1. How a Mackerel's Food Changes as It Grows
The food that mackerel eat shifts significantly as they develop. According to the Tokyo Metropolitan Islands Agriculture, Forestry and Fisheries Center, newly hatched fry eat small zooplankton, but as they grow, they become more carnivorous, eating other fish. Adult mackerel are said to eat fish like sardines and horse mackerel, squid, and krill. [1]
A more detailed breakdown from the Fisheries Research and Education Agency’s stock assessment notes that during the fry stage, mackerel eat copepod eggs and nauplii larvae (the first form crustaceans take after hatching). During the juvenile stage, they consume small copepods, Noctiluca (a type of plankton), appendicularians, and salps. [2]
Growth rates also vary by age. The same center reports that at one year, body length is 20 to 28 centimeters; at two years, 28 to 35 centimeters; and at three years, 35 to 42 centimeters. [1] If you average the first year’s growth over 12 months, the fish grows about 1.7 to 2.3 centimeters per month. However, actual growth is not constant and depends on the season, so this is just a rough guide. Over that first year, the diet shifts from eggs and larvae to small plankton, and finally to fish. This raises a key question: how crucial is that first food for survival?
2. Why Copepod Larvae Are Important Food for Newborn Fish
For many marine fish, survival as fry depends on the density of copepod nauplii available as food. The Hiroshima University Takehara Station explains that the presence of these larvae is a critical factor for whether fish fry can survive. [6] It is important to note that this explanation applies to marine fish in general, not just mackerel.
By combining the fact that mackerel fry eat copepod eggs and larvae [2] with the general importance of nauplii density for fish survival [6], we can infer that having enough of this first food is likely a vital condition for baby mackerel. However, no direct study linking copepod density specifically to mackerel survival rates is cited here. Those newborns that survive the first few days grow by eating small plankton, eventually reaching a size where they can eat other fish. This leads to the next question: where do they go to find that food?
3. Krill and Anchovies as Mackerel Food in Sanriku Waters
For juvenile and adult mackerel, the diet varies by region and season. The Fisheries Research and Education Agency notes that crustaceans like krill and copepods, fish like Japanese anchovies and lanternfish, and salps are central to their diet. In the Sanriku region, hornless krill and Japanese anchovies are considered the main prey. [3]
This shows that we cannot simply say "mackerel eat small fish" as a blanket rule. In Sanriku, small shrimp-like crustaceans called krill and the fish known as Japanese anchovies share the spotlight as primary food sources. The Fukuoka City Biodiversity Web Center also describes chub mackerel as carnivorous, eating zooplankton, small fish, and krill. [4] The general direction is the same, but the specific details depend on "which sea" and "which season." If the food changes by location, does the mackerel itself move across the seas?
4. How Mackerel Migrate North in Summer to Feed
A 2019 paper by Yatsu et al. in *Fisheries Science* outlines the annual movement of Pacific chub mackerel. After spawning and wintering, they move north. Mainly in summer, they feed in waters from Sanriku to off Hokkaido. This movement in search of food is called *sakujii* (foraging migration). In autumn, both adult and immature mackerel move south along the Sanriku coast and offshore. Adults spawn in the south, while immature fish mainly winter in the Boso and Joban waters. [5]
The Fukuoka City explanation also describes chub mackerel as migratory fish, moving north in spring and summer, and south in autumn and winter. [4] The northern waters they travel to include the Sanriku areas rich in krill and anchovies mentioned earlier. However, the paper states the fact that they forage mainly in Sanriku and off Hokkaido during summer; it does not explicitly state in the same sentence that they go north *because* of the food. It is safer to interpret this as the northern waters being the place where they go to eat. Does eating in the north change the mackerel’s body?
5. Mackerel Body Fat and Condition from Autumn to January
How does the mackerel’s body change during the season when they return from the north? Yatsu et al. studied chub mackerel caught from the eastern Hokkaido coast to Kashima Beach between 2012 and 2017. They reported that crude fat content (the percentage of fat in the body) and condition factor (a number indicating how plump the fish is) were high from September to January, dropping slightly in February. [5]
Few individuals were sampled from March to August, but the fat content was lower than in February, and the condition factor was similar to February levels. Thus, within the data range, autumn and early winter are when fat and plumpness numbers are highest. While people often say "mackerel are tastiest in autumn," this report specifically shows seasonal differences in fat and body condition. It does not directly show the relationship to specific food sources. Since studying wild mackerel diets is difficult due to the vast ocean, researchers use farmed mackerel to test these relationships.
6. Farming Tests Show Oil Added to Feed Helps Mackerel Grow
In farming, researchers can control the feed. A Tottori Prefecture aquaculture trial fed chub mackerel commercial compound feed, comparing it to feed with 4% added oil. Survival rates were about 94% in all groups, with no major difference based on the type of oil. There was no significant difference in average weight, but the group fed soybean oil had the highest weight gain. The cost of feed and oil per unit of weight gained was also lower in this group. [7]
The goal of farming is to balance growth and cost. This trial looked at mackerel growth not through the natural "plankton to fish" axis, but through the axis of "what to add to compound feed." Sashimi-grade farmed mackerel production has also expanded. A 2010 news report noted that in places like Nagashima Town, Kagoshima, farming of fatty, fresh sashimi mackerel became popular because fishermen could stably secure juvenile fish from purse seine nets. [8] However, this was a 2010 report, so current scales are unknown. We have now seen how mackerel food changes with growth stage, location, season, and human intervention.
7. What Mackerel Eat in the North Before They Reach Your Table
Take out a ruler and mark 20 and 28 centimeters (about 8 and 11 inches). This is the size range of a chub mackerel in its first year. [1] During this growth, their food changes from eggs and larvae to small plankton, and then to fish. When you see a pack of mackerel at the store, read the label for the catch location and date. If the Sanriku and Hokkaido waters are the summer dining grounds, then mackerel caught in autumn and winter may have fed in the north. Of course, the label alone does not tell you exactly what they ate, so this is just a starting point for imagination. For more details, you can check the Fisheries Research and Education Agency’s stock assessments [2][3] or the Tokyo Metropolitan Islands Agriculture, Forestry and Fisheries Center’s explanation [1]. When preparing fish at home, an adult should handle the knife.