Skipjack tuna

The skipjack tuna population in the western and central Pacific Ocean is assessed using the MULTIFAN-CL software, which has been used since 2000. The last assessment was conduced in 2016. The model is age (16 quarterly age classes) and spatially structured for quarterly time periods from 1972-2014 and includes catch, effort, size composition and tagging data. This new assessment included a modified tagging input file, changes to the relative weighting of the CPUE, legnth composition and tagging data and modifications to some model parameters {McKechnie et al. 2016}.

The Scientific Committee advised the Commission in 2016 that managers should be aware of the total effort in the WCPO region and should take action to keep the spawning biomass near the target reference point (McKechnie et al. 2016).

Last updated on 16 Jan 2018

*current represents 2011-2014 and latest represents 2014
(Rice et al. 2014)

The current biomass is above levels needed to produce the maximum sustainable yield (MSY) and fishing mortality rates are below MSY levels. The population of skipjack tuna in the WCPO is neither overfished nor undergoing overfishing (McKechnie et al. 2016).

Last updated on 16 Jan 2018

Skipjack tuna populations are strongly linked to environmental factors such as El Niño. This is especially true in the eastern equatorial region (region 3), where the performance of the fishery is strongly influenced by environmental conditions.

The population of skipjack tuna is currently moderately exploited and fishing levels are sustainable. Fishing levels have been increasing through time but are still below levels needed to produce the maximum sustainable yield (MSY) {McKechnie et al. 2016}.

Tuna fisheries in the Central and Western Pacific are managed through the recently established Western and Central Fisheries Commission (WCPFC), which is headquartered in Micronesia. The members of the Commission have agreed that only vessels flying the flag of the members of the Commission may be authorized to fish in the Western and Central Pacific. The Commission is tasked with managing the largest industrial tuna fishery in the world.

The most recent management measures for this species were adopted in 2017 (WCPFC 2017). These measures apply to the longline, purse seine and other surface fisheries.  The WCPFC has implemented several management measures specific to the purse seine fisheries. For purse seine fisheries, there is a three month prohibition (July, August and September) on setting on fish aggregating devices (FAD’s) for all purse seine vessels in EEZ’s and the high seas in the area between 200 N and 200 S. In addition, member nations (except Kiribati and Philippines) must iprohibit FAD fishing (deployement and service as well) during an additional two sequential months, either April-May or November-December (WCPFC 2017). Coastal CCM's must also adhere to purse seine effort limits in their EEZs (WCPFC 2017). Other CCMs (non Small Islands Developing States and Indonesia) must limit the number of purse seine vessels larger than 24 m operaring between 200 N and 200 S to the level required under CCM 2013-01 (WCPFC 2017)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 and target reference points have been adopted by the WCPFC for skipjack tuna and are used to determine the status of tuna populations (WCPFC 2015). However, there are no harvest control rules (WCPFC 2017). The WCPFC does have a working group that is working on identifying potential target reference points {WCPFC 2013c}.

Skipjack tuna populations are healthy in the western and central Pacific Ocean, so no recovery plans are needed. However, they are covered under the current Conservation and Management Measures adopted by the Western and Central Pacific Ocean for bigeye, yellowfin and skipjack tuna (WCPFC 2017).

Although some compliance, monitoring and enforcement measures have been put into place by the Western and Central Pacific Fisheries Commission, it is often difficult to determine their success.

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 2012b}.

There are specific reporting requirements in place to monitor compliance with the FAD set limiting options {WCPFC 2013a}. Vessel Monitoring Systems are required on all vessels fishing for highly migratory species in the Western and Central Pacific Ocean south of 20N and east of 175E.The area north of 20N and west of 175W will have an activation date for VMS’s set at a later time {WCPFC 2012d}.There are measures in place allowing for the boarding and inspection of vessels in the Convention Area {WCPFC 2006b} and the WCPFC maintains a list of illegal, unreported and unregulated vessels {WCPFC 2010b}.

Purse seine fisheries do incidentally capture non-target species.Bycatch rates are much lower in unassociated compared to associated fisheries. Bycatch ratios in associated sets in the western and central Pacific Ocean (WCPO) region are 1.7% and for unassociated 0.3% {Dagorn et al. 2012}. In associated fisheries, marine mammals are most often caught during sets made in the western section of the tropical western and central Pacific Ocean, specifically near Papua New Guinea (north-east of EEZ) and the Solomon Islands (north-western EEZ).Sets made on floating objects (logs, dFADs, FADs, whales and whale sharks) caught the most marine mammals.In most instances it was not recorded whether marine mammals were alive or dead when returned, but when it was recorded, the majority were alive.Based on the catch per unit effort of incidental catches, less than 3,500 marine mammals are caught per year in the entire purse seine fleet and the mortality rate is estimated to be less than 10% {Molony 2005}.The purse seine fishery is thought to have little impact on the sustainability of marine mammals in this region. Observer records from the tropical region of the WCPO suggest most interactions between marine mammals and purse seines occurred during sets made on floating objects {Molony 2005}. Sea turtle interactions with the purse seine fishery in the western and central Pacific Ocean are not common, with an estimated encounter frequency (1995-2007) of 0.1% on FAD and 0.8% on log sets {Hall and Roman 2013}. The most commonly caught sea turtles, in descending order, are olive ridley, hawksbill and green {Hall and Roman 2013}.Sea turtle interactions in animal associated sets are the highest, 1.6%, resulting in around 105 captures per year. However the majority are released alive {Hall and Molony 2013}. It is estimated that total turtle captures in the purse seine fishery are 200 per year, with fewer than 20 moralities {Molony 2005}.

Observer records from the tropical region of the WCPO for unassociated purse seine fisheries suggest most interactions between marine mammals and purse seines occurred during sets made on floating objects.In addition, observer records from the same region suggests a very low incidence rate, 0.36%, of sea turtles and purse seine fisheries {Molony 2005}.

The Western and Central Pacific Fisheries Commission (WCPFC)has adopted several management measures to protect vulnerable bycatch species incidentally captured in purse seine fisheries. For example, 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 did not become mandatory until January 1, 2014 {WCPFC 2012e}.In addition, vessels are restricted from making a set on a school of tuna associated with a cetacean or encircle sea turtles and if this does occur they must take measures to ensure its safe release and to report the incident {WCPFC 2012f}{WCPFC 2008b}.

Purse seine fisheries, primarily associated fisheries, do capture additional non-target species, including other tunas, billfish, sharks and bony fish. Within the western and central Pacific Ocean, silky sharks are the most commonly caught shark species, followed by whales sharks – which are incidentally captured in unassociated fisheries. Oceanic whitetips are also commonly captured in purse seine fisheries. Both silkyand oceanic whitetip sharks are overfished and undergoing overfishing in this region {Rice and Harley 2012a,b}. Recently the Western and Central Pacific Ocean banned the retention of both silky and oceanic whitetip sharks {WCPFC 2012g}{WCPFC 2013f} and setting around whale sharks is now prohibited {WCPFC 2012e}. Starting on 1 January 2010, WCPFC coverage of purse seine vessels is intended to be 100% within the area bounded by 20oN and 20oS, and has adopted a target of 5% coverage by 30 June 2012 of all trips by longline and other vessels (WCPFC 2008). Blue and black marlin are reported as the most common billfish bycatch species, but they are caught in very low numbers. The most common bonny fish bycatch species in this region is the rainbow runner, followed by mahi-mahi and wahoo {Hall and Roman 2013}.

The major gears used to capture skipjack tuna in the western and central Pacific Ocean do not typically come in contact with bottom habitats.

Last updated on 02 Jun 2008

There are some areas in the western and central Pacific Ocean where some types of purse seine fishing are prohibited.