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Are skipjack stocks in WCP on a decline?

Urgent steps are needed to restrain purse-seine capacity on a global scale    
Dr. Ziro Suzuki, Tuna Scientist

  
The author is concerned that the state of skipjack stock may be approaching its limit, although the stock has generally been believed to be in a healthy condition. A yellow light might start flashing for the skipjack resources in the Western and Central Pacific (hereafter referred to as the “WCPFC area,” i.e. the area under the jurisdiction of the Western and Central Pacific Fisheries Commission), in which nearly 60% of tunas in the world are produced.
   In recent years, concerns have been rising among fishermen and other stakeholders that skipjack  resources in the waters near Japan have been on a decline. This concern is based on the view that the stock migrating to Japan’s near-shore area may most probably be decreasing because they are caught in large quantities by purse-seine fishing vessels in the southern tropical area. Although a number of uncertainties exist about the migratory relations of skipjack  between Japan’s near-shore area and the tropical area, the catch of skipjack  by purse-seine fishing vessels in recent years has reached 1.5 million-1.7 million tons a year partly helped by the progress in the use of Fish Aggregating Devices (FADs). (The FAD is a fishing method using floating reefs, and is capable of catching juveniles of skipjack, yellowfin and bigeye in a very efficient manner). Not only fishermen but also researchers and administrators thus embrace anxiety over the future of the skipjack  population in the WCPFC area, including Japan’s near-shore waters. 
   The first yellow light was shown in the joint research by the South Pacific Commission (SPC — an international organization to which stock assessment of tunas in the WCPFC area is entrusted) and Japan’s National Research Institute of Far Seas Fisheries. This research is still in a preliminary stage and is subject to modifications in the future. Therefore, before drawing a final conclusion, we need to wait for the results of analyses of large-scale tagging program for tunas in the WCPFC area, which has just been completed.
   The outline of this research is as follows. In the stock assessment for skipjack , stock size indicators (such as catch per unit of effort (CPUE); hook rate in the case of longling) cannot be used. The reason is that major fishing methods targeting the skipjack  are purse-seining and pole-and-line fishing and only  increasing trend can be shown by calculating CPUE (e.g. catch in weight per fishing day) that can be obtained from these fisheries. In other words, those data do not convey accurate information regarding the stock status because the apparent CPUE is increasing as fishing effficiency continues to be improved. In the case of purse-seining, it is highly possible that the decline in stocks, even when it occurs, cannot be known because fishing efficiency is increasing at a conspicuous pace by the use of the devices such as FADs, sonar and bird radar. Similar situation can be found for pole-and-line fishing as well although pole-and-line boats do not depend on the FADs .
   Detailed analysis of operation records in pole-and-line fishing was carried out in this joint research. Specifically, CPUE was calculated in detail on a vessel-to-vessel basis, in addition to the examination of the installation of fishing equipments onboard the vessels. Although the number of pole-and-line fishing vessels is decreasing, it is possible that the vessels with good catch records and having competent fishing masters and crew will survive a longer period of time. This situation leads to making an oversight of the stock decline because only CPUE of excellent vessels tends to be calculated recently.
   Therefore, attempts were made to remove the rising factors for CPUE that have resulted from the increasing fishing efficiency, by calculating to what extent the current CPUE has declined when the ordinary fishing vessels with average catch continue to operate at the conventional fishing efficiency. The CPUE, thus corrected, seem to be generally showing declines of nearly 25% compared with a decade ago, although the figures may differ from area to area.
   In the stock assessment of the skipjack  at the WCPFC Scientific Committee meeting in August this year, these corrected CPUE values were used, together with other information. As a result, in a considerable change from the optimistic stock status to date, it was shown that cautious approach should be taken with regard to further increase in skipjack  catch, although the stock is found to be in a healthy condition. The estimated maximum sustainable yield (MSY) for the entire WCPFC area totals about 1.4 million-1.8 million tons, which is close to the current catch level.
   The recent skipjack  catch has been supported by the increasing trend in recruitment of this species. But, when the recruitment returns to the average level, then catch may exceed the MSY level, thus resulting in the over-exploitation of the resources. The stock management target of the WCPFC is given as one attaining the MSY level. But the view is increasingly gaining ground that the MSY should be regarded as the limit value (i.e. the value that should not be exceeded as the excess may cause over-exploitation), rather than as a target value.
   Observing the tuna resource management organizations throughout the world, we find that regulations have always been introduced belatedly only after the stock status exceeded the MSY level. In the WCPFC area, purse-seining, which accounts for nearly 80% of the total tuna catch, is the main fishing practice.  Regulations have already been introduced for bigeye and yellowfin tunas, but there are no regulations in place with regard to skipjack tuna. As skipjack  is the last species of vital importance in this area, it seems that precautionary measures should be implemented, taking utmost caution in stock management into consideration. If catch regulations on skipjack  can be introduced at an early stage in the WCPFC area, it would certainly mark an epoch-making achievement in the history of tuna stock management.
   The results of most recent stock assessment need to be considered in a comprehensive manner, jointly with the analysis of large-scale tagging program that has just been completed. The results may be revised in the future. But, amid continued increase of purse-seining capacity, the indicators of corrected CPUE, as stated in the foregoing, should be taken as an important warning. Notably, there is a need to implement immediately the reduction and control of overcapacity of purse-seine fishing on a global scale.