Bigeye tuna

A updated stock assessment of bigeye tuna in the Eastern Pacific Ocean (EPO) was conducted during 2018. The assessment was conducted using an age structured model called Stock Synthesis, which has been used in previous assessments.  Because this is an 'updated' assessment, the same base case model from the 2016 assessment was used and only data was updated. Data includes information on catch, discards, catch per unit effort and size composition. New or updated longline data for this assessment were availalbe from China, Japan, Korea, Chinese Taipei, the US, French Polynesia and Vanuatu. The assessment assumes a single stock of bigeye tuna in the EPO, with minimal net movement between the EPO and western and central Pacific Ocean (Xu et al. 2018). 

In 2013 the Inter-American Tropical Tuna Commission (IATTC) Scientific Committee made the following recommendations to the Commission in 2013 for tunas and sharks: 1. The current tuna management plan should be continued through 2014 because there is evidence that fishing mortality for bigeye tuna may still be too high. 2. A harvest control rule that requires effort to be reduced once fishing mortality exceeds the maximum sustainable yield should be adopted {IATTC 2013a}.

In 2014, the Scientific Committee made several additional recommendations including conducting a feasibility study for sampling lengths from adult tuna on a regular basis, combining observer data into a central database, and analyze movement patterns of bigeye tuna (IATTC 2014). In 2016, the Scientific Committee indicated that the current tuna conservation and management plan should be maintained with the addition of additional purse seine closure days (87 instead of 62)  (IATTC 2016).

The Scientific Committee was unable to provide new management advice for bigeye tuna in 2018 due to considerable uncertainty in the updated stock assessment results (Xu et al. 2018). Instead they suggested a limit on the number of purse seine sets be adopted for the entire fishery (IATTC 2018). 

In 2019, due to issues with the yellowfin tuna stock assessment, the IATTC scientific staff was unable to make any management recommendations specific to yellowfin tuna. They instead provided a similar recommendation to that from 2018 to maintain the current management resolution and limit the total number of purse seine (associated and unassociated) sets in 2020 to 15,723  (IATTC 2019). The IATTC Scientific Advisory Committee did not endorse this recommendation from the IATTC staff and provided no additional management advice for yellowfin tuna in 2019 (final report not published yet, personal communication from the meeting).

The 2017 updated assessment indicated a recovering trend for bigeye tuna in the EPO between 2005-2009 but the rebuilding trend was not sustained between 2010-2013. The spawning biomass ratio (SBR) declined gradually to a historically low value of 0.15 at the start of 2013. The SBR has increased since to 0.23 at the start of 2016. Bigeye tuna are not currently considered overfished in the eastern Pacific Ocean (Xu et al. 2018).

Fishing mortality rates are above the level corresponding to the maximum sustainable yield (MSY). Bigeye tuna in the eastern Pacific Ocean are undergoing overfishing based on this increased fishing mortality rate (Xu et al. 2018)

Management measures specific to pelagic longline fisheries operating in the Eastern Pacific Ocean include catch limits (combined) for bigeye and yellowfin tuna by set type for large purse seine vessels. There are additional catch limits for longline caught bigeye by China, Japan, Korea and Taiwan. IATTC currently uses an interim limit reference point for bigeye tuna but target reference points and harvest control rules are not used. Management measures specific to the purse seine fisheries include a mandatory closure for 72 days during one of two predefined time periods and there is an additional purse seine closure between October 9th and November 8th in the area of 96⁰ and 110⁰W and between 4⁰N and 3⁰S (IATTC 2017). If a fisheries observer is onboard from the On-Board Observer Program of the Agreement on the International Dolphin Conservation Program (AIDCP), the vessels (182-272 metric tons carrying capacity) can make one 30 day trip during the specified closures dates. An additional time/area closure off the coast of Central and South America for purse seine vessels is also in place {IATTC 2013b}. Discarding bigeye, skipjack and yellowfin tuna is prohibited {IATTC 2013}. Purse seine vessels are also prohibited from setting on data buoys {IATTC 2010}. 

The Inter-American Tropical Tuna Commission (IATTC) has a multi-annual conservation program in place to monitor bigeye tuna populations. The plan was last updated in 2017 for the 2018-2020 fishing seasons (IATTC 2017).

Individual countries that have been given a catch limit for longline caught bigeye tuna, China, Japan, Korea and Chinese Taipei, have abided by those limits. Other countries have kept their catches below 500 t or at 2001 levels (Xu et al. 2018).

The longline fisheries operating in the eastern Pacific Ocean (EPO) that capture bigeye tuna can also incidentally catch several species of sea turtles currently listed under CITES Appendix I. Purse seine fisheries have some interactions with sea turtles, but far less than in the longline fisheries. The troll and pole fisheries for bigeye tuna do not incidentally capture any of these species.

Green, hawksbill, leatherback, loggerhead and olive ridley sea turtles have been reported as incidentally captures in longline fisheries operating in the EPO. The International Union for the Conservation of Nature (IUCN) classifies green, leatherback and loggerhead turtles as Endangered, hawksbill as Critically Endangered, and olive ridley as Vulnerable (www.iucn.org).

Marine mammal interactions are not common in this fishery.

Several species of seabirds, including black-footed, laysan and waved albatross. Black-footed, laysan albatross are considered Near Threatened by the IUCN and waved albatross as Critically Endangered.

Purse seine (associated) fisheries also incidentally capture a variety of bycatch species including some sea turtles and shark species. Unassociated purse seine fisheries typically have less bycatch associated with them.

The Inter-American Tropical Tuna Commission (IATTC) has put several management measures aimed at bycatch species into place. IATTC member countries are to implement an International Plan of Action for Seabirds. Two seabird mitigation methods are required on vessels larger than 20 m fishing in specific areas. A 3 year program to reduce the impact of fishing on sea turtles has been put into place. This plan requires reporting of any interaction and carrying of proper handling and release gears. Shark finning is banned (5% rule) and oceanic whitetip sharks are prohibited from being retained (IATTC 2011b)(IATTC 2011c)(IATTC 2005)(IAC 2012). Purse seine fisheries fishing on fish aggregating devices (FADs) must use specific methods designed to avoid entangling sea turtles or other bycatch species. Any interactions must be reported and sea turtles are to be released (IATTC 2012)(IATTC 2007).

The dolphin set fishery has historically had interactions with spinner and pan tropical dolphins. The Agreement of the International Dolphin Conservation Program (AIDCP) was put into place to track this fishery and any dolphin moralities (IATTC 2003).

Eastern Pacific Ocean (EPO) longlines fisheries that capture bigeye tuna also catch a number of other species of fish, including billfish and other tuna species, and sharks. Purse seine fisheries also catch a number of fish and shark species. The troll fishery catches small amounts of other tuna species and fish.

Other common bycatch species in the longline fishery include blue and silky sharks, indo-Pacific sailfish, dolphinfish and swordfish. Blue shark populations are currently healthy in the north Pacific region of the EPO but populations in the south Pacific appear to be in much worse condition. The current status of silky sharks, despite an assessment being conducted, is unknown in this region. The status of indo-Pacific sailfish is also uncertain. Swordfish populations are healthy in both the northern and southern region of the EPO {ISCSWG 2014}{IATTC 2013c}{IATTC 2014b}.

In the purse seine fishery (floating object), yellowtail, mahimahi, rainbow runner and wahoo are common bycatch species. Mahimahi and rainbow runner are also caught in the unassociated fisheries. Assessments have not been conducted on these species, so their status is unknown. Silky and oceanic white tips sharks, along with manta rays (unassociated) are also incidentally caught (Hall and Rowman 2013). No assessments of oceanic whitetip sharks or manta rays have been conducted. Oceanic whitetip sharks are prohibited from being retained and shark finning (5% rule) is prohibited (IATTC 2011c).

The gears used to capture tuna have no impact on bottom habitats.