Stock Status of Atlantic Bluefin Tuna

The rebuilding plan for western Atlantic bluefin tuna began in 1999. A decade later, the population has declined still further rather than rebuilt, according to the latest assessment completed in September 2010. The situation in the East Atlantic is also tenuous, with continued illegal fishing and failure by the majority of nations to report landings data.  Nevertheless, data has improved since the 2008 assessment, and the egregious pirate fishing of 2007 and years prior has been curtailed somewhat.

The status of the western and eastern populations is assessed separately, using different methodologies. The key end results are an estimate of population size, called biomass, and relative fishing mortality. The 2010 assessment estimated the size of the spawning stock (i.e., age 4-5+ for the East Atlantic and age 9+ for the West Atlantic) for 1950-2009 for the east and 1970-2009 for the west.  It should be noted that the western population had already decreased by 1970, and there is a desire among scientiests to take the next assessment back to 1950 as has been done for the East Atlantic. The following graphics clearly illustrate the significant population declines and great difference in population size of the eastern and western stocks.

 

Overfished vs. Overfishing

Before discussing the stock assessment results, it is important to clarify a bit of fisheries jargon. Both western and eastern bluefin tuna are classified by NOAA Fisheries as overfished and as experiencing overfishing. So what's the difference?
As with most fisheries, the western bluefin fishery is managed for maximum sustainable yield (MSY), defined as the largest average fish catch that a population can support over time without declining. BMSY is the population size (or biomass) that will support MSY, and FMSY is the associated fishing mortality. If the population size drops below BMSY (and therefore B/BMSY is < 1.0; see figure at right), it is considered to be overfished, meaning that the population can no longer support the target fishing rate (i.e., FMSY). If the fishing rate is above FMSY (and therefore F/FMSY is > 1.0; see figure at right), overfishing is occurring, and more fish are being caught than can be replaced by reproduction.
The western and eastern bluefin populations would be in the upper left quadrant of this schematic since overfishing is occurring AND the populations are overfished.

Stock Status of Western Bluefin

ICCAT assesses the population size of western bluefin relative to maximum sustainable yield and the population size in 1970; it should be noted that the bluefin population was already significantly depleted by 1970. Rather than estimating the size of the entire population, ICCAT estimates only the size of the spawning stock, or SSB, which is assumed to include all fish age 9+.

Western bluefin are assessed using two different scenarios of reproductive success. The "low recruitment" scenario assumes that environmental conditions have changed such that the number of young produced by the same number of spawning fish is lower than in the past, while the "high recruitment" scenario assumes that the number of young produced is low only due to the lower number of spawning fish. Compared to the low recruitment scenario, the high recruitment scenario would allow for a higher catch rate upon rebuilding but is less optimistic about the current state of the population since the target population size is bigger.

The final results of the 2010 assessment are below, showing that western bluefin are overfished and overfishing is still occurring under the high recruitment scenario but that they are not overfished and overfishing is not occurring under the low recruitment scenario:

High Recruitment Low Recruitment
MSY
6,329 MT
2,585 MT
SSB2009/SSBMSY
0.15
1.1
SSB2009/SSB1970
0.27
0.27
F2006-08/FMSY
1.88
0.73

To download the full report of the 2010 ICCAT Atlantic bluefin tuna stock assessment, click here. The Executive Summary can be found in the full annual report of the SCRS, ICCAT's scientific body, available here. Additional information can be found in the annual Stock Assessment and Fishery Evaluation (SAFE) Report for Atlantic Highly Migratory Species released by NOAA Fisheries.

Stock Status of Eastern Bluefin

Maximum sustainable yield is not calculated for eastern bluefin. The stock is assessed in comparison to the size of the spawning stock biomass that will allow F0.1 (i.e., the fishing mortality rate at which the yield of young fish will not increase very much if fishing mortality is increased). The fishing mortality rate was estimated at 2.9 times the sustainable level. Eastern bluefin are overfished and overfishing is still occurring.
SSB2009/SSBF0.1
High Recruitment
0.19
Medium Recruitment
0.35
Low Recruitment
0.51
F2009/F0.1
2.9

To download the full report of the 2010 ICCAT Atlantic bluefin tuna stock assessment, click here. The Executive Summary can be found in the full annual report of the SCRS, ICCAT's scientific body, available here.