In order to achieve sustainable use of fishery resources, efforts are being made to set scientific management reference values and fishing control rules based on resource assessment. Here, we will explain why resource management is necessary for sustainable use of fishery resources, while touching on the concept of resource management.
In order to realize sustainable fisheries resources, it is necessary to keep the resources in a "desirable state." So, what exactly is this "desirable state"?
When considering fishery resources, there is a definition of "surplus production." Surplus production is "the number of new recruits to the resource."*1It is expressed as "resource status + increase in grown fish - natural mortality," and in sustainable resource management, it is essential to maintain the resource status at a level that does not exceed surplus production. Therefore, if we simply define the "desirable state," it is one in which parent fish are at a level where they can reproduce, and are not overfished relative to surplus production. In fisheries resource science, the maximum estimated catch that can be obtained by maintaining this desirable state and without reducing the resource is called "MSY (maximum sustainable yield)," and is the ultimate resource management goal.
In order to maintain sustainable resource levels, it is necessary to understand two characteristics of fishery resources: "autonomous renewal" and "ownerless preemption." "Autonomous renewal" refers to the characteristic of a resource that allows a certain fish species to maintain its population by repeating reproduction (producing the next generation) even if a certain level of fishing pressure is applied to it. "Ownerless preemption" refers to the characteristic that fishery resources do not belong to anyone until they are harvested, which creates the risk of competition to get first to claim them.
As long as the resource abundance does not exceed the MSY level, there will be a tendency for it to return to its original state even if fishing pressure increases to a certain extent. However, if excessive fishing continues against the backdrop of "occupation of unclaimed land" to a level that exceeds the "autonomous renewal," the fishing pressure will increase, and the resource will fall into a "state of depletion" where reproduction becomes difficult.
This condition is called "overfishing," and once this occurs, it becomes difficult for resources to recover.
So, if a fishery becomes overfished, how can we restore the stock? To do this, it is important to look at the sustainability of the fishery from the perspectives of both parent fish stocks and fishing pressure.
This is shown in the "Kobe Chart"*2(See figure below)
The x-axis represents the abundance of parent fish. If it is greater than the central 1.0 (MSY level), the abundance of parent fish is underfished, and if it is less than 1.0, the abundance of parent fish is overfished.
The y-axis represents fishing pressure. A value greater than 1.0 (the center) indicates high fishing pressure (overfishing), and a value less than 1.0 (MSY level) indicates low fishing pressure (underfishing).
The Kobe Chart divides stocks into four categories based on parent fish abundance and fishing pressure, making it easier to take the necessary measures for resource management.
Category 1 (Red): Overfishing is occurring despite the decline in parent fish stocks...There are cases where a fishing ban is necessary.
Category 2 (yellow in the upper right): Resources exceed MSY, but overfishing is occurring... It is necessary to reduce catches and efforts, and take care not to reduce parent fish resources.
Category 3 (orange in the bottom left) Fishing is restricted in order to restore depleted parent fish resources. It is necessary to increase parent fish resources by applying restrictions on catch size, etc.
Category 4: The resource abundance exceeds the MSY level and the fishing pressure is appropriate. It is necessary to maintain the current fishing pressure and manage the parent fish resource abundance so that it does not decrease.
For example, if the resource status is in the yellow category ②, the resource is not yet depleted, but fishing pressure is excessive, so measures to reduce the catch to a certain extent as a fishing control rule to ease fishing pressure would be taken.
So far, we have explained the desirable state of fishery resources and the characteristics of the resources. Based on this, the following three points are considered to be important for resource management.
1. Do not thin out (fish) excessively
If we fish beyond the reproductive capacity of a resource, it will become depleted. We need to take strict measures, such as setting catch limits according to the situation and banning fishing when the situation is extremely deteriorating.
2. Protecting small and juvenile fish
In order to increase fishing income, small fish with low unit prices are caught, but this only leads to the depletion of resources. Resource management is necessary, such as by restricting the mesh size of the net.
3. Protecting spawning stock
We can increase resource abundance through appropriate resource management, such as by banning fishing during the spawning season.
Many people may be surprised to learn that overfishing is not just about overfishing, but can be divided into four categories depending on the situation. In order to achieve sustainable fisheries resources, it is necessary to maintain the resources in a "desirable state," but to do so, it is necessary to take appropriate measures according to the detailed circumstances.
It is important to pass on a bountiful ocean to future generations by taking a long-term view of resource management rather than making decisions based on short-term circumstances.
<References>
-Tanaka, Eiji (2015) "Fisheries Resource Management: Sustainable Use and Management of Fisheries Resources"
・Hasegawa Akira (2002) "Hasegawa Akira Collected Works Volume 1: Fisheries Management"
-Fisheries Economic Newspaper, ""Kobe Chart" makes resources visible, Fisheries Agency launches new evaluation method", 2019-3-11
-"Current status and future of fisheries resource assessment" FRA NEWS, vol.59
・Tanaka, Shoichi (1985) "Modeling the dynamics of fishery resources and predicting changes"
Text and illustrations: Nao Nagasawa
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Key terms and concepts to understand seafood sustainability.
