Stock Assessment Highlights Decline of Pacific Bluefin Tuna Stock

The Pacific Bluefin Tuna Working Group (PBFWG) recently completed a scientific assessment of the highly-migratory Pacific bluefin tuna stock, a prized source of sashimi in Japan and other countries, and concluded that the stock is overfished and overfishing is occurring. In December 2012, the group reported its findings to its parent body, the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC).

The assessment assumed a single pan-Pacific stock of Pacific bluefin tuna (PBF), and estimated the stock's population dynamics using a fully-integrated age-structured numerical population model fitted to PBF data on total catch, size composition of the catch, and catch-per-unit of effort (CPUE) from 1952 to 2011 provided by ISC member fishing nations to the PBFWG. To study how uncertainties concerning PBF life history and fishery dynamics affect conclusions about the stock dynamics, the working group constructed 20 different models, each with different weightings of input data and alternative assumptions about model structure. No single model scenario provided a good fit to all sources of data deemed reliable, but all scenarios were in general agreement about key metrics of population dynamics. Spawning stock biomass fluctuated throughout the assessment period (1952-2011) but has been declining steadily for the past decade (Fig. 1). However, there is no evidence of reduced recruitment. Fishing mortality, estimated for each age group of fish (Fig. 2), has increased in the most recent period for which the computation can be made (2007-2009) relative to the current baseline period (2002-2004). Finally, the current fishing mortality rate, averaged over the 2007-2009 period exceeds all target and limit biological reference points commonly used by fishery managers to assess overfishing.

Figure 1.  Spawning biomass (SSB) for the base case model run (black line). Dashed red lines and solid red line indicate 80% 
               confidence intervals for SSB and median SSB estimated from bootstrapping the base case model. Dashed blue line indicates the overall 
               median SSB.
Figure 1. Spawning biomass (SSB) for the base case model run (black line). Dashed red lines and solid red line indicate 80% confidence intervals for SSB and median SSB estimated from bootstrapping the base case model. Dashed blue line indicates the overall median SSB.
Figure 2.  Average age-specific fishing mortality during 2002-2004 and 2007-2009.
Figure 2. Average age-specific fishing mortality during 2002-2004 and 2007-2009.

Model projections of Pacific Bluefin tuna stock spawning biomass and catch from 2011 to 2030 were computed assuming 4 alternative harvest scenarios:

  1. constant fishing mortality at the most recent levels ( denoted F2007-2009);
  2. constant fishing mortality at current baseline levels (F2002-2004);
  3. constant fishing mortality at F2007-2009 along with limitations on BFT catch by purse seine fleets in the eastern and western regions of the Pacific Ocean; and
  4. constant fishing mortality at F2002-2004 along with limitations on catch by the purse seine fleets.

Under harvest scenario 1, the future projections indicate that the median SSB is not expected to increase substantially from the present median SSB. Under scenarios 2, 3, and 4 the median SSB is expected to increase by about 41,000—83,000 mt by 2030.