Yellowfin tuna

The first assessment for yellowfin tuna in the western and central Pacific Ocean was conducted in 1999, with assessments being conducted annually until 2007 and every two-three years since. The overall objectives of the assessment are to estimate population parameters, such as time series of recruitment, biomass and fishing mortality, which indicate the status of the stock and impacts of fishing on that stock. 

The 2017 assessment, similar to past assessments, used the stock assessment model and computer software known as MULTIFAN-CL. This updated assessment included a complete update of the 2014 reference model, with inputs extended for the time period 2012-2015. Catch rate series were updated using the Pacific wide longline database, a new regional structure was used and modifications to recruitment estimates were made(Tremblay-Boyer, L. et al. 2017).

The current scientific advice for yellowfin tuna is the same as recommended since the 2014 assessment. The WCPFC should consider measures to reduce fishing mortality from fisheries that take juveiles and measures should be implemented to maintain current spawning biomass levels until the Commission can agree on a target reference point (WCPFC 2017).

There is currently a limit but no target reference point adopted by the Western and Central Pacific Fisheries Commission for yellowfin tuna (Tremblay-Boyer, L. et al. 2017).

The estimates of the latest (2015) and recent (2011-2014) spawning biomass are both above levels necessary to produce the maximum sustainable yield (MSY). This indicates that that the population is not overfished. Fishing mortality levels have been increasing over time but are still below levels needed to produce the maximum sustainable yield. Therefore overfishing is not occurring (Tremblay-Boyer, L. et al. 2017).

The biomass of yellowfin tuna has declined over time and fishing mortality rates for juveniles and adults have increased over time (Tremblay-Boyer, L. et al. 2017). Fishing mortality rates have increased considerably on juvenile and adult fish since industrial fishing was introduced. The spawning potential has shown a slight increase in recent years, but is far below levels seen at the beginning of the time series (Tremblay-Boyer, L. et al. 2017).

Yellowfin tuna in the western and central Pacific Ocean are managed by the Western and Central Pacific Fisheries Commission (WCPFC). The most recent management measures for this species were adopted in 2013 and put into place during 2014. These measures apply to the longline, purse seine and other surface fisheries. For longline fisheries, members agreed to not increase fishing effort aimed at yellowfin tuna. The WCPFC has implemented several management measures specific to the purse seine fisheries. For purse seine fisheries, there is a three month prohibition on setting on fish aggregating devices (FAD’s) for all purse seine vessels in EEZ’s and the high seas. In addition, member nations must institute additional measures to reduce FAD sets, such as an annual limit on the number of sets or additional closures. During 2017, vessels from countries, other than those that achieved verifiable reductions in bigeye catches, are prohibited from setting on FADs in the high seas. Coastal states must reduce purse seine effort to 2010 levels if they participate in the Parties to the Nauru Agreement, otherwise to 2001-04 average or 2010 levels and the number of purse seine vessels fishing for other countries cannot be increased. Member nations must have a FAD management plan in place to help reduce the capture of small bigeye and yellowfin tuans, and implementing FAD closures and discarding bigeye, skipjack or yellowfin tuna is prohibited {WCPFC 2012a}{WCPFC 2013b}{WCPFC 2016b}. In addition, member countries of the Parties to the Nauru Agreement have agreed to use a regional fishing vessel register, abide by a high seas pocket area closures, are prohibited from fishing on FAD’s, utilize a Vessel Day Scheme and retain all catch {PNA 2013}{PNA 2012} [PNA 2010}{WCPFC 2016b}{WCPFC 2016c}. 

In addition, biomass based limit reference points have been adopted by the WCPFC for yellowfin tuna and are used to determine the status of tuna populations. Target reference points are not yet in place and there are no harvest control rules {ISSF 2013}. However, the WCPFC has a working group that is currently working on identifying potential target reference points and a harvest control rule {WCPFC 2016d}.

Although there are no recovery plans in place, yellowfin tuna are included in the conservation and management measures (CMM) for tuna adopted by the Western and Central Pacific Fisheries Commission. The most recent CMM asks members not to increase fishing pressure on yellowfin tuna and discarding of yellowfin tunas in the purse seine fisheries is prohibited (WCPFC 2016b).

The Western and Central Pacific Fisheries Commission (WCPFC) has a compliance monitoring scheme in place that assess’s members compliance with obligations, identifies areas of conservation and management that may need refinement, responds to non-compliance and monitors and resolves non-compliance issues.The Commission evaluates compliance by members annually with respect to: catch and effort limits and reporting for target species, spatial and temporal closures, observer and Vessel Monitoring Systems (VMS) coverage and provision of scientific data {WCPFC 2012c}. Historically, the compliance assessment process has been closed to the public and the WCPFC has no ways of addressing non-compliance {Koehler 2013}. However, in 2013, the WCPFC released information on compliance with certain management measures in it’s report from the annual Commission Meeting {WCPFC 2013b}.No TAC was set for yellowfin tuna in Western Central Pacific Ocean, so compliance cannot be calculated.

Bycatch of seabirds, sea turtles, marine mammals and sharks in purse seine and pelagic longline tuna fisheries threatens some populations with extinction.

Yellowfin tuna are harvested with a diverse variety of gear types, from small-scale artisanal fisheries in Pacific Island and Southeast Asian waters to large, distant-water longliners and purse seiners that operate widely in equatorial and tropical waters. Purse seiners catch a wide size range of yellowfin tuna, whereas the longline fishery takes mostly adult fish (Langley et al. 2011).

A review by Birdlife International in 2007 found that 16 species of albatross and 60 species of petrel potentially overlapped WCPFC longline fisheries. This included species with IUCN classification of Critically Endangered (6), Endangered (7), Vulnerable (26) and Near Threatened (7) for both albatrosses and petrels. The remainder were classified by the IUCN as Least Concern. (Waugh 2006 in Black 2008). Analysis of albatross distribution within WCPFC waters and their overlap with longline fisheries (BirdLife International 2007, ACAP 2008) identifies that highest overlap with albatross distribution occurs in waters South of 30°S and North of around 20°N.

Historical bycatch rates for vessels fishing in Hawaiian waters have been used to model seabird bycatch in the Asian distant water fleets operating in the North Pacific (Crowder and Myers 2001). This study estimated that the Japanese fleet was likely to catch 14,540 birds per year (7,200 Laysan albatross, Phoebastria immutabilis, and 7,340 black-footed albatross, P. nigripes) and the Chinese Taipei fleet was estimated to catch 2,945 birds per year (1,630 Laysan albatross, and 1,315 black-footed albatross). These estimates cover both the WCPFC and IATTC areas. However, satellite tracking data indicates that over 90% of Laysan and 50% Black-footed Albatross distribution is within the WCPFC Convention Area (ACAP 2008).

Interactions with sea turtles also include incidental capture during longline operations, particularly when they actively take bait, or become entangled in the fishing gear. In the Western and Central Pacific Ocean (WCPO) five species are generally encountered in longline fisheries, namely: green (Chelonia mydas), loggerhead (Caretta caretta), leatherback (Dermochelys coriacea), hawksbill (Eretmochelys imbricata) and olive ridley (Lepidochelys olivacea) turtles. These species are generally long lived and reach sexual maturity at between 6-30 years old (SPC 2001 in Brouwer 2009). Large turtles have few natural predators and longline bycatch can result in high levels of fishing mortality on the large sub-adults and adults.

Kaplan (2005) in Brouwer (2009) calculated point estimates of longline bycatch based on turtle catch rates from the US Hawaii-based fleet and used effort data for the international Pacific longline fleet. His estimates suggest that in the Western and Central Pacific Ocean, coastal sources lead to a 13% annual mortality rate, compared with a point estimate of 12% from longlining. In the eastern Pacific Ocean, coastal sources account for a 28% annual mortality rate, compared with a point estimate of only 5% from longlining. Others have estimated longline associated mortality to be between 17 and 27% (Aguilar et al. 1995; McCracker 2000 in Brouwer 2009).

The Western and Central Pacific Fisheries Commission (WCPFC)has adopted several management measures to protect vulnerable bycatch species.For example, WCPFCmembers are asked to implement the International Plan of Action for Reducing Incidental Catches of Seabirds in Longline Fisheries.Vessels fishing north of 23N in the western and central Pacific Ocean (WCPO) are required to use at least two mitigation measures including at least one of the following: side setting, night setting, tori line or weighted branch line.Members must submit annual reports detailing the mitigation measures used and are encouraged to undertake additional mitigation research {WCPFC 2012a}.Members of the WCPFC are also to implement the FAO Guidelines to Reduce Sea Turtle Mortality in Fishing Operations. Proper handling and release guidelines should be used when hard-shell turtles are incidentally captured and longline vessels must carry line cutters and de-hookers to allow for the safe handling and release of turtles.Longline fisheries are also urged to research mitigation techniques such as the use of circle hooks {WCPFC 2008}.In addition, fisheries observers (5% required level) record and report interactions with seabirds and turtles {WCPFC 2012b}{WCPFC 2008}.

Purse seine vessels in the western and central Pacific Ocean are prohibited from setting on a school of tuna with a whale shark, although members that fish north of 30N can implement this measure or a comparable measure.If a whale shark is incidentally encircled, the vessel must take reasonable steps to ensure its safe release and report the incident.However, these measures will not become mandatory until January 1, 2014 {WCPFC 2012d}.

The western and central Pacific Ocean (WCPO) longlines fisheries that capture bigeye tuna also catch a number of other species of fish, including billfish and other tuna species, and sharks (Molony 2005). The status of these species and the impact of this fishery on them varies greatly and is often times unknown. Examples of commonly other commonly captured species include blue, black and striped marlin, swordfish, dolphinfish, opah, and oceanic, silky, blue and shortfin mako sharks {OFP 2012}{{Molony 2007}.

Purse seine sets on anchored and drifting Fish Aggregating Devices (FADs) and natural floating objects (logs, flotsam) is widespread, with about half of tropical tuna catches coming from FAD sets (Fonteneau et al., 2000). FAD sets have high catch rates of small and juvenile bigeye and yellowfin tunas and unmarketable species and sizes of other fish species relative to unassociated sets (i.e. Fonteneau et al., 2000). Networks of thousands of artificial drifting and anchored FADs aggregate tunas from surrounding waters and possibly act as ‘ecological traps’ of pelagic species by altering their natural spatial and temporal distributions, habitat associations, migration patterns and residence times (i.e. Marsac et al., 2000; Bromhead et al., 2003;).

Smaller swordfish are often discarded in pelagic longline tuna fisheries due to minimum size requirements or low market value (Cramer, 2003, Ward et al. 2008).

There are extremely low bycatch levels in pole-and-line fisheries, where bycatch that does occur generally consists of juvenile kawakawa tuna (Euthynnus affinis), frigate mackerel (Auxis rochei), mahimahi (Coryphaena hippurus), and rainbow runner (Elagatis bupinnulata). Discards are believed to have high post release survival rates due to the use of barbless hooks and flick-off practices (FAO, 1997). However, concern over bycatch of reef fish and juvenile classes of target species in baitfish fisheries that supply live bait to pole-and-line fisheries has been raised, as have other ecological issues (ecosystem effects of removal of baitfish species, overexploitation of target baitfish species, habitat degradation) and socioeconomic issues (food security impacts with coastal communities) (FAO, 2008; Gillett, 2010). .

Members of the WCPFC are prohibited from retaining, transshipping, storing or landing oceanic whitetip sharks and any incidentally caught sharks should be released, the incident recorded and reported {WCPFC 2012e}. Members are also to implement the FAO International Plan of Action for the Conservation and Management of Sharks and National Plans of Action should have policies in place to reduce waste and discarding of sharks. Information on catch and effort for key species is to be reported and shark finning is banned (5% ratio) {WCPFC 2010b}. In the South Pacific, the WCPFC limited the number of vessels targeting swordfish and catches to levels from any year between 2000 and 2005 and required this information to be reported to the Commission {WCPFC 2009}.

Primary gear used to catch yellowfin tuna, including purse seine, pelagic longline and pole-and-line gear, do not come in direct contact with the seafloor. Lost and discarded gear can damage coastal habitats.

WCPFC has a time/area closure on purse seine sets on FADs and other floating objects by purse seine vessels for three months annually in the area bounded by 20ºN and 20ºS (WCPFC, 2008a, 2009a).In 2008, the Parties to the Nauru Agreement, a regional agreement establishing terms and conditions for foreign access to the Exclusive Economic Zones of eight Pacific Island Countries, closed to purse seine fishing two areas of high seas international waters that are enclosed by the Parties’ domestic waters (PNA, 2008).