Tuna numbers sustained using research and training
The Oceanic Fisheries Programme of the Pacific Community (SPC) monitors and assesses the populations (called stocks) of Pacific tuna and other fisheries. It provides scientific advice to the WCPFC. It also provides data, scientific analysis and advice to PNA members. This information helps them assess tuna stocks and develop measures to manage fisheries.
Graham Pilling explains SPC’s role in conducting stock assessments on the four major tuna species in the Western and Central Pacific, and how that feeds into decisions made by the WCPFC (1.39 mins).
Hear SPC’s Andrew Hunt, who collects data for the Oceanic Fisheries Management Project (OFMP2), explain how that data feeds into stock assessment (32 secs).
Current status of tuna stocks
FFA describes the status of tuna stocks annually in a report card, and publishes the report cards and related documents on its website.
The Tuna Fishery Report Card 2020 contains the most recent assessment of the status of major tuna species. The abundance of a species is estimated against a benchmark called a target reference point (TRP), which is a desirable level of stock to maintain the healthy functioning of the environment and the sustainability of fishing. An interim TRP exists for albacore tuna. TRPs for the other three species are being developed.
All four species are assessed as being in biologically healthy numbers. The current report card is summarised here:
- Bigeye tuna is not overfished, and overfishing is not occurring, although there is a 1 in 8 chance of bigeye being overfished. The biomass of this tuna continues to decline, pointing to the need to strengthen management measures.
- Skipjack tuna is not overfished, and overfishing is not occurring. The stock is above the TRP. Improved understanding of skipjack biology has been incorporated into modelling of stock health.
- South Pacific albacore tuna is not overfished, and overfishing is not occurring. However, as it is estimated to be below the interim TRP, the catch must be reduced. This will help improve catch rates so they are economically viable.
- Yellowfin tuna is not overfished, and overfishing is not occurring.
For more details see:
- SPC’s overviews of the tuna fisheries assessments ppublished in 2020 and 2019
- individual assessment reports for bigeye (2020), South Pacific albacore (2018), skipjack (2019), and yellowfin (2020) tuna,
among all assessments of fish stocks for the WCPO
- a summary of the state of health of global tuna stocks, including in the WCPO
- Baseline study and performance indicators for OFMP2, a 2017 report that includes data on tuna stocks.
The SPC’s Dr John Hampton explains the change in the way bigeye is assessed, and what it means (3.29 mins).
Models are used to help assess tuna stocks
Estimating tuna populations in the Western and Central Pacific Ocean is a challenge. Tuna are always on the move, driven by ocean currents, changes in the availability of their prey, and other processes that vary naturally. There are many gaps in knowledge about their biology and lifecycle, which further complicates assessments.
Tuna are caught by fishing fleets that record and report their catches with varying degrees of accuracy. Sampling is limited to a tiny fraction of this commercial catch, and is often not representative of the tuna population at large.
Not only must the numbers and weight of each species be estimated, but these values have to be compared with the population size that would produce the maximum yield in the future. SPC’s tool over the past decade is a computer model called MULTIFAN-CL. It integrates information from three main sources.
The first is length or frequency data, which is gained by counting and measuring a sample of the fish caught by commercial vessels. From this, it is possible to estimate the relative numbers of fish of different ages in different areas of the Pacific, and to understand recruitment (additions to the population) and mortality (losses from the population) among tuna.
The second is the relationship between the fishing effort (the number of days spent fishing) and the catch, captured using log sheets that commercial vessels are required to complete. As the number of fish in the sea is reduced, they become more sparsely distributed and an average fishing trip will have smaller catches. Understanding this relationship helps researchers track changes in the size of the fish population.
The third source of information comes from tagging programs, in which tuna are caught, marked with small numbered tags, and released. When tagged fish are re-caught and reported, estimations can be made of fish populations.
The results of this modelling are provided to the WCPFC.
SPC also assesses tuna stocks using another model, SEAPODYM (Spatial Ecosystem and Population Dynamics Model). It uses data on climate, other environmental factors, and fish populations, among other factors. This model is important for understanding the potential impacts of climate change.
The SPC’s Oceanic Fisheries Programme Manager, Dr John Hampton, explains the use and benefits of the two stock-assessment models, and their basic differences (2.46 mins).
Tuna tagging contributes to assessments
Tuna tagging is also used to collect information that is used in stock assessments. SPC conducts a region-wide tuna-tagging project. The data collected are fed into SEAPODYM modelling.
Tagging helps scientists get information about the growth, movements, natural mortality and fishing mortality of the tuna, which helps estimate the status of the stocks and the impacts of fishing.
SPC has a dedicated tuna-tagging portal that contains information about all the tagging programs, including details of what to do if you catch or find a tagged tuna.
Research improvements flagged
The following have been identified as critical to research to improve our knowledge of tuna stocks in the WCPO:
- Improve estimates of tuna growth, including regional differences.
- Continue tagging and analysis to support future assessments.
- Assess otoliths (inner-ear structure in fish) to work out fish age and growth.
- Continue to analyse pole-and-line catch rates, longline efficiency, longline models, and purse-seine indices.
- Improve estimates of natural tuna mortality and use parts of longline-caught fish to do so.
- Consider biological markers to improve estimates of east–west movement.
- Refine modelling approaches for maturity and sex of tuna.
Tagging tuna at sea
Training on how to assess tuna stocks
The Oceanic Fisheries Management Project works with SPC to deliver annual stock-assessment training workshops. The workshops, which have been run since 2006, help fisheries professionals know how to interpret the very complex stock assessment data. This contributes to ensuring that regional decisions about tuna stocks are made in an informed way, and are based on the best available science.
Officers learn how to:
- understand and interpret the results from the regional oceanic fisheries’ stock assessments
- communicate this information to fishery managers within their countries
- increase their confidence to participate in scientific discussions of the WCPFC – in particular, during meetings of its Scientific Committee.
The courses have attracted people from 28 countries. By the end of 2020, 141 people had attended an average of 2 courses each: 41% (58) being women, and 59% (117) men. In May 2021, the first online workshop will be run.
Participants in the 2019 Advanced Stock Assessment Workshop run by SPC collaborate on an exercise on using a Majuro plot, a graphic that illustrates tuna stock status. Photo: SPC.
Workshop facilitator Steven Hare explains the content and format of the workshops (3.41 mins).