1. Why flounder larvae drift in the sea and then settle onto shallow sandy bottoms
Flounder larvae float in the sea for a while after hatching. Once their body shape is complete, they descend to the bottom and begin life on the seabed. This move is called "settlement." [3]
For stone flounders, a document from the National Institute for Land and Infrastructure Management states that they settle on sandy and muddy bottoms less than 1 m (about 3.3 ft) deep in estuaries and surf zones in February and March. [3] Since 1 m is about waist height for an adult, their first home on the seabed is very shallow, where one can stand.
In the case of the Japanese flounder, a database for Sanbanze in Chiba Prefecture reports that settled juvenile fish are seen from February through June. [1] While the timing varies by species and location, the common point is that they descend to shallow areas from late winter to spring.
2. How the food of Japanese flounder juveniles changes as their bodies grow
According to the Sanbanze database, Japanese flounder juveniles eat animal plankton when they are about 10 mm (about 0.4 in) long. When they reach 16–18 mm (about 0.6–0.7 in) and settle, they start eating small creatures living on the seabed. At 80 mm (about 3.1 in) or more, they also eat bivalves. [1]
The size of 16–18 mm is smaller than the diameter of a 1-yen coin (20 mm, or about 0.8 in). At this time, their food switches from "things floating in the sea" to "things on the bottom." Changing where they live and changing what they eat happen at the same time.
Adults stay on the bottom. For the black flounder, a fishing guide by Honda explains that they prefer sandy or muddy bottoms on the continental shelf within 100 m (about 328 ft) of depth. They eat polychaete worms and crustaceans. [8] Life on the bottom continues into adulthood.
3. How Japanese flounders move from shallow coastal areas to deeper offshore waters as they grow
A survey document for Tokyo Bay in Chiba Prefecture reports that Japanese flounder juveniles appear in coastal areas in spring and gradually move to deeper areas or southward. The density of juveniles peaks in April and May, then drops sharply around June and July. [2] It seems their stay in shallow places does not last forever.
Stone flounders move to deeper places as they grow. Adults are distributed at depths of less than 70 m (about 230 ft) and move between deep and shallow areas depending on the season. [3] A document from Ibaraki Prefecture also states that their main habitat is at depths of less than 150 m (about 492 ft), and they are often found on sandy and muddy bottoms less than 70 m (about 230 ft) deep within the prefecture. [4]
The figures of 70 m and 150 m are different ranges cited by different sources for the same species. The national document describes the distribution of adults, while the Ibaraki document describes the main habitat. This article lists them separately without mixing them.
4. How the living depths of different flounder species compare
Let’s compare the addresses listed in various documents. According to Honda’s fishing guide, the sand flounder lives on sandy and muddy bottoms up to 30 m (about 98 ft) deep. [7] The Japanese flounder is found on sandy or muddy bottoms at depths of less than 100 m (about 328 ft). [1] The brown sole lives in sandy and muddy areas at depths of 50–100 m (about 164–328 ft). [6]
The baba flounder has a habitat depth range of 50–450 m (about 164–1,476 ft). In surveys in Ibaraki Prefecture, they are caught at depths of 80–350 m (about 262–1,148 ft), mainly using bottom trawls. [5]
Comparing the shallowest sand flounder at 30 m (about 98 ft) with the deepest baba flounder found in Ibaraki surveys at 350 m (about 1,148 ft), the latter is about 11.7 times deeper. If you stood a 25 m (about 82 ft) pool vertically, 30 m would be 1.2 pools, and 350 m would be 14 pools.
5. Where natural Japanese flounder eggs were found in inner Tokyo Bay
In surveys of inner Tokyo Bay by Chiba Prefecture, natural eggs of Japanese flounders were found most often from late December to early January. They were found in muddy sand to mud bottoms at depths centered around 5 m (about 16 ft), ranging from 4.5 to 11 m (about 15–36 ft). The eggs sink and stick to the bottom, and hatching takes about one week. [2]
On the other hand, the Sanbanze database lists the spawning period for Japanese flounders as December to April, and the spawning location as bottoms at depths of 10–60 m (about 33–197 ft) outside the bay or near the bay mouth. [1] This differs from the 5 m depth found in inner Tokyo Bay. The sources describe different locations; they do not give one depth for all spawning sites. Neither is wrong. The spawning location of Japanese flounders cannot be summarized as a single depth.
Spawning times also differ by species. The sand flounder spawns in May and June along the Sea of Japan coast of Hokkaido, which is different from winter spawners like the Japanese flounder. [7]
6. Why some flounder species need shallow areas to complete their life cycle
A document from the National Institute for Land and Infrastructure Management states that in Tokyo Bay, the catch of stone flounders has decreased as tidal flats disappeared. It identifies the stone flounder as an indicator species because it needs tidal flats and shallow areas to complete its life cycle. [3]
The document says "along with the disappearance of tidal flats." It does not directly state that the disappearance caused the decrease in catch. The document does not attribute the cause of catch changes solely to tidal flats. What is known is that larvae settle on sandy and muddy bottoms less than 1 m (about 3.3 ft) deep, and adults are distributed at depths of less than 70 m (about 230 ft).
Babies that settle on sand less than 1 m (about 3.3 ft) deep eventually go to depths of around 70 m (about 230 ft). The starting point of this long journey is shallow water where a person can stand.
7. How to feel the depth of flounder habitats by walking steps
In a wide area with no cars (like a schoolyard), walk assuming one step is about 50 cm (about 20 in). The 30 m (about 98 ft) depth of the sand flounder is about 60 steps. The depth of up to 100 m (about 328 ft) for the Japanese flounder is about 200 steps. The deeper side of the baba flounder surveyed at 350 m (about 1,148 ft) is about 700 steps.
However, depth is a vertical length. Imagine standing these lengths vertically on the ground. By visualizing the same length as depth rather than horizontal distance, you can understand how far away flounder homes are.
You can read about these species on the fisheries pages of Chiba, Ibaraki, and Hokkaido prefectures. Choose one species from this article and compare how depth, spawning period, and food are described. You may notice that even for the same Japanese flounder, the depth ranges differ between inner Tokyo Bay eggs and the Sanbanze database. If you want to know more, proceed to the source documents.