WCPO bigeye and yellowfin tuna stocks remain healthy

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Republished from FFA Trade and Industry Newsvolume 13, issue 4, July–August 2020

WCPFC’s 16th Scientific Committee (SC16) was held virtually from 12–19 August 2020. During this meeting new WCPO bigeye and yellowfin stock assessments were presented by the Secretariat of the Pacific Community (SPC), indicating that both stocks remain healthy.

The previous full stock assessment for bigeye was conducted in 2017. It indicated a positive change in WCPO bigeye stock status to ‘healthy’ from ‘overfished’, with overfishing occurring. For the 2020 assessment, median values of relative recent spawning biomass (2015–2018) and fishing morality (2014–2017) indicate that the bigeye stock remains not overfished (with 100% probability) and likely continues to not be experiencing overfishing (with 87.5% probability).

However, levels of bigeye fishing mortality and depletion differ among the nine regions used in SPC’s stock-assessment models, with higher impacts in the four tropical regions, particularly on juvenile bigeye. Hence, overall bigeye stock status is buffered by lower catches in the temperate regions.

Similarly, the 2020 stock assessment indicates that the WCPO yellowfin stock is not overfished, nor subject to overfishing (both with 100% probability). Like bigeye, yellowfin exploitation is higher in tropical regions where fishing effort is concentrated by the equatorial purse-seine fishery and ‘other’ fisheries (e.g. pole-and-line and handline vessels operating in Indonesia); there is low yellowfin exploitation in temperate regions.

Hence, for both bigeye and yellowfin, SC16 recommended that WCPFC17 continue to consider management measures that reduce fishing mortality from fisheries that take juveniles (i.e. purse-seine fishery) to increase bigeye and yellowfin fishery yields and reduce any further impacts on spawning biomass in the tropical regions.

SC16 also recommended that a precautionary approach be maintained, with bigeye and yellowfin fishing mortality kept at a level that maintains spawning biomass at 2012–2015 levels until the Commission can agree on appropriate target reference points (i.e. the optimal level of spawning biomass or fishing mortality that ensures long-term sustainability of the stock).

The WCPO continues to be the only ocean where stocks of the four key tuna species – skipjack, yellowfin, bigeye and albacore – are deemed to be in a healthy state.

2020 tuna research at sea conducted, but cut back to follow COVID-19 rules

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Two important tuna research trips have gone ahead in the Western and Central Pacific Ocean (WCPO) this year, although the research programs and the routes had to be curtailed dramatically.

Both programs, which are run by the Pacific Community (SPC), were cut back to comply with regional practices put in place to prevent the transmission of COVID-19.

For the Pacific Tuna Tagging Programme, scientists continued routine tagging of tuna and tested new sampling methods that will help scientists analyse the structure and behaviour of tuna populations. 

The research was conducted in the high seas and the exclusive economic zone (EEZ) of the Kiribati groups of Line Islands and Phoenix Islands. 

This year’s cruise was originally to have focused on understanding more about the habit of tuna to gather around drifting fish-aggregating devices (FADs). It was to have been conducted in waters around Tuvalu, which have one of the highest densities of drifting FADs in the WCPO.

Three men on deck of research vessel Gutsy Lady 4 being prepared at Kewalo Basin, Honolulu, for the tuna tagging cruise in the central Pacific. Photo SPC.
Research vessel Gutsy Lady 4 being prepared at Kewalo Basin, Honolulu, for the tuna tagging cruise in the central Pacific. Photo: SPC.

SPC research scientists Bruno Leroy and Valérie Allain reported on this year’s tuna tagging cruise in issue 162 of SPC’s Fisheries Newsletter.

The tagging cruise left from Honolulu, Hawaii, in mid-August, and returned to the same port in early October, staying clear of any other ports. Mr Leroy and Dr Allain said that protocols to protect Pacific Islands people from COVID-19 meant that, before the research vessels could depart, the crew and scientists had to stay isolated for 14 days and return negative test results to the disease.

Mr Leroy said that, since the Pacific Tuna Tagging Programme began in 2006, about 455,000 tuna had been tagged. If SPC tagging campaigns carried out since the end of the 1970s were included, the number was more than 800,000. On the 2019 cruise, researchers tagged nearly 17,000 tuna.

Second study to develop better ways to sample small marine animals 

Bruno Leroy and Valérie Allain also reported on a second SPC research cruise, which was also curtailed to comply with COVID-19 rules. 

The SPC’s scientific team conducted a four-day trial to develop a new sampling method to collect micronekton from the surface of the ocean to a depth of 600 m. (Micronekton are marine animals such as fish, crustaceans, jellies and squids that measure 2–20 cm in length. They are the main source of food for seabirds, tuna and marine mammals.)

As the research vessel Alis is based in New Caledonia, the trial was conducted inside the country’s EEZ.

The original cruise planned for this year in the waters of New Caledonia, Solomon Islands and Federated States of Micronesia has been postponed until 2021. It is part of a long-term program to study the ecosystem of the open oceans in the Western and Central Pacific. 

Crew on the research vessel Alis haul in a net full of micronekton during the 2020 research cruise in New Caledonian waters. Photo: Valérie Allain, SPC.
Crew on the research vessel Alis haul in a net full of micronekton during the 2020 research cruise in New Caledonian waters. Photo: Valérie Allain, SPC.