1. The Flounder Is a Sand-Colored Fish That Eats Small Fish
In shallow sandy areas of the sea, you might see a flat fish that matches the color of the sand, barely moving. This is the flounder. According to documents from Miyagi Prefecture, flounders hide in the sand on the sea floor and prey on small fish that pass by [5]. They do not chase their prey; instead, they wait for the food to come to them in an ambush hunt. For adult flounders, small fish make up about 90% of their diet, according to encyclopedia estimates [9]. Explanations from Chiba Prefecture note that they also eat crustaceans like shrimp, in addition to small fish such as sand eels and horse mackerel [1]. It is also said that they often hide in the sand during the day and are active at night [9]. However, the figure of 90% is a general estimate from an encyclopedia, so it may vary depending on the sea area and the season [9]. Does this hunter of small fish eat fish from the time it is a baby?
2. A Flounder's Food Changes in Three Stages as It Grows
The key answer to the question in the title lies here. A flounder’s food is divided into three main stages based on its growth. First, during the floating period when they drift in the sea, they eat animal plankton [2]. Next, when the young fish begin living on the sea floor, mysid shrimp become their main food [2]. Mysids are small creatures that look like tiny shrimp [2]. As they grow, they begin to eat fish and squid, and adults show a strong preference for fish [2]. Explanations from Chiba Prefecture also describe this flow: from plankton in the floating period to small fish and crustaceans for adults living on the bottom [1]. The food changes from plankton to mysids to fish and squid. So, what is the very first meal?
3. Newly Hatched Flounder Larvae Must Start Eating Within a Few Days
Flounder eggs hatch about 3 days after fertilization, depending on the water temperature [7]. The newly hatched larvae spend a short time using the egg yolk in their bellies, but this time is very brief [7]. In rearing experiments at Kyoto University, it was shown that after absorbing the egg yolk, the larvae began eating within 2 days at 15°C, and within 1 day at 18°C and 21°C [7]. This means that the warmer the water, the shorter the time they have before they must eat [7]. So, what do they eat first in the natural sea? A report on natural larvae in Mutsu Bay found that larvae at the start of feeding were eating tiny organisms called chinchin and copepod larvae [6]. Both are tiny plankton [6]. The same idea applies in artificial rearing. In Shimane Prefecture, larvae are fed rotifers, brine shrimp, and formulated feed in order as they grow, switching to formulated feed after 30 days [8].
4. Young Flounder on the Sea Floor Grow Fast by Eating Mysid Shrimp
The main food for young fish that have settled on the sea floor is mysid shrimp [2]. According to a survey by the Fisheries Research and Education Agency in Sendai Bay, this area is very rich in mysids, which serve as food for flounder juveniles up to about 25 cm long one year after settling [4]. In the survey, the benchmark for maximum juvenile growth is 100 mysids per square meter, and most survey sites exceeded this number [4]. The reported growth speeds were 1.9 mm per day in 2012 and 2.0 mm per day in 2013 [4]. At this speed, the fish would grow about 2 cm in 10 days. However, if you calculate this for a full year, it would be about 70 cm, which is much larger than the "30-40 cm after 1 year" mentioned in Chiba Prefecture documents [1]. This means this speed is specific to the juvenile period and does not last all year. Note that the number of 100 is a benchmark for maximum growth, not the number eaten per day. When did the flounders eating mysids switch to eating fish?
5. Flounder Switch to Eating Fish at Sizes Between 3 cm and 5 cm
The size at which they switch to fish is not a fixed number. In surveys by Aomori Prefecture, the size at which the main food changed from mysids to fish varied by year. In one year, it was 5 cm in total length, and the smallest switch size was 3 cm [3]. There are also reports that the proportion of eating fish like whitebait increases when the total length exceeds 4 cm [3]. Putting these together, we can say they shift from a mysid-centered diet to a fish-centered diet somewhere between 3 and 5 cm [3]. 3 cm is about the length from the tip of a person’s finger to the first knuckle. In some years, such a small flounder is already on the side of eating fish. Considering they start living on the sea floor at about 14 mm in total length [1], they begin hunting fish within a few centimeters of settling. Compared to their maximum body size of about 85 cm [1], this is a very early stage. These figures come from Aomori Prefecture's surveys and may not apply to other sea areas. The documents do not explain why the switch size varies by year, so read 3 to 5 cm as a rough range.
6. Comparing the Size When Flounder Start Eating Fish (3 cm) with an Adult (85 cm)
First, draw lines for 3 cm and 5 cm on paper with a ruler. This is the size where they switch to eating fish [3]. Next, cut a string to 85 cm and lay it straight on the floor. This is the maximum size of a flounder [1]. If you place the 3 cm line at the end of the string, it is only about 1/28th of the string’s length [3]. Thinking that in some years, a fish this small was already eating other fish changes how you see it. Marking the position for 30-40 cm after 1 year [1] helps you see the growth pattern. Documents from Chiba Prefecture [1] and Aomori Prefecture [3] are available in Japanese.