Tuna belong to the mackerel family, Scombridae. They are large, oceanic fish and are seasonally migratory, some making trans-oceanic journeys. Able to tolerate both warm and cool temperatures, bluefin tuna range throughout the entire north Atlantic and adjacent seas, (primarily the Mediterranean Sea) and can frequent depths to 1000m. Despite this thermal tolerance, a recent analysis of present vs. historical ranges concluded that Atlantic bluefin tuna has shown range contractions of 46% since 1960 - more than any other pelagic species . Despite poorly understood movements from east to west, a distinction in populations is made between the two regions. Interestingly, life history characteristics differ greatly between them. In the Mediterranean, bluefin tuna is assumed to mature at approximately 25 kg (age 4), whereas in the Gulf of Mexico in the West Atlantic, maturity occurs at approximately 145 kg (age 9). Northern bluefin grow slowly compared with other tunas and billfish but can reach more than 450cm in length and 680kg in weight with a maximum age of approximately 40 years. Spawning occurs from April to June in the Gulf of Mexico and June to August in the Mediterranean.
The bluefin tuna stock in the East Atlantic and Mediterranean are assessed by the International Commission for the Conservation of Atlantic Tunas (ICCAT). Catches in the East Atlantic and Mediterranean reached an estimated peak of 61,000 t in 2007, severely overfishing the stock. Catches were seriously under-reported between 1998 and 2007, and it is estimated that realized catch during this period was around 50,000-60,000 t per year. The introduction of fattening and farming activities into the Mediterranean in 1997 and good market conditions resulted in rapid changes in the Mediterranean fisheries for bluefin tuna, mainly due to increasing purse seine catches. From 2007 to 2011, reported catches steadily decreased to 11,781 t, and subsequently increased to the provisional 2017 catch of 23,616 t (just above the Total Allowable Catch (TAC) of 23,155 t).
The latest stock assessment was carried out in 2017 and is considered to be more reliable than previous assessments. There have been considerable improvements in the data quality and quantity over the past few years, but there remain gaps in the data for several fisheries prior to 2014, especially in the Mediterranean. The Scientific Committee does not expect that there can be further improvement in historical statistics. The current assessment uses the estimated catches from 1996-2007 rather than the declared ones. It was not possible to calculate biomass-based reference points (e.g. Maximum Sustainable Yield, MSY, and FMSY) in the latest assessment and in the absence of such knowledge, a fishing-effort-based (F) proxy has been used for the Eastern stock since 2008. The results indicate that fishing mortality is well below the Maximum Sustainable Yield (F approximately at 0.34Fmsy proxy). The modelled state of the biomass is highly dependent on the historical productivity of the stock. Scenarios suggest that the stock is no longer overfished under medium and low recruitment levels, but remains overfished under the high recruitment scenario. The state of the biomass is therefore unknown.
Projections indicate that annual constant catches of up to 36,000 t have higher than 60% probability of maintaining fishing effort at sustainable levels, and catches of 28,000 t or less have a higher than 50% probability of allowing a continued increase in the stock.
It still remains unclear how much of the Eastern Atlantic stock mixes with and supports the Western Atlantic stock. The species is listed as Endangered by the International Union for the Conservation of Nature (IUCN) Globally and in the Mediterranean (2011) and listed as Near Threatened in Europe (2015), yet as the stock status has rapidly changed, there is a need to update these assessments.
Most tuna stocks range across and are accessed by numerous coastal states, making harmonised and effective management of these individual stocks very difficult. To try and address this, Intergovernmental Regional Fisheries Management Organisations (RFMOs) have been established. There are five main tuna RFMOs worldwide and it is their responsibility to carry out data collection, scientific monitoring and management of these fisheries. This stock is managed and assessed by the International Commission for the Conservation of Atlantic Tunas (ICCAT). Whilst the RFMOs are responsible for the development of management and conservation measures, the degree to which they are implemented, monitored and enforced still varies significantly between coastal states. There remain large data deficiencies in most tuna and billfish fisheries, particularly with regards to fine scale spatial and temporal data for both target and especially for vulnerable bycatch species. For this reason, it is important to choose tuna that has been caught by vessels that are well regulated by their flag state.
It is currently illegal to catch bluefin tuna in UK waters. This section discusses the management of Atlantic Bluefin tuna across the entire Atlantic ocean and Mediterranean sea.
There is a 15-year recovery plan for Eastern and Mediterranean bluefin, starting in 2007 and continuing to 2022, which is on track to achieve its target of Biological Maximum Sustainable Yield with at least 60% probability. The combination of size limits and the reduction of catch has certainly contributed to a rapid increase of the abundance of the stock. Given this increase, the scientific committee advises moving from the current rebuilding plan to a management plan.
TACs (Total Allowable Catches) have increased from 16,142 t in 2015 to 23,155 t in 2017, and continue to increase: 28,200 t for 2018; 32,240 t for 2019; and 36,000 t for 2020. Whilst this has largely been in line with scientific advice, constant annual catches over approximately 32,000 t are projected to lead to a reduction biomass. Individual catch limits are assigned to each country. Countries must submit fishing plans, establishing quotas for each type of fishing gear. Unlike for other stocks, no carry-over of quota from one year to the next is allowed. Bluefin caught as bycatch cannot exceed 5% of the total catch of a vessel and must form part of the overall quota.
To protect spawning grounds (the precise location of which are still being identified), open seasons are set for defined periods and these windows vary depending on gear and location: Longliners over 24m may fish for this species for a 5 or 6 month period, depending on location. Purse seiners may fish for 1 or 4 months, depending on location. Bait boats and trolls, pelagic trawlers, and sport & recreational boats may fish for 4 months. Other gear is prohibited.
Aerial vehicles can not be used to help vessels find bluefin tuna.
Bluefin below 30kg or 115cm may not be caught, with a few exceptions.
Sport & recreational vessels are limited to 1 bluefin per day.
Capacity is limited, and countries must submit management plans to demonstrate how they will meet this requirement. The number and tonnage of vessels fishing for bluefin is limited to that of 2007/8, although the number of purse seiners is limited to numbers from 2013 or 2014. The number of traps is limited to that of 2008. Farming capacity is limited to that of 2008, and input of fish into farms is limited to that of any one year from 2005-2008.
Countries must keep a list of vessels and traps authorised to fish for, and facilities authorised to farm, this species.
Transhipment at sea is prohibited, and may only take place in designated ports with prior authorisation.
Vessels over 24m must have Vessel Monitoring Systems. Minimum observer coverage is higher than for other stocks and varies by gear: 20% for longliners, pelagic trawlers, and bait boats over 15m; 100% of purse seiners, towing vessels, and harvesting operations from traps.
There is a catch documentation scheme which from 2018 will be fully electronic to improve tracking of bluefin from catch to farm to export.
Other measures for bluefin include:
As of 2006, catch of Atlantic bluefin by longliners from the central Atlantic is frozen to levels caught in 1999/2000 as mixing of eastern and western stocks in this area is not well understood.
Countries must carry out research to support better understanding of the species.
Currently commercial and recreational capture of Atlantic Bluefin Tuna is prohibited in UK waters. The information below relates to the capture of this stock in other areas of its huge range across the Atlantic ocean.
Approximately 61% of the catch is made by purse seiners that set on free schools (as opposed to using Fish Aggregation Devices, FADs). These fish are live captured to then be transported to sea pens where they are held and fattened for later sale on the Asian sashimi market. Purse seining on free schools encounters less bycatch compared with FAD associated fisheries, though both still have interactions with vulnerable species.
15% of the catch is made by longlining, a method associated with the bycatch of vulnerable species, including sharks, sea turtles and seabirds.
17% of the catch is made by fixed traps that have minor impact on sensitive species.
3% of the catch is made by pole-and-line fisheries. This is a very selective method of fishing yet relies on significant amounts of baitfish. Assessment of these bait fish fisheries is generally needed.
2% is also taken in recreational sports fisheries.
To a lesser extent, bluefin is also captured in illegal gill net fisheries. For EU Member States, driftnet fishing for tuna has been banned since January 2002, yet remains a problem in some Italian fisheries and is still officially permitted in Morocco. Gill netting, especially offshore drift netting, encounters a very high proportion of bycatch.
ICCAT aims to take an ecosystem-based and precautionary approach to fisheries management. In order to minimize the ecological impact of FADs, in particular the entanglement of sharks, turtles and other non-targeted species, and the release of synthetic persistent marine debris, countries must use non-entangling FADs and phase out non-biodegradable FADs. Countries must have FAD Management Plans that improve understanding of FADs and limit their impacts on the ecosystem. In order to protect juvenile swordfish, a closure period applies to longline vessels targeting Mediterranean albacore from 1 October to 30 November each year.