should be added to create hiding spots [5]. In the wild, water plants likely soften the current and provide cover. But what do they eat in these quiet places?
2. What Do Rice Fish Eat and When Do They Breed?
Rice fish are omnivores. They eat water plants and algae, such as green algae, as well as mosquito larvae and water fleas [1]. The Kyoto Prefectural Red Data Book also confirms their omnivorous nature and notes that in captivity, they lay eggs all year round [3].
In the wild, the breeding season is from April to August. Egg-laying begins when the water temperature reaches 18°C (about 64°F) or higher. The number of days until eggs hatch depends on the water temperature. For example, at 18°C, it takes 20 days, while at 25°C (about 77°F), it takes 10 days [1]. This suggests that for every 7°C increase in temperature, the hatching time is roughly halved. These numbers are estimates; the key point is that higher temperatures lead to faster hatching.
The name "medaka" (eye-high) is explained in some city documents as coming from their eyes being positioned high on their heads, making it easier to spot small insects falling onto the water surface [4]. However, this is just one theory about the origin of the name.
3. Can Rice Fish Live in Slightly Salty Water Near River Mouths?
Rice fish have a tolerance for salt that allows them to live in brackish water areas near river mouths [1]. Brackish water is a mixture of river water and seawater, with low salinity.
It is important to note that they have not become sea fish. Rice fish normally live in freshwater rivers, ponds, and rice paddies. Their salt tolerance is best understood as a trait that slightly widens the range of habitats they can occupy.
4. Why Were Japanese Rice Fish Split into Two Species in 2012?
In 2012, Japanese rice fish were divided into two species. The southern group is known as *Oryzias latipes* (Minami-medaka), and the northern group is a new species called *Oryzias sakaizumi* (Kita-medaka) [6]. Kita-medaka are found on the Sea of Japan side, from Aomori Prefecture to the eastern part of the Tango Peninsula in Kyoto Prefecture.
The boundary between the two species is the Yura River system in Kyoto Prefecture. The Japan Science and Technology Agency reports that even where the two species meet, they do not interbreed and maintain independent populations [6]. This research was published in a German ichthyology journal that year.
The two species look very similar. To tell them apart, one must look at fine differences, such as the notch in the dorsal fin. This means that among the fish we have always called "medaka," there were differences that had gone unnoticed until recently.
5. Why Have Rice Fish Numbers Decreased?
The Ministry of the Environment cites the deterioration of water environments, rural development, and predation by non-native fish as causes for the significant decline of rice fish nationwide. They describe the rice fish as one of the most familiar wild creatures to Japanese people, known from the song "Medaka School" [7].
In the Ministry’s Red List 2020, the Minami-medaka is classified as Vulnerable (Category II) [7]. Specific reasons include the filling in of ponds and swamps, the improvement of rice paddies and irrigation channels, and competition with non-native species like the eastern mosquitofish [2].
Researchers at Mie University note that construction projects, specifically land consolidation for agriculture, turned small streams and irrigation channels into concrete structures and filled in reservoirs and channels [8]. This reduced the habitats available for aquatic life. For rice fish, rice paddies and channels were places with gentle currents. The loss of these specific habitats is considered linked to their population decline.
6. Is It Okay to Release Pet Rice Fish into Ponds?
The Kyoto Prefectural Red Data Book lists the release of commercially sold rice fish as a factor in their decline, alongside development, water pollution from pesticides, predation by largemouth bass and bluegill, and the introduction of mosquitofish [3]. Releasing commercial rice fish can risk destroying the genetic uniqueness of local wild populations.
As seen earlier, rice fish are now divided into two species, with further regional variations. A well-intentioned release might mix local fish with fish from other areas. If you want to help declining rice fish, the guidance from the Ministry of the Environment and Kyoto Prefecture suggests that it is better to first check if the water body has gentle currents and water plants, rather than simply releasing fish.
7. How Can You Safely Observe Rice Fish Habitats?
If there are rice paddies or small channels nearby, observe them from a safe distance with an adult, without entering the water. Watching how fast leaves float on the surface can help you understand the current. Look for water plants as well. It is okay if you do not see any rice fish. Recording the current and plant conditions in a notebook allows you to verify the "conditions" described in this article with your own eyes. Be careful not to get too close, as the ground near water bodies can be slippery.
SAFETY NOTE: When checking water bodies, observe nearby channels or ponds with an adult, without entering the water.