Status Assessment 2024 - Atlantic Bluefin Tuna

Confidence

High

Background information

• Year added to OSPAR List: 2003.

• The original evaluation of bluefin tuna against the Texel-Faial criteria referred to decline and sensitivity criteria, with information also provided on threat.

• OSPAR Regions II−V constitute a key feeding ground for Atlantic bluefin tuna, which distributes throughout the Atlantic and adjacent seas.

• Decline: the eastern Atlantic stock was evaluated as significantly declined at the time of listing.

• Sensitivity: the species was considered sensitive at the time of listing due to its relatively slow growth, long life span and late age at maturity.

• Anthropogenic pressures and biological factors: fishing was the most important anthropogenic pressure and fishery data was uncertain, affecting management advice and management action.

• Last status assessment: 2003. A background document was published in 2014. 

Geographical range and distribution

Atlantic bluefin tuna are widely distributed mostly across the north Atlantic and adjacent seas with two populations, a larger eastern population, spawning in the Mediterranean Sea, and a smaller western population, spawning largely in the Gulf of Mexico. Atlantic bluefin tuna are most abundant in the Mediterranean and temperate waters of the North Atlantic. Recently they recolonised Nordic waters (where they had been very rare for at least 50 years) and this may be due to the increased population size, environmental change, or both (MacKenzie et al., 2014; Failletaz et al., 2019; Nottestad et al., 2020; Aarestrup et al., 2022). The two populations mix when not spawning, mostly on the western side of the Atlantic.

Population/Abundance

Most bluefin tuna in OSPAR Regions belong to the eastern population (Fraile et al., 2015; Rodriguez-Ezpeleta et al., 2019). The western population is an order of magnitude smaller than the eastern one (ICCAT, 2023a) and remains mostly in the western Atlantic (to the west of OSPAR Region V), as indicated by tagging and genetic evidence (Dedman et al., 2003; Rodriguez-Ezpeleta et al., 2019).

The latest (2022) ICCAT assessment of the eastern population indicates a strongly increasing population with Spawning Stock Biomass (SSB) at the highest value since the 1960s (Figure 1). No biomass reference points are determined. The fishing mortality for juveniles and adults has decreased substantially and is now below fishing mortality reference points.

Figure 1: East Atlantic and Mediterranean spawning-stock biomass (SSB, left panel) and fishing mortality (F, right panel) trends estimated by three different stock assessment models: VPA (blue lines), Stock Synthesis (green lines), and ASAP (orange lines). Source: ICCAT (2023a).

Condition

Recruitment estimates for East Atlantic and Mediterranean Atlantic bluefin tuna are uncertain and show considerable variability, especially over the recent period. In general, however, there are two distinct periods, one with low recruitments before 1990 and the other with higher recruitments thereafter (ICCAT, 2023a).

Threats and impacts

Historically, fishing has been the main threat to the Atlantic bluefin tuna. This generated high and largely uncontrolled fishing rates causing population declines. Following the implementation of the recovery plan in 2006, fishing mortality was reduced and has remained within sustainable levels since the early 2010s.

Climate change effects on bluefin tuna are not fully understood. The preferred feeding grounds are expected to move to higher latitudes (Erauskin-Extramiana et al., 2019). Conversely, important spawning and nursery areas in the Mediterranean might become thermally limiting, implying that spawning in the open Atlantic (as western bluefin already do, Trueman et al., 2023) would need to occur to sustain the eastern population.

Measures that address key pressures from human activities or conserve the species/habitat

ICCAT adopted a Recovery Plan in 2006 and, as the stock recovered, this evolved into the current Management Plan (ICCAT, 2022) that includes a wide range of measures, including TACs and quotas, closed seasons, minimum sizes, and diverse control measures.

In addition, a management procedure was adopted in 2022 (ICCAT, 2023b). This procedure explicitly considers the mixing of the eastern and western bluefin tuna populations and is robust to main uncertainties, including alternative productivity regimes. 

Conclusion (including management considerations)

Although the current management plan accounts for mixing between the eastern and western populations, within the OSPAR regions bluefin tuna are expected to originate almost exclusively from the eastern population.

Based on the last ICCAT assessment, the status of eastern Atlantic bluefin tuna has clearly improved, and SBB for the last year (2020) in the current assessment is the largest since the 1960s. Bluefin tuna has expanded their distribution to recolonise Nordic waters after more than 50 years.

The large reduction in fishing mortality and a shift in the pattern of selectivity to large fish is expected to have increased the mean age of the population.

The assessment is considered data rich, as it includes many sources of information including some fisheries independent surveys. Still, some key data sources (e.g. historical total catch and its size composition) are uncertain, and assessment results are sensitive to many assumptions. However, the management procedure has been tested to be robust to the main uncertainties, so there is good confidence in that the catch levels based on this management procedure will keep both populations in good status.

Knowledge Gaps

Further research on stock structure and the location of spawning grounds will improve the understanding of stock mixing and potential climate resilience.

The persistence and relative importance of newly found spawning grounds in the Slope Sea warrants further investigation (Hernandez et al., 2022; Diaz-Arce et al., 2024).

The current management procedure is expected to be revised in 2028. Further research on the effects of climate change on the productivity of the stock, would be an important consideration during the next review of the management procedure. The maturity schedule and natural mortality are important biological parameters for improving the assessments that deserve additional research.

Sheet reference:

BDC2024/Bluefin_tuna