Yellowfin tuna

The yellowfin tuna stock in the Indian Ocean is currently no overfished or subject to overfishing.This stock is managed and assessed by the Indian Ocean Tuna Commission (IOTC). Indian Ocean yellowfin has been caught since the 1950s. Catches peaked at around 540,000 tonnes in 2004, dropped to around 270,000 t in 2009 and in recent years have stabilised at around 425,000 t. A new stock assessment was carried out in 2024, using data up to 2023. The next stock assessment is expected after 2027.The 2024 stock assessment was carried out using an updated model that included a series of revisions compared to the model used in the 2021 assessment.Spawning biomass (SSB) has been declining since the 1980s. It is estimated that the stock in 2023 was 44% of unfished levels and 32% above target levels (Maximum Sustainable Yield, SBMSY). Therefore, it is in an overfished state, but above the lower limit (Blim) of 40% of MSY. This is an improvement from the previous assessment in 2021, when the stock was estimated to be 87% of SBMSY. Therefore, the stock is not subject to overfishing.Fishing mortality (F) in 2023 was 75% of target levels (FMSY). This is an decrease from the previous assessment, when F was 132% of FMSY. Therefore, F is no longer approaching, the upper limit of 140% of MSY and there is no concern for fishing pressure.Projections indicate that if catches are maintained around MSY levels, 421,000 tonnes (range 416,000–430,000 tonnes), there is a greater than 50% probability that the stock will remain above SBMSY in 2033. The five‑year average catch for 2019-2023 (423,143t) falls within this range, while the 2023 catch (401,400 tonnes) falls below it, these are in line with projections. Additionally, under recent catch levels, the probability of breaching either Blim or Flim by 2033 is projected to be 0%.

Few appropriate management measures are in place for Indian Ocean yellowfin tuna. Stock rebuilding plans have been in place since 2016. Catches in previous years have been too high, and monitoring and enforcement needs to be improved.Tuna, marlin, and swordfish are highly migratory species, found on the high seas and in numerous countries' waters. This makes harmonised and effective management challenging. Regional Fisheries Management Organisations (RFMOs) are responsible for monitoring and managing these stocks on behalf of the countries that access them. However, the degree to which management is implemented, monitored and enforced by each country varies significantly.Indian Ocean yellowfin tuna is managed and assessed by the Indian Ocean Tuna Commission (IOTC). The stock in 2023 is considered to be underfished and not subject to overfishing. Although interim rebuilding plans have been in place since 2016 to recover the stock to target levels. The Yellowfin Management Strategy Evaluation has been inactive for several years, pending a stock assessment revision. The rebuilding plans for 2016-2021 aimed to reduce catches by 20% from 2014 levels (405,000 tonnes), with variations by gear type. However, even with full compliance, the measures were estimated to deliver only a 10% reduction. Some countries, particularly those operating longline fleets, achieved the required reductions during this period and again in 2023. The Maldives decommissioned its longline yellowfin fleet in 2019, removing catches of around 3,000 tonnes per year to support stock recovery. These efforts were offset by increased catches elsewhere, especially by gillnet fleets, indicating that the plans were inadequately enforced and overall management remained insufficient to protect the stock. Total catches declined in 2023 to 401,400 tonnes, below the five‑year average of 423,143 tonnes (2019-2023). Projections indicate that maintaining catches within the recent 20‑year maximum sustainable yield range (416,000-430,000 tonnes) gives a greater than 50% probability that the stock will remain above the biomass target level (SBMSY) in 2033. However, Higher catch levels are predicted to lead to stock decline over the long term. Due to uncertainty in the recent assessment, it has been recommended that an initial one‑year total allowable catch (TAC) in 2026, not exceeding 421,000 tonnes.Several concerns have been raised about the 2016-2021 rebuilding plans. Catch reductions only applied to countries reporting catches above 5,000 tonnes in 2014, creating incentives for under‑reporting. Countries were also free to choose how they achieved reductions, leading to inconsistent and often ineffective approaches. In practice, fishing pressure was displaced towards purse seine fisheries using fish aggregating devices (FADs), increasing catches of skipjack tuna and juvenile yellowfin and bigeye tuna.The interim rebuilding plan introduced in 2022 included further catch reductions and catch freezes based on historical catch periods. If fully implemented, it is projected to reduce catches to around 401,000 tonnes, though it is too early to assess its effectiveness. Several high‑catch countries objected to the measure and are therefore exempt. While overall catch reductions remain essential, targeted action is needed to reduce catches of juvenile yellowfin. There are currently no minimum size limits, and some gears, particularly gillnets, FAD‑associated purse seines, and pole‑and‑line fisheries, take high proportions of juveniles and may require further restrictions. Improved monitoring and compliance are critical, as rebuilding plans are unlikely to succeed without effective enforcement. In addition to catch limits, measures are in place to constrain fishing capacity, particularly for the purse seine fleet, which accounts for around 30% of total catches. This includes limits on the number of supply vessels and progressive restrictions on FAD use, which declined from 550 FADs per vessel in 2015 to 300 from 2019 onwards. Countries using FADs must report their use and submit FAD management plans describing how they will minimise impacts on juvenile tuna and vulnerable non‑target species such as sharks, turtles and rays. Only purse seine vessels and their associated supply vessels may deploy drifting FADs (DFADs), and these must be fitted with activated, registered instrumented buoys. From 1 January 2026, vessels may follow no more than 250 instrumented buoys at any one time and may acquire a maximum of 400 per year, with further reductions from 2028. A DFAD Register applies from 1 January 2026 to all purse seine vessels fishing on DFADs.A general freeze on fleet capacity at 2003 levels is also in place to prevent further expansion. However, this measure is broad, applies across all fleets, and has reportedly been poorly enforced. More targeted approaches, such as spatial and temporal closures and quota allocation, may be more effective.Monitoring and compliance in the IOTC region remain weak. Some countries repeatedly fail to submit catch data. In 2018, the IOTC introduced measures to improve reporting, including prohibitions on retaining species when catch data are not reported, yet data gaps persist. Mandatory observer coverage remains at just 5% for vessels operating outside their Exclusive Economic Zones, well below the 20% coverage generally recommended by scientists. Proposals to increase coverage have so far failed to achieve consensus.Some catches may be misreported due to uncertainty over the boundary between Atlantic and Indian Ocean stocks. Genetic evidence suggests that much of South Africa’s reported Atlantic catch may in fact originate from the Indian Ocean stock.Other IOTC conservation and management measures of note include:Discarding bigeye, yellowfin, skipjack tunas, and non-target species at sea is banned.To help address Illegal, Unregulated, and Unreported fishing (IUU), the IOTC maintains an active vessel register and an IUU Vessel List. In 2021, all transhipments at sea were banned, except for large scale tuna longliners, which must be pre-approved, monitored by an observer and the vessel uses a Vessel Monitoring System (VMS).In 2012 IOTC banned the use of driftnets on the high seas. In 2022 this was extended to the entire IOTC area (i.e. including countries' EEZs).Using aircraft and unmanned aerial vehicles as fishing aids, which significantly contribute to fishing effort by helping to detect fish, is banned.Using artificial lights to attract tuna and tuna-like species beyond territorial waters is banned.

Some yellowfin tuna from the Indian Ocean is caught by purse seining. This method is associated with bycatch of species such as sharks, rays, and turtles.Purse seining accounts for 33% of Indian Ocean yellowfin tuna catches (around 132,000t in 2023). Approximately two thirds of purse seine catches are from Fish Aggregating Devices (FADs) and the rest is on free schooling tuna, but it is not always possible to verify which catches are from which method.Purse seining is associated with bycatch of species such as sharks, turtles and marine mammals, although less so than longlining. Bycatch is higher when FADs are used. In the Indian Ocean, bycatch per ton of tuna averages 0.8% on free school sets and 3% on FADs. FADs are floating objects that tuna and other species tend to aggregate around. FADs can include hanging elements such as ropes and nets, which can entangle vulnerable species. FADs can also become lost at sea, continuing to ghost fish and be a source of marine debris.Some mitigation measures are in place. The IOTC is reducing the number of FADs that countries can use, and working on ways to reduce impacts through development of biodegradable and non-entangling FADs. Countries must submit FAD management plans outlining how they will minimise mortality of juvenile yellowfin and bigeye tuna and vulnerable non-target species such as sharks, turtles and rays.However, concerns remain. Several countries have failed to implement national plans of action (NPOAs) for sharks, seabirds and turtles as required. Monitoring and reporting of bycatch in fisheries managed by IOTC is poor. Most data come from scientific observers on vessels. Scientific recommendations are that 20% of a fishery should be observed for accurate data. IOTC only requires coverage of 5%. Many fleets are not reaching this level. This makes it difficult to assess the impact of these fisheries on vulnerable species.In 2022, around 80,000t of sharks were caught, with rays totalling around 1,500t. Gillnets are responsible for around 34% of the catch, while line methods such as longline, handline, and trolling, contribute to around 45% of retained shark and ray catches. It is estimated that around 24,000t is blue shark, which is not overfished or subject to overfishing. Most of the rest of the data is not reported to species levels, so data on interactions is poor. However, species such as the critically endangered oceanic whitetip and scalloped hammerhead, and the endangered shortfin mako and pelagic thresher are known to be caught at significant levels in the Indian Ocean. The available evidence indicates considerable risk to these species. Mobulid rays are also bycaught and are declining across the Indian Ocean. Gillnets are responsible for around 40% of the catch, followed by longline, handline, and trolling. Data on bycatch by purse seiners is poor, and a 2021 study indicated that it could be double the quantity that is currently recorded. There are some mitigation measures, such as restrictions on finning, and bans on retaining some species. There are no gear-specific measures.The IOTC reports that flatback, green, hawksbill, leatherback, loggerhead and olive ridley turtles are caught in its fisheries. Of these, hawksbills, southwest Indian Ocean leatherbacks, and northwest Indian Ocean loggerheads are critically endangered. Data on interactions is not regularly submitted, so the extent of bycatch is unknown. Gillnetting is the biggest concern for turtles, with estimates ranging from 11,000-52,000 individuals being caught annually. Longlining is estimated to catch around 3,500 turtles annually. Turtles can also be entangled by Fish Aggregating Devices used by purse seiners, and around 250 turtles are estimated to be caught by purse seiners annually. An estimated 75% of turtles are released alive from longliners and purse seiners. The scientific committee advises that maintaining or increasing fishing effort in the Indian Ocean without appropriate measures in place will likely result in further population declines. Current mitigation measures include requirements to release turtles wherever possible.Most bycatch of cetaceans is by gillnetting. Purse seine nets have the potential to encircle or entangle the animals, although interactions are thought to be low. It is illegal to intentionally set a purse seine net around a cetacean. These interactions are not well recorded and the scale of impact of IOTC fisheries is unclear. The IOTC scientific committee notes that maintaining or increasing fishing effort in the Indian Ocean without appropriate mitigation measures in place will likely result in further declines in a number of cetacean species. In 2023, it was noted that there are tuna-dolphin associations for yellowfin tuna. The dolphins are not targeted, but can be caught as bycatch. This association appears to be widespread around the Indian Ocean, and is used by coastal country fishermen in Maldives, Sri Lanka, Oman and elsewhere to target yellowfin tuna.FADs are also of concern due to the unknown impacts such gear might have on other tuna and fish species in relation to species composition of schools, migratory patterns, growth rates and predation rates. Juvenile yellowfin and bigeye tuna are caught by purse seining, and scientific recommendations are for these impacts to be reduced to protect the stocks.