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By Catherine Norwood 23 May 2022 5 min read

Scanning electron microscope image of a whole tuna otolith. Photo: Naomi Clear, CSIRO

A new study using fish ear bones, known as otoliths, to age tuna in the western and central Pacific Ocean (WCPO) has produced surprising findings about fish size and age, helping to improve fisheries management in the region.

CSIRO research has found that bigeye tuna in the western Pacific live more than twice as long as previously thought – not six years but 14 years. And they don’t grow as big as their counterparts in the eastern Pacific. The average size of the largest bigeye tuna in the west is about 1.6 metres, rather than the 1.8 metre average for the east, even at age 14.

Juvenile bigeye tuna. Photo: David Itano.

CSIRO’s fisheries scientist Jessica Farley is a global leader in fish population biology and has been developing fish ageing techniques for more than 20 years. Her team’s recent work to verify the age of bigeye tuna builds on previous work with other tuna and billfish species.

“Otoliths have bands, the same way that trees have bands that mark their growth each year,” Farley says. “But you need to be able to prove that the bands you’re counting are deposited on an annual basis. We know this is true for southern bluefin tuna. But people didn’t believe it applied to tropical tunas such as bigeye and yellowfin tuna.”

She says seasonal changes are thought to trigger the formation of annual bands in otoliths. Because these changes are more subtle in tropical environments, people thought they would not be enough for tropical fish to deposit annual bands that could be reliably interpreted.

Bigeye tuna were known to form daily otolith rings for the first year or two of life. But it was uncertain how these rings related to age beyond that. To estimate the age of older fish, Farley and her team first had to verify whether the larger-scale bands that appear in bigeye otoliths were actually formed annually.

Scanning electron microscope image of sectioned bigeye otolith showing the daily band zones. Photo: Karsten Goemann, Central Science Laboratory, University of Tasmania

Gathering evidence

A seven-year tuna tagging program dating back to the late 1990s provided the basis for doing this. The Pacific Community (SPC) is the international scientific service provider that assesses fish stocks in the WCPO. It partnered with the CSIRO to catch and tag bigeye and some yellowfin tuna in the Coral Sea. They also injected fish with a non-toxic salt that is absorbed by the otoliths before releasing them back into the ocean.

“When they were recaptured, we knew how long they’ve been at liberty,” Farley says. “Looking at the fish’s otolith under a scanning electron microscope, we could see the tagging mark in the otoliths and count the bands afterwards. If the bands formed annually, the number of bands after the mark would equal the years at liberty.” And they did.

These otolith markings from the tagging and recapture program have verified the accuracy of the ageing technique Farley and her team have developed for bigeye.

This light microscope image of a bigeye tuna otolith shows the annual growth bands of an individual fish aged 12. The yellow crosses indicate each annual growth band. Photo: Kyne Krusic-Golub, Fish Ageing Services.

CSIRO, in collaboration with Fish Ageing Services, has now analysed otoliths from more than 1200 bigeye tuna collected over almost 10 years from fish across the western and central Pacific Ocean.

“Our ageing technique uses a combination of daily age estimates for young fish and annual age estimates for older fish to estimate a growth curve. And instead of just using counts of annual bands to provide estimates such as five years old, we have developed methods using otolith size measurements to refine the annual counts to provide decimal age estimates,” explains Farley. “So it could be 5.3 or 5.4 years. This produces a much finer and more precise growth curve for the fish than we had before.”

This chart shows the revised growth curve for bigeye tuna (green line) after new research conducted by the CSIRO team. The black circles show the length of bigeye tuna at different age estimates. The red line is the growth curve previously used in the bigeye tuna 2014 stock assessment.

Improved tuna modelling

Previous growth curves for bigeye tuna in the western Pacific were based on data from the eastern Pacific tuna fishery, where large bigeye tuna are common.

While few, if any, western fish reach the large sizes seen in the eastern Pacific, Farley’s research shows that there are, in fact, many older fish present. They just don’t grow as big. It also means there are more fish actively reproducing in the fishery than previously thought.

SPC has incorporated CSIRO’s findings into the modelling it uses to assess fish stocks for the WCPO tuna fishery, which is the world’s largest tuna fishery. It accounts for 55% of the global tuna catch and is worth US$5 billion per year to fishers. The Western and Central Pacific Fisheries Commission (WCPFC) manages the fishery.

Principal fisheries scientist with SPC, Dr Simon Nicol, says the CSIRO findings have had significant implications for the region. Since the early 2000s there had been concerns that the bigeye fishery was close to being overexploited, based on the best available information.

“By 2016 we were facing a significant increase in fisheries restrictions to manage stocks,” Nicol says. This would have had “huge economic consequences” for many Pacific nations who rely on fishing access fees for income. For some nations, this equates to more than half of their primary industry-related income.

But with the new fish age and growth information, SPC has been able to assess all tuna stocks in the WPCO as sustainable. And while there have been no moves to relax existing restrictions, Nicol notes there have been no additional restrictions either.

SPC is now working with Farley and her team on a similar collaboration to validate estimates of age and growth rates of yellowfin tuna in the WCPO.

Future techniques

Farley is also working on another international project, using bomb-produced radiocarbon to further verify the age estimates for bigeye and yellowfin tuna.

When atomic bombs were released in the Pacific in the 1950s and 1960s, it led to a rapid increase and subsequent decrease in carbon-14 in the world’s atmosphere and oceans. Carbon-14 can be measured in a range of carbonate structures in corals and fish otoliths.

In this new project, Farley’s team estimates the age of a fish using the current technique and calculates the birth year of the fish.  It then compares the carbon-14 level recorded in the fish’s otolith during its birth year with the known levels of carbon-14 in the environment for that year. If the carbon-14 values match, it validates the aging technique. Farley says results so far are promising.

“But bomb radiocarbon dating is very expensive. Our aim is to use this as a benchmark to take the next step, which is to establish epigenetic ageing for tunas and other species.”

Farley says this kind of genetic ageing uses fish muscle tissue rather than reading otoliths and would be faster and more affordable. This would help to expand the reach of fisheries science and, potentially, also help researchers and managers better understand how environmental factors combine with genetics to influence fish growth.

This kind of information, she says, is increasingly important for the sustainable management of fisheries in a rapidly changing world.

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