Stock Status of Atlantic Bluefin

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 July 2008. The situation in the East Atlantic is also tenuous, with excessively high fishing mortality threatening the potential for complete collapse of the population.

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 2008 assessment estimated the size of the spawning stock (i.e., age 4-5+ for the East Atlantic and age 8+ for the West Atlantic) for 1955-2007 for the east and 1970-2007 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 1975; it should be noted that the bluefin population was already significantly depleted by 1975. 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 8+.

Until 2006, western bluefin were assessed using two different scenarios of reproductive success. The "low recruitment" scenario assumed that environmental conditions had changed such that the number of young produced by the same number of spawning fish was lower than in the past, while the "high recruitment" scenario assumed that the number of young produced was 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 was less optimistic about the current state of the population. In 2006, ICCAT stopped using the different scenarios and assessed the stock relative to one estimate of maximum sustainable yield.  MSY was set at 3,200 MT, a target even lower than the low recruitment scenario target in the 2002 assessment, which estimated 3,500 MT for low recruitment and 7,200 MT for high recruitment. Importantly, this significantly improved the optimism of the assessment.

The final results of the 2006 assessment are below, showing that western bluefin are overfished and overfishing is still occurring:

MSY	3,200 MT
SSB2004/SSBMSY	0.41
SSB2004/SSB1975	0.18
F2004/FMSY	1.7

The above numbers will be updated to reflect the results of the 2008 assessment once the final guidance is issued in October 2008. To download the full report of the 2008 ICCAT Atlantic bluefin tuna stock assessment, click here. The Executive Summary will be posted in October 2008. Additional information can be found in the 2007 Stock Assessment and Fishery Evaluation 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 solely in comparison to the size of the spawning stock biomass (SSB) in 1970. The 2006 assessment estimated that the eastern bluefin population has declined to 48% of the 1970 level, a significant decrease from the 2002 estimate of 86%. The fishing mortality rate was estimated at 3.1 times the sustainable level. Note that as with western bluefin, eastern bluefin are overfished and overfishing is still occurring.

SSB2000-04/SSB1970	0.48
F2004/Fmax	3.1

The above numbers will be updated to reflect the results of the 2008 assessment once the final guidance is issued in October 2008. To download the full report of the 2008 ICCAT Atlantic bluefin tuna stock assessment, click here. The Executive Summary will be posted in October 2008.