An updated assessment was conducted this year, including information up to 2007. This assessment is consistent with previous analyses in that spawning stock biomass (SSB) declined steadily between the early 1970s and 1992. Since then, SSB has fluctuated between 18% and 27% of the 1975 level (Figure 3). The stock has experienced different levels of fishing mortality (F) over time, depending on the size of fish targeted by various fleets (Figure 3). Fishing mortality on spawners (ages 8 and older) declined markedly between 2002 and 2007.

Estimates of recruitment were very high in the early 1970s (Figure 3), and additional analyses involving longer catch and index series suggest that recruitment was also high during the 1960s. Since 1977, recruitment has varied from year to year without trend. The Committee noted that a key factor in estimating MSY-related benchmarks is the highest level of recruitment that can be achieved in the long term. Assuming that average recruitment cannot reach the high levels from the early 1970s, recent F (2004-2006) is about 30% higher than the MSY level and SSB is about half of the MSY level (Figure ). Estimates of stock status are more pessimistic if a high recruitment scenario is considered (F/FMSY=2.1, B/BMSY=0.14).

One important factor in the recent decline of fishing mortality on large bluefin is that the TAC has not been taken during this time period, due primarily to a shortfall by the United States fisheries that target large bluefin. Two plausible explanations for the shortfall were put forward previously by the Committee: (1) that availability of fish to the United States fishery has been abnormally low, and/or (2) the overall size of the population in the Western Atlantic declined substantially from the level of recent years. While there is no overwhelming evidence to favor either explanation over the other, the base case assessment implicitly favors the first hypothesis (regional changes in availability) because a large recent reduction in SSB is not estimated. Nevertheless, the Committee notes that there remains substantial uncertainty on this issue and more research needs to be done.

The SCRS cautions that the conclusions of this assessment do not capture the full degree of uncertainty in the assessments and projections. An important factor contributing to uncertainty is mixing between fish of eastern and western origin. Limited analyses were conducted of the two stocks with mixing. Depending on the types of data used to estimate mixing (conventional tagging or isotope signature samples) and modeling assumptions made, the estimates of stock status varied considerably. However, these analyses are preliminary and more research needs to be done before mixing models can be used operationally for management advice. Another important source of uncertainty is recruitment, both in terms of recent levels (which are estimated with low precision in the assessment), and potential future levels (the "low" vs "high" recruitment hypotheses which affect management benchmarks). Finally, the growth curve assumed in the analyses may be revised based on new information that is being collected. If the curve changes substantially, it may impact the assessment results as well as management benchmarks.

Figure 3 Median estimates of spawning biomass (age 8+), fishing mortality on spawners, apical fishing mortality (F on the most vulnerable age class) and recruitment for the base VPA model. The 80% confidence intervals are indicated with dotted lines.

Figure 4 Estimated status of stock relative to the Convention objectives (MSY) by year (1970 to 2007). The lines give the time series of point estimates for each recruitment scenario and the clouds of white symbols depict the corresponding bootstrap estimates of uncertainty for the most recent year.

In 1998, the Commission initiated a 20-year rebuilding plan designed to achieve Bmsy with at least 50% probability. The current assessment indicates that the stock has not yet rebuilt as projected under the plan initially. The 2007 SSB is estimated to be 7% below the level of the Plan’s first year.

Based on a strict interpretation of the base case projections and the Western Atlantic Rebuilding Plan [Rec. 98-07], the Commission is faced with TAC options that range between 2,400 t and zero depending on its choice of recruitment scenarios and choice of the probability of rebuilding. The Committee notes that making decisions based on the low recruitment scenario when in fact the high recruitment scenario is true, could be riskier in terms of stock rebuilding. In light of the uncertainty about recruitment and other uncertainties not taken into account in the projections, the Committee strongly advises against an increase in TAC. Analyses conducted during the Joint ICCAT-Canada Precautionary workshop as well as two subsequent analyses reviewed by the Committee (SCRS/2008/089, SCRS/2008/175) suggested that the projections made during past assessments were too optimistic. This is reinforced by the observation that, halfway through the rebuilding program, biomass is still below what it was at the beginning. The Committee notes that there is a provision of the rebuilding plan to adopt harvest levels that provides a 50% or greater chance of meeting rebuilding targets. Given this possibility and the lack of progress on rebuilding, the Committee recommends that the Commission adopt more conservative catch levels that will result in a higher probability (for example, 75% chance) that BMSY is achieved by the beginning of 2019. Under the more optimistic "low recruitment" scenario, this target could be achieved with a TAC of 2,000 t. However, if the assessment and estimates of future yield are positively biased or if there is implementation error (both of which have occurred in the past), the TAC should be lower (for instance, based on the assessment results without the Gulf of St. Lawrence CPUE index, the TAC would need to be reduced to less than 1,500 t in order to achieve Bmsy by 2019 with 75% probability).

As noted previously by the Committee, both the productivity of western Atlantic bluefin and western Atlantic bluefin fisheries are linked to the eastern Atlantic and Mediterranean stock. Therefore, management actions taken in the eastern Atlantic and Mediterranean are likely to influence the recovery in the western Atlantic, because even small rates of mixing from East to West can have significant effects on the West due to the fact that Eastern plus Mediterranean resource is much larger than that of the West.

WEST ATLANTIC BLUEFIN TUNA SUMMARY (Catches and Biomass in t)
Current (2007) Catch (including discards)	1,624 t
Assuming Low Potential Recruitment
Maximum Sustainable Yield (MSY|R1)	2,852 (2,680-3,032)2
Relative Spawning Stock Biomass:
B2007/B1975	0.25 (0.16-0.38)2
B2007/BMSY|R	0.57 (0.46-0.70)2
Relative Fishing Mortality3:
F2004-2006/FMSY|R	1.27 (1.04-1.53)2
F2004-2006 /F0.1	2.23 (1.82-2.72)2
F2004-2006 /Fmax	1.27 (1.04-1.53)2
Assuming High Potential Recruitment
Maximum Sustainable Yield (MSY)	6,201 (4,887-9,142)2
Relative Spawning Stock Biomass:
B2007/B1975	0.25 (0.16-0.38)2
B2007/BMSY|R	0.14 (0.08-0.21)2
Relative Fishing Mortality3:
F2004-2006 /FMSY|R	2.18 (1.74-2.64)2
F2004-2006 /F0.1	2.23 (1.82-2.72)2
F2004-2006 /Fmax	1.27 (1.04-1.53)2
Management Measures:	[Rec. 06-06] TAC of 2,100 t which began in 2007, including dead discards

¹ MSY calculated conditional that recruitment remains at recent (1976-2004) levels.
² Median and approximate 80% confidence interval from bootstrapping from the assessment.
³ 2004-2006 refers to the geometric mean of the estimates for 2004-2006 (a proxy for recent F levels).