How we know how much tuna there is and whether fishing is sustainable

by Claire Heath | 11 May 2021 | News

Tagging tuna for scientific research at sea. Photo Pacific Community (SPC)

The peoples of the Western and Central Pacific Ocean (WCPO) can celebrate having the healthiest stocks of tuna in the world. 

It doesn’t happen by chance. Protecting populations of tuna so that they remain in healthy numbers and provide food for generations takes a lot of effort, skill, and teamwork. 

One of the most important pieces of work that supports the success of sustainable tuna management in the WCPO is stock assessment: the detailed research and analysis of how healthy a tuna species is, and whether it is being overfished or fished sustainably.

The work is conducted by a team of fisheries experts led by scientists of the Division of Fisheries, Aquaculture and Marine Ecosystems (FAME) at the Pacific Community (SPC). The team is made up of scientists from different fields, official observers, and fishers. The scientists generally come from the fields of fisheries biology, ecosystem science, oceanography, climate science, and statistical modelling. 

The highly technical reports they produce are used by the Western and Central Pacific Fisheries Commission (WCPFC) to decide how to manage the stock and the fishing so that populations remain healthy. It also allows them to make formal statements about the status of each species. SPC also publishes an overview that gives an excellent summary of the status of the tuna populations. 

Graph of numbers of four species of tuna in eastern Pacific, western Pacific, Atlantic and Indian oceans, and state of health of tuna populations using green, yellow and red code. Graph: Pacific Community
Science and monitoring well used … tuna populations in the WCPO (second column) are much healthier than in other oceans. Catch and stock status for albacore (ALB), bigeye (BET), skipjack (SKJ) and yellowfin (YFT) tuna in the eastern Pacific Ocean (EPO), western Pacific Ocean (WPO), Atlantic Ocean (AO) and Indian Ocean (IO). Figure: Pacific Community.

Assessing a species takes a year

A fisheries scientist with FAME, Dr Nicholas Ducharme-Barth, said that it took a full year to assess an individual species.

He led the south-west Pacific striped marlin stock assessment in 2019, the 2020 assessment of western and central Pacific bigeye tuna, and is now leading the 2021 assessment of southwest Pacific swordfish. As well as assessing stocks, Dr Ducharme-Barth is interested in looking at the socio-economic factors that drive the decisions behind where, how, and why people chose to fish. 

The four most important tuna species – albacore, bigeye, skipjack and yellowfin – are assessed every 3 years, and other species, such as swordfish, striped marlin and several kinds of sharks are assessed every 4 or 5 years.

“It begins with a post-mortem of the previous assessment. We consider any recommendations on improvements we can make over the previous report , and any advances in modelling techniques that can be applied,” Dr Ducharme-Barth said. 

“We assess whether our knowledge of the biology of the species has improved, and also our understanding of the structure of the population, for example if there is new genetic or tagging information available to suggest separate components of the population.”

The team works out a broad structure for the assessment, and then they begin to organise the data they will use.

“There are a lot of data inputs, coming from many different sources,” Dr Ducharme-Barth says. “They include the catch or other removals from each fishery, which would include deepset and shallow-set longline fishing, and purse-seining on fish-aggregating devices and free-swimming schools, as well as more artisanal types of fishing such as handlining,” he said.

Data inputs also included lengths and weight measurements of tuna (which allow FAME to correctly model the size at which fish are removed from the population), tagging data (which tells scientists about where tuna move), and data on catch rates (which reveal trends in the abundance of tuna).

Two tuna lie in a cradle on a fishing vessel at sea. They are being tagged for scientific research by two men. Photo Pacific Community.
Tagging tuna on a pole-and-line vessel during a Pacific Community research voyage in the WCPO. Photo SPC.

“We also analyse biological factors, such as individual growth rates, maturity (or spawning) rates, and natural mortality rates,” Dr Ducharme-Barth said.

“Finally, once all the data inputs are gathered, the work of the assessment scientists can really begin! 

“We fit and tune the assessment model to evaluate all of these different components in a statistical framework. The framework allows us to estimate the current abundance and historical trajectory of the stock by balancing what we know about the biology and ecology of the species against what has been observed.”

“Lastly, we calculate the level of uncertainty in key management reference points and provide this to managers.”

The uncertainty calculation provides a plausible range of outcomes for the stock. Managers can use this information when they set regulations. A narrower range of uncertainty can be achieved through better understanding tuna life-history and improved scientific data collection. 

FAME also runs stock assessment workshops every year to build the skills of fisheries professionals around the Pacific so they can interpret stock assessment results, which are very technical. 

It seems fitting that such rigorous accountability is needed for an animal so important to the health of the ocean environment and to the diet and culture of human societies.

Majuro plot showing that populations of albacore, bigeye, skipjack and yellowfin tuna in the western and central Pacific Ocean are healthy and not being overfished. Data from 2020 report by Pacific Community. Figure: Pacific Community.
Stocks of albacore, bigeye, skipjack and yellowfin tuna are healthy and not being overfished. Stock health is indicated by the area in green in this Majuro plot. Confidence about the good conditions of WCPO tuna stocks is due to the exacting data collection and analysis of the stock assessment team. Figure: Pacific Community.