1. Why Farmed Sea Bream Can Look Blackish Instead of Red
A healthy sea bream has a distinct red body with a pale belly. Small dots ranging from blue-green to cobalt blue are scattered across the upper side of its body. A black edge on the rear of the tail fin is another key feature [4]. However, in aquaculture (fish farming), this vibrant red can fade, causing the fish to take on a blackish hue. The research report from Hiroshima University uses this problem as a starting point to investigate how to maintain the red color [5]. Why does the color change depending on how the fish are raised? To find clues, we first look at what wild sea bream eat.
2. What Sea Bream Eat: A Wide Variety of Food and Their Favorites
Sea bream have a very broad diet. An explanation from a fishing tackle manufacturer lists copepods (small planktonic crustaceans), mysids, seaweed, crustaceans, starfish, and small fish as food sources [1]. Other sources also mention shrimp and crabs. Among these, crustaceans—specifically shrimp and crabs—are considered their favorite [1]. A guide from Kanagawa Prefecture states that shrimp are a favorite, while also noting they eat various other items like shellfish, ragworms, starfish, and crabs [3]. Tokyo’s resources state that adult sea bream primarily prey on crustaceans such as shrimp and crabs [4]. Some explanations say this preference applies regardless of age [1]. However, as we will see later, the diet of young fish differs slightly from adults, so this claim should be read with some flexibility. Since shrimp and crab shells are hard, how do sea bream manage to eat them?
3. How Sea Bream Use Their Mouth and Teeth to Crush Hard Shells
Fishing encyclopedias note that sea bream have quite sturdy teeth, which they use to bite and crush the hard shells of shrimp and crabs [2]. Tokyo’s resources describe sea bream as having two rows of well-developed molars on both jaws [4]. The Japanese character for "molar" contains the character for "mortar," a tool used for grinding or crushing things. As the name suggests, these are back teeth suited for breaking down hard objects. If adult sea bream have these teeth, what do young sea bream eat?
4. How the Food of Sea Bream Changes as They Grow Up
During the larval stage, when they live floating in the water, sea bream eat zooplankton. As they grow, the types of food they eat gradually change, and by adulthood, shrimp and crabs become the main part of their diet [4]. When they reach about 1 cm (about 0.4 inches) in body length, they settle in seaweed and seagrass beds, which are rich in seaweed and seagrass like kajime, sargassum, and eelgrass [3]. Juvenile fish are said to live in shallow seagrass beds and rocky areas, moving to deeper waters after 2 to 3 years [2]. As they grow, both their food and their habitat change. So, is the red color of their body related to their food? Here, we return to the story about fish farming.
5. A Hiroshima University Study: Adding Crayfish Shell Powder to Sea Bream Feed
In the Hiroshima University study, researchers aimed to improve the red color tone of farmed sea bream that had faded and turned blackish. They added carotenoids (a group of red and yellow pigments) extracted from American crayfish shells, along with shell powder, to artificial feed. The results showed that the red coloration of the sea bream improved [5]. The study suggests that a pigment called astaxanthin ester contributes significantly to the red color [5]. However, this was a test conducted on farmed sea bream. This research alone does not determine whether the red color of wild sea bream is created by eating shrimp and crabs. Also, blackening is not caused by food alone. The Kanagawa Prefecture Fisheries Technology Center explains that there have been cases where fish turned completely black after being exposed to sunlight in shallow tanks, due to an accumulation of a black pigment called melanin [3]. Both diet and environment seem to be involved in the background of the red color.
6. What the Proverb "Fishing for Sea Bream with Shrimp" Means and Why Small Shrimp Are Used as Bait
The Japanese proverb "Fishing for sea bream with shrimp" means gaining a large profit from a small investment. A page introducing sea bream fishing notes that small shrimp are used as bait for catching sea bream [6]. It is thought that the sea bream’s preference for shrimp forms the background of this proverb. The connection between sea bream and shrimp has long been familiar in human life. However, from fishing stories alone, we cannot conclude that eating shrimp makes the body red. While a link between red color and food was seen in the farming test, there are still unknowns regarding wild sea bream.
7. How to Spot a Healthy Sea Bream When Buying a Whole Fish at the Market
If you find a whole sea bream at a supermarket or fish shop, look for the signs mentioned in this article: the pale color of the belly, the blue dots on the side of the body, and the black edge on the rear of the tail fin [4]. If there is a fishmonger, you might ask, "What did this sea bream eat to grow so big?" Do not use knives; just look. To verify the information, the Kanagawa Prefecture "Fish Encyclopedia" and the Tokyo Islands Agriculture, Forestry and Fisheries Comprehensive Center’s explanation of sea bream provide short summaries of diet and physical features [3][4]. The research on farmed sea bream color is available in university paper repositories [5].