Overall Assessment Results
A full stock assessment was carried out by the Pacific Bluefin Working Group of the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC) in 2012. The assessment was conducted with Stock Synthesis 3, an integrated statistical age-structured stock assessment model. Uncertainties were found in the assessment, and these were characterized through a series of 20 models, each with alternative data weightings and structural assumptions. While no single model scenario provided a good fit to all sources of data deemed reliable, long-term fluctuations in spawning stock biomass (SSB) occurred throughout the assessment period (1952-2011), and the SSB has been declining for more than a decade; however, there is no evidence of reduced recruitment. Age-specific fishing mortality has increased 8-41% in the recent period (2007-2009) relative to the baseline period (2002-2004) used in recent WCPFC and IATTC conservation measures.
A model configuration was chosen as the representative model to determine stock status and provide management advice, acknowledging that while it represents the general conclusions above, the model was unable to reconcile all key data sources. According to this model, estimated age-specific fishing mortalities for the stock in the recent period (2007-2009) relative to 2002-2004 (the base period for the current WCPFC conservation measures) show increases of 4, 17, 8, 41 and 10% for ages 0, 1, 2, 3 and 4+, respectively. Although no target or limit reference points have been established for the Pacific bluefin stock, the current F
(2007-2009 average) is above all target and limit biological reference points commonly used for management. The current (2010) Pacific bluefin SSB level is near historic low levels, and the ratio of SSB in 2010 relative to unfished SSB is low.
Stock projections of spawning biomass and catches of Pacific bluefin tuna from 2011 to 2030 were conducted assuming alternative harvest scenarios. Recent WCPFC and IATTC
conservation and management measures that entered into force in 2011 and 2012, respectively, combined with additional Japanese domestic regulations aimed at reducing mortality, if properly implemented and enforced, are expected to contribute to improvements in the stock status of Pacific bluefin tuna.
The ISC stock assessment
was updated in 2014 using data up to and including 2013. The results of the updated assessment generally followed those of the previous assessment. The IATTC staff conducted an alternative analysis of the data outside the stock assesment model (document SAC-05-10a
). This analysis confirmed the results of the ISC update assessment. The average recruitment for the last five years was estimated to be below the historical average. Estimated age-specific fishing mortalities on the stock during 2009-2011 relative to 2002-2004 increased for ages 0-6 and decreased for ages 7+. Although no target or limit reference points have been established for the Pacific bluefin stock under the auspices of the IATTC, the average fishing mortality during 2009-2011 exceeds all target and limit biological reference points (BRPs) commonly used by fisheries managers except one, and the depletion ratio (ratio of SSB in 2012 relative to unfished SSB ) is less than 6%. In summary, based on reference point ratios, overfishing is occurring and the stock is overfished. Based on projection results
, the recently-adopted conservation measures, if continued in to the future, are expected to increase the SSB even if the recent low recruitment continues. A full stock assessment of bluefin tuna will be carried out by the ISC in March 2016.
The total catches of bluefin have fluctuated considerably during the last 50 years (Figure E-1).
|Figure E-1: Retained catches of Pacific bluefin tuna. |
The consecutive years of above-average catches (mid-1950s to mid-1960s) and below-average catches (early 1980s to early 1990s) could be due to consecutive years of above-average and below-average recruitments.
The IATTC has adopted resolutions to restrict the catch of bluefin tuna in the EPO. Resolutions C-12-09, C-13-02, and C-14-06 limit the commercial catches in the IATTC Convention Area by all CPCs to 10,000 metric tons during 2012-2013, 5,000 metric tons in 2014, and 6,600 during 2015-2016, respectively.