This paper addresses a fundamental question in conservation ecology, which is the balance between rebuilding of a species’ population and exploiting them, by examining an appropriate sustainable management regulation for Pacific Bluefin tuna. The population of Pacific Bluefin tuna has been heavily depleted to just 2.6% of its historic unfished size by many years of overfishing. In order to rebuild the population, an immediate implementation of a moratorium on all commercial fishing was proposed by NGOs such as the Pew Charitable Trusts and Greenpeace. The primary objective of this paper is to examine the necessity of the moratorium as a sustainable stock management regulation. The paper concludes that an additional 10% reduction in catch limit of fish less than 30 kg could be a better alternative management regulation than a total ban on all fishing. This option can increase the probability of the stock recovery, while allow ing to catch as many large fishes as under the current management regulation. Through the examination of sustainable stock management for Pacific Bluefin tuna, it can be said that it is important to consider the balance between the recovery of the population and utilization of them. Since millions of people rely on the consumption of the species, it is more realistic to gradually rebuild the population with an intermediate target and an efficient additional catch limit. This consideration can be appl ied to conservation of other species such as whales and eels.
The initial purpose of this paper is to examine the necessity of a moratorium on all commercial fishing as sustainable stock management of Pacific Bluefin tuna (Thunnus orientalis). Pacific Bluefin tuna is a highly migratory species of great economic importance found throughout the North Pacific Ocean. They mature at 3 to 5 years and live for more than 20 years. The latest stock assessment for Pacific Bluefin found that the population has been heavily depleted to just 2.6% of its historic unfished size by nearly a century of overfishing [
In the following sections, the paper examines pros and cons of the current management plan, rationales for a moratorium, and the possibility for alternative management regulation. The paper reveals that gradual rebuilding of the population with an additional catch limit is more realistic way as sustainable stock management. The paper concludes with a statement that it is important to consider the balance between the recovery of the stock and exploitation of them rather than a simple calculation of conservation since millions of people rely on the consumption of Pacific Bluefin.
Until very recently, it was believed that Pacific Bluefin tuna was not in an overfished condition, even though overfishing was occurring [
ISC, the group responsible for assessing the status of this stock, conducted a full assessment of Pacific Bluefin stock with data from 1952 to 2012 in 2014, and concluded that the stock was experiencing overfishing and in an overfished condition. According to their estimation, the current fishing mortality rate of Pacific Bluefin tuna exceeds all biological reference points commonly used by fishery managers. The 2014 spawning stock biomass (hereinafter called “SSB”) of Pacific Bluefin tuna was around 17,000 tons. The ratio of the 2014 SSB to the theoretical unfished SSB (hereinafter called “SSB current, F = 0”) is less than 2.6%. SSB current, F = 0 can roughly be defined as the theoretical SSB size without fishing assuming recent levels of recruitment. These estimated results suggest that overfishing is occurring and the stock is overfished, although specific reference points have not been adopted for this stock. Moreover, as
The stock of Pacific Bluefin tuna is managed by both the Western and Central Pacific Fisheries Commission (hereinafter called “WCPFC”) and the Inter- American Tropical Tuna Commission (hereinafter called “IATTC”). Both WCP- FC and IATTC base their Bluefin tuna management and conservation measures by consensus on science proposed by ISC. Currently, WCPFC set the initial goal of rebuilding the SSB of Pacific Bluefin to the historical median (hereinafter called “SSBMED”) within 10 years with at least 60% probability. For this goal, WCPFC members, including the US and Japan, are required to ensure that total fishing effort for Pacific Bluefin tuna shall stay below the 2002-2004 annual average levels. Also, they shall reduce all catches of “small fish” (≥30 kg) to 50% of the 2002-2004 annual average levels [
According to ISC [
The future projection is based on a model which is composed of estimates of natural mortality, growth, and fecundity as well as observed quantities such as
catch and size-composition. Thus, it is inevitable that the projection results sometimes have statistical errors and sampling errors. In fact, ISC’s latest assessment estimated the 2014 SSB of Pacific Bluefin as around 17,000 tons, which is about 9000 tons below the 2012 SSB estimated in the 2014. This is not because the SSB suddenly declined from 2012 to 2014, but because of improvements to the input data and refinements to the assessment model which scaled down the estimated value of SSB.
Although ISC scientists cannot precisely estimate how few spawning Pacific Bluefin tuna would be too few to sustain the population due to the limitation of the projection, the population is likely to be reaching that point. It might be too late when a recruitment decline is evident in the data and there are few juvenile fish to be caught. The earlier catch limits take place, the more quickly the depleted stock can rebuild.
In order to begin the process of rebuilding the population for the future, some NGOs such as The Pew Charitable Trusts and Greenpeace proposed to implement a recovery plan that could rebuild the population to 20% SSB current, F = 0 within 8 years (two generations) and then further recover the stock to at least 40% SSB current, F = 0 by 2030 [
Thus, The Pew Charitable Trusts and Greenpeace proposed to immediately implement an initial 2-year moratorium on all commercial fishing for Pacific Bluefin tuna and, if necessary, to extend the moratorium until there is adoption of appropriate recovery planning. Furthermore, they called for an international trade ban if appropriate recovery planning is not in place by 2018.
Since Pacific Bluefin tuna is an important source of protein for millions of people, it would be necessary to think about this issue from a perspective of sustainable utilization of the stock rather than total ban on all catching. Actually, as the convention clearly states, the principle of WCPFC is also to adopt measures designed to maintain stock levels capable of producing maximum sustainable yield (hereinafter called “MSY”) and promote their optimum utilization [
From the perspective of maximum sustainable utilization of the stock, it is difficult to say that SSB current, F = 0 is an appropriate reference point. 20% SSB current, F = 0 is used as the biomass depletion-based limit reference point for other tuna stocks, including skipjack, yellowfin, bigeye, south Pacific albacore and north Pacific albacore. On the other hand, as
years from 1952. Moreover, since SSB current, F = 0 is an estimated value as the theoretical SSB size without fishing assuming recent levels of recruitment, the value fluctuates so widely due to changes in the input data and the assessment models that it is not suitable to a reference point in practice.
Also, a smaller number of spawning Bluefin does not necessarily mean that recruitment will be low. Pacific Bluefin tuna are very productive. The females can spawn millions of eggs in one year. Given the many eggs that each mature female can spawn, it is entirely possible to see many juvenile fish while the quantity of spawning fish is still very low [
In light of the balance between the conflicting requirements, namely rebuilding of Pacific Bluefin tuna population and consuming them, it is more realistic to pursue SSBMED as an intermediate goal, rather than unprecedented SSB current, F = 0 by implementing a moratorium on all commercial fishing. Unlike widely fluctuating SSB current, F = 0, the value of SSBMED is stable because it is a historical median for several decades. For example, if 1952-2012 is chosen, then SSBMED is estimated to be 41,069 tons, and if 1952-2014 is chosen, SSBMED is 40,994 tons [
According to ISC’s projection results, the probability of achieving SSBMED by 2024 would increase if more conservative management measures are implemented. The prospect of recovery is highly dependent on the definition of “small fish” and catch limit. If the definition of “small fish” is changed from the current definition (≥30 kg) to the larger size, it has positive impact on the stock recovery. This is because the impact of fishing on a stock depends on both the number and size of the fish caught by each fleet. Catching a high number of smaller juvenile fish can have a greater impact on future SSB than catching the same weight of larger mature fish. Also, an additional reduction in catch limit would provide a higher probability of reaching a higher biomass [
Among several scenarios, as shown in
In light of the balance between the conflicting requirements, it seems to be more practical to pursue SSBMED as an intermediate goal, rather than unprecedented SSB current, F = 0 by implementing a moratorium on all commercial fishing. According to ISC, SSB of Pacific Bluefin can recover to the historical median by
Current Management (Scenario 1) | Additional 10% reduction in catch limit of fish ≥30 kg (Scenario 2) | Additional 10% reduction in catch limit of fish <30 kg (Scenario 3) | Additional 10% reduction in catch limit of all fish (Scenario 4) | |
---|---|---|---|---|
Probability that SSB is more than historical median | 69.3% | 90.0% | 75.3% | 90.3% |
2024, the initial WCPFC goal, with 69% probability under the current management regulation. On the other hand, the NGOs called for the moratorium in order to rebuild the population to 20% of unfished SSB. Pacific Bluefin, however, has yet to achieve this stock level for the last 6 decades. On the other hand, it should be still noted that 69% probability is not statistically high enough to secure the recovery to SSBMED taking into consideration uncertainties in estimation of model parameters. Thus, an additional 10% reduction in catch limit of fish less than 30 kg could be a better alternative management regulation. This option can increase the probability of achieving SSBMED by 2024 to 90%, while allowing to catch as many large fishes as under the current management regulation.
Through the examination of sustainable stock management for Pacific Bluefin tuna, it can be concluded that it is important to consider the balance between the recovery of the population and exploitation of them. Although it is easy to say that a moratorium on catching is necessary to rebuild the population of a species, we should also take into consideration that millions of people rely on the utilization of the species. This conclusion can be applied to conservation of other species, such as whales and eels, of which there are controversies over the sustainable use [
Sekiyama, T. (2017) An Examination of Sustainable Management of Pacific Bluefin Tuna Stock. Journal of Environmental Protection, 8, 26-33. http://dx.doi.org/10.4236/jep.2017.81003