1. Sardines and Anchovies: Two Fish That Swap Places
There are two types of fish: the Japanese sardine and the Japanese anchovy. These two have an interesting relationship. When one is abundant, the other is scarce. It is like a seesaw, where the amounts switch back and forth. This swap from sardines to anchovies is called "species replacement." Why does this happen?
2. How Sardine Catches Rose and Fell in Japan
In the 1980s, the amount of Japanese sardines caught across Japan exceeded 4 million tons per year [2]. However, when the 1990s began, they suddenly became hard to catch. By the early 2000s, the level had dropped to less than 100,000 tons. This was less than 1/40th of the peak period [2]. Since the 2010s, the resource amount has recovered, and catch volumes are increasing again [3]. Why do they increase and decrease so drastically?
3. Regime Shifts: Sudden Changes in Ocean Temperature and Wind
Sea water temperature and wind patterns can change suddenly over periods of decades. This is called a "regime shift" [2]. Once a regime shift occurs, the new state continues for another several decades. The sea surface temperature changes by only a few degrees before and after a regime shift. However, the catch volume of schooling fish, such as sardines, can change by tens or hundreds of times [2]. Why does such a small difference in temperature create such a huge difference?
4. Why Colder Water Helps Baby Sardines Survive
Baby sardines grow from winter to spring in an area called the Kuroshio Extension region. In years when the water temperature in this area is lower, the survival rate of baby sardines tends to be higher [1]. This is thought to be because lower water temperatures make the surface of the sea easier to mix, which increases the plankton that serves as food [1]. In other words, a slight difference in water temperature greatly affects the survival rate of baby fish.
5. Why Sardines and Anchovies Take Turns
Japanese sardines and Japanese anchovies eat similar plankton. When the marine environment changes, the places that are good for laying eggs also change for each fish. It is thought that when the spawning grounds for anchovies become narrow, the spawning grounds for sardines expand in the following period [4]. Research reports show that for anchovies, when the plankton that baby anchovies eat decreases, it becomes harder for the babies to survive [5]. Thus, changes in water temperature influence which fish increases through spawning grounds and food amounts. In this way, sardines and anchovies have swapped places according to the marine environment. How important are these sardines to the entire ocean?
6. Why Sardines and Anchovies Matter to the Food Chain
Sardine relatives are among the most heavily fished fish in the world [6]. In many years, the Peruvian anchoveta (a type of anchovy) is the number one species by catch volume. However, in poor fishing years, other fish like pollock may surpass it, so the ranking changes by year (according to FAO statistics) [8][9]. Although they are small fish, they eat plankton and are eaten by larger fish, birds, and whales. They play an important role in supporting the marine food chain. Therefore, the increase or decrease of sardines affects the entire ocean and our lives. In Japan, the price and variety of canned sardines can change from year to year. This is also related to the increase and decrease of this resource.
7. How to Investigate Sardines and Anchovies Yourself
Let's check it out at the supermarket or fish shop. Look at "chirimenjako" (dried baby fish) or "niboshi" (dried small fish) at home. These are mainly baby Japanese anchovies. Sometimes baby sardines are mixed in [7]. At the supermarket or fish shop, look at the types and prices of sardines. Observe which is more common: Japanese sardines or Japanese anchovies. On the Fisheries Agency website, look at the resource assessment results for Japanese sardines. Check the graph to see the trend in catch volumes [3]. For safety, be careful of other people and shelves when walking inside the store.