High tech, complex science and deep knowledge: assessing WCPO tuna populations

Detailed scientific assessments are conducted regularly to calculate how well tuna populations are coping with fishing and changes in the oceanic environment.

They are rigorous and exhaustive: it takes a full year to assess one species. The results form the basis for the management of the tuna fisheries in the Western and Central Pacific Ocean (WCPO). Scientists of the Division of Fisheries, Aquaculture and Marine Ecosystems (FAME) at the Pacific Community (SPC) conduct assessments on the four commercially important tuna – albacore, bigeye, skipjack and yellowfin – and on other important species, such as swordfish, striped marlin, and several sharks.

Each of the tuna is assessed every three years, and the other species every four or five years.

The WCPFC uses the highly technical assessments to make formal statements about the status of each species, and to make management decisions aimed at keeping tuna populations – and tuna fishing – sustainable for many generations. Decisions encompass fishing rules, including limits, to protect the viability of the tuna species, and rules to protect the environment and other species.

The assessment reports and data files are available to everyone. SPC also publishes an overview that gives an excellent summary of the status of the tuna populations. It includes some other matters researched. The most recent overview, for 2019, included the effects of fishing on marlin stocks, some endangered species of sharks, the effects of El Niño Southern Oscillation (ENSO) and the potential impact of the climate on fisheries.

Determining how healthy the populations are, and whether they are being overfished, involves a team of people with a range of skills. A scientific committee decides which species will be assessed, what modelling approaches to use, and what particular inputs should be used to account for uncertainty. Others involved are:

  • fisheries biologists, who provide information on age, growth, and reproduction of species
  • ecosystem scientists, who collect and analyse primary tagging data (relying on input from commercial and subsistence fishers when they capture tagged fish)
  • oceanographers and climate scientists, who provide data on what’s happening in the ocean environment and climate, what may be changing and why
  • statisticians, who design how the sampling is done, and who process the collected data to make sure it is representative of the population being modelled
  • stock assessment scientists, who combine all the data collected into a sophisticated statistical model that estimates the current abundance of the stock, and compares it with historical data and trends to calculate its sustainability.

SPC’s research shows that all four tuna in the WCPO are in a healthy state, and that none of them are being overfished. The status of tuna in the region is much better than for any other ocean basin.

Bar graph showing catch of main tuna species and their health in four ocean basins, 2020. Figure: SPC.
Tuna populations in the WCPO in 2020 are much more abundant that those in the eastern Pacific (EPO), Atlantic (AO) and Indian (IO) oceans. The WCPO is the only ocean basin where all four species are at sustainable levels (coloured green). The status of tuna stocks is calculated from the proportion of tuna that have reached spawning age (maturity) and how much fish is caught. Figure: Pacific Community.

Many types of data are used

What goes into the assessments is also complex. It requires information from log sheets supplied by fishers, data provided by observers. These kinds of data may include, for example, how much fish is caught using different gear (e.g. longline hooks both shallow set and deepset, purse-seine nets set on fishing-aggregating devices (FADs) purse-seine fishing on free-swimming schools).

The assessments also use data from tuna tagging research cruises and from tagged tuna that are reported when caught. Scientists also include estimates of age and other biological information that collaborating research teams contribute from around the world, and sampling data (of length and weight) taken during transhipping or offloading in ports. Genetic sampling gives information about population structure, and how populations are connected.

The stock assessment team is exploring ways to incorporate information from the WCPFC’s Vessel Monitoring System and from the tracking of FADs.

Two models, SEAPODYM and MULTIFAN-CL, are used to manipulate the data in different ways, and build understanding of the stock’s population dynamics.

Majuro plot shows that populations of albacore, bigeye, skipjack and yellowfin tuna in western and central Pacific Ocean are healthy and not being overfished. Image: SPC 2020.
Summary of the status of the four tuna stocks in the WCPO. All four are in the green segment, which means that they are in a healthy state and not being overfished. Image: SPC.

FAME helps fisheries professionals understand the science

FAME also runs a two-week stock assessment workshop to help fisheries professionals around the Pacific to learn how to interpret the stock assessment data. The workshop is run every year, and is funded by GEF.
Students around a table at the 2016 Stock Assessment Workshop interpret implications for their country from the regional tuna stock assessments. Photo: SPC.
Students attending the 2016 stock assessment workshop were tasked with interpreting implications for their country from the regional tuna stock assessments. Photo: SPC.