Recovery of Resource by Catch Management

Resources usually run out after use.
However, since organisms can reproduce by giving birth, biological resources can be used sustainably if they are used in a moderate manner.
Here, we will use southern bluefin tuna as an example to explain efforts to limit catches and the current state of resource recovery.

Southern Bluefin Tuna

Southern bluefin tuna are distributed in the southern hemisphere in the cold waters around 50 to 30 degrees south latitude.Southern bluefin tuna, like bluefin tuna and Pacific bluefin tuna, is considered a luxury item in the market because of its high toro (fatty tuna) content. To meet the demand, many southern bluefin tuna were caught from the sea.As a result, in the late 1950s, the global annual catch was more than 80,000 tons, but the catch declined rapidly thereafter, and by the mid-1980s it was internationally recognized as being at a level requiring conservation and management measures (Figure).The decline in southern bluefin tuna stocks continued, and in 1996 it was listed as CR (Near Threatened) by the IUCN.

Commission for the Conservation of Southern Bluefin Tuna (CCSBT)

In 1985, Japan, Australia, and New Zealand launched an effort to limit the catch of southern bluefin tuna in order to protect the resource. In 1994, these three countries expanded their efforts into the Convention for the Conservation of Southern Bluefin Tuna, and Commission for the Conservation of Southern Bluefin Tuna (CCSBT) was established in Canberra, the capital of Australia, as its secretariat. The CCSBT has since gradually increased the number of member countries, and as of 2022, eight countries and regions are members.
The CCSBT aims to utilize southern bluefin tuna as a sustainable fishery resource by surveying its abundance and allocating fishing quotas to member countries according to their abundance.
CCSBT is explained again in " For Sustainability → RFMOs"

Recovery of southern bluefin tuna stocks

Since the beginning of fishing restrictions based on catch quotas by the CCSBT, the downward trend in southern bluefin tuna abundance has been halted.On the other hand, the abundance of southern bluefin tuna has not yet reached the level of the pre-1980s (Figure).However, an increase in the number of egg-laying parent fish (spawning parent fish) has been observed (upper left of the figure). It is expected that the increase in the number of egg-laying parents will lead to an increase in the overall number of southern bluefin tuna in the future.The latest IUCN Red Book for 2021 has therefore assessed the species as being at lower risk of extinction, from CR (Near Threatened) to EN (Endangered).

Figure. Trends in catch and spawning parent index of southern bluefin tuna and stock recovery

I am supplised that the international management by CCSBT has stoped the declining trend of natural resource of southern bluefin tuna and some recoverly of fisherly productions. I was also impressed to learn that the degree of endangerment is also determined by the number of parent fish.

Reference

Commission for the Conservation of Southern Bluefin Tuna（CCSBT）
　　https://www.ccsbt.org/

Fisheries Agency and National Fisheries Research and Education Organization, "Status of International Fishery Resources in FY2021".
　　https://kokushi.fra.go.jp/index-2.html
　　(in Japanese)

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