As every year at this time before the WCPFC meeting, the good people of SPC releases this jewel of a report that present the overview of the tuna fishery of the last years and the status of the stock.
The Pacific Community (SPC) is the principal scientific and technical organisation in the Pacific region, owned and governed by the 26 country and territory members in the pacific. In pursuit of sustainable development to benefit Pacific people. The Fisheries, Aquaculture and Marine Ecosystems (FAME) Division includes the Oceanic Fisheries Programme (OFP) and Coastal Fisheries Programme (CFP). The goal of the OFP is to ensure fisheries that exploit the region’s resources of tuna, billfish and related species are managed for economic and ecological sustainability using the best available scientific information. In pursuing this goal, the OFP provides scientific support for the management of fisheries for tuna and associated species, with a strong focus on stock assessment and modelling, fisheries and ecosystem monitoring and analysis and data management. The OFP works closely with member countries and territories, the WCPFC, FFA and PNA, so it does this work as the science provider for the WCPFC.
So if you want to know what is the situation of the tuna fishery in the region, this report is the ultimate reference, is good, concise, well illustrated, and has many reference to further research. (I wish articles like this one in the otherwise quite reputable “the conversation” website were use it as reference)
Majuro plot stock status summary for the four WCPO target tuna stocks, the grid median value is shown as a large dot, the ellipses closely approximate the distribution of values from grid models
So here is snapshot of the situation
Bigeye and yellowfin tuna stocks were assessed in 2020, the skipjack tuna stock was assessed in 2019, and the South Pacific albacore stock was assessed in 2018. Due to uncertainty in the fisheries data for the most recent year due to COVID, data from the year immediately preceding the assessment year is not included in the bigeye, yellowfin and albacore assessments. Thus, the bigeye and yellowfin tuna assessments include data through 2018, while South Pacific albacore currently includes data through 2016. Skipjack, with its shorter lifespan and importance of young fish to the fishery, includes the most recent year of data; thus the 2019 assessment included fisheries data through 2018.
Summary across target tuna stocks
To summarise the most recent stock assessments for the four target tuna stocks, stock status for all four species are plotted together on a single Majuro plot, along with grid model uncertainties. All four are considered to be in a healthy, sustainable status as none are considered to be overfished. Yellowfin, skipjack and albacore are estimated to have a 0% probability of currently experiencing overfishing, while bigeye is estimated to have a 12.5% probability. To place these results in context, a summary of stock status for these same four stocks assessed in other ocean basins by the three other tuna Regional Fisheries Management Organizations (RFMOs) are illustrated in in the figure below. As most of the other tuna RFMOs report stock status relative to MSY-based reference points (i.e., SB/SBMSY and F/FMSY ), we based the WCPFC status on the same criteria.
comparison of stock status for the same four tuna species in the other major ocean basins
And while the status of the tuna stock os good (the best of all tuna fisheries in the world), the situation for sharks is bad, and particularly for the oceanic whitetip
Catch and status of billfish and sharks
The status of silky and oceanic whitetip sharks is of concern as assessments have shown that stocks are subject to overfishing and, in the case of oceanic whitetip, severely overfished. A WCPFC ban on the use of either shark lines or wire traces in longline sets is in place, which is hoped will reduce the catch of silky and oceanic whitetip sharks. Over the past several years stock assessments have been undertaken for several billfish and shark species, in addition to the main tuna species. The SC recommendations to the WCPFC are broadly outlined below.
Stabilise stock size or catch/ensure no increase in fishing pressure
Southwest Pacific swordfish
Pacific blue marlin
Reduce catch and/or rebuild the stock and/or reduce effort and/or enhance data collection efforts
Pacific bluefin tuna
Southwest Pacific striped marlin
Western and central north Pacific striped marlin
Blue shark
Silky shark
Oceanic whitetip shark
Two shark (oceanic whitetip and sillky) and two billfish (Southwest Pacific striped marlin and Southwest Pacific swordfish) species have been assessed by SPC staff in recent years (Figure 18). Stock status for these species is based on the Kobe plot, where overfished status is judged relative to spawning stock size at MSY3. There is considerable uncertainty in the estimates of F/FMSY and SB/SBMSY for all four species. Based on the assessment model grid medians, Southwest Pacific striped marlin and oceanic whitetip are likely in an overfished state, while overfishing is likely occurring for silky shark as well as oceanic whitetip.
Kobe plot stock status summary for four species of billfishes and sharks assessed at SPC over the past decade and for which stock status has been determined. Note that this plot differs from that presented for the target tuna (the \Majuro" plot), because the WCPFC has not yet decided on LRPs for these species and therefore MSY-based reference points are used as a default.
The other worry is of course climate change…
The historical simulations reflect key features of the ecology and behaviour of the four tunaspecies and match the total historical catch in terms of both weight and size frequency distributions. The projections show an eastern shift in the biomass of skipjack and yellowfin tuna over time, with a large and increasing uncertainty for the second half of the century, especially for skipjack tuna. The impact is weaker for bigeye tuna and albacore, which predicts a wider and warmer range of favorable spawning habitat. For albacore, a strong sensitivity to sub-surface oxygen conditions resulted in a very wide range of projected stock sizes. Historical fishing pressure was estimated to have reduced the adult stocks of all four tuna species by 30-55% by the end of 2010. The effects of fishing on biomass strongly outweighed the decreases attributed to climate change in the short- to medium-term. Thus, fishing pressure is expected to be the dominant driver of tuna population status until the mid-century. The projected changes in abundance and redistribution of these tuna associated with climate change could have significant implications for the economic development of Pacific Island countries and territories, and the management of tuna resources, at basin scale. In particular, larger proportions of the catch of each species is increasingly expected to be made in international waters.