For a long time, I have been recommending people to eat sardines, which are more plentiful in number than tuna and have a higher nutritional value. Even if populations of high-grade fish decrease, there are still many other types of fish we can use for food. However, Japanese pilchard and Japanese anchovy appear to have repeatedly undergone "species replacement" over several decades at a time from before recorded history began. In the 1980s, fish meal used as feed for farmed fish was made from Japanese pilchard, with the majority supplied domestically. However, Japanese anchovy has undergone species replacement since the 1990s, with fish meal becoming reliant on imports. Even when fisheries are managed appropriately, a system of processing, distribution, and consumption based on the premise of a repeated cycle of good and bad catches over time is crucial.

Yellowtail is inexpensive at present, however, as with Pacific saury and Okhotsk Atka mackerel, which have risen in cost from their low prices a decade prior, it will not always be available at such low prices. Neighboring countries may also notice its value and significantly increase the size of their catches. We should seriously start considering resource management during periods of abundance.

Wild-catch fisheries catch wildlife. If caught in moderation with an expectation of an organism's ability to repopulate with a compound increase, this method can be used for many years to come. However, overfishing has been occurring around the world for a long time.

Tuna and other high-level predators have a low rate of natural increase (r). Even with the maximum level of stocks (K = carrying capacity) under a constant environment, in a simple reproduction model known as the logistic formula, catches over a quarter of rK (amount of natural increase) constitute overfishing. This upper limit value is called the "maximum sustainable yield" (MSY). However, if r is lower than the rate of economic increase of other industries, it becomes more profitable to catch fish all at once, then invest elsewhere. In other words, the value of future catches is considered to be discounted by the expected profit compared to that of the present. This is referred to as the discount rate (d), which is commonly set at about 4% per year. If r is smaller than d, then overfishing will be more profitable.

Another cause of overfishing is the "tragedy of the commons." Marine resources do not belong to a specific nation or fisherman. Even if an individual only takes a modest catch, other people who are overfishing will take any immediate profits, while everyone loses future resources. The more that the same resources are shared, the more likely that overfishing will occur.

The discount rate and the tragedy of the commons are also applicable to many other environmental issues. Climate change countermeasures will cost vast amounts of money right now, but the benefits will work to protect the environment half a century from now. And even if economic losses from global warming half a century from now are enormous, their present value will be low. The discount rate should not only be determined by market economics. As for mitigating measures to reduce greenhouse gas emissions, the total amount of emissions across the entire planet will determine the future, regardless of which country they come from. "Adaptive measures," which assume that climate change will occur and prevent its negative impacts, are effective in individual regions. It is therefore more advantageous to leave mitigation measures to other countries, and instead work on adaptive measures. This is also part of the tragedy of the commons.

The trading system of fishing quotas can be said to be a prototype of emissions trading, which is gaining traction as an important framework for decarbonization. Fisheries science, which includes the concept of sustainability, came about prior to the emergence of several other important environmental issues.