Out on the western Pacific in October, SPC fisheries scientist Dr Valerie Allain found herself back in a familiar rhythm: nights built around nets, sensors, and the quiet urgency of collecting the ocean’s smallest clues about tuna.
Dr Allain, a veteran of numerous Pacific research voyages, was aboard the Japanese scientific vessel Kaiyo-Maru for the second time, part of an expedition split into two legs and designed to answer a deceptively simple question with big consequences for the Pacific and for tuna-loving nations like Japan: where are tuna larvae and juveniles, how many are there, and what might those patterns be telling us?
The voyage was a collaboration between The Pacific Community (PC) and Japan’s Fisheries Research and Education Agency (FRA), and came with an unusual advantage. Japan carried out comparable tuna sampling about 20 years ago and is now restarting the work, creating the kind of long-time gap that makes real environmental change easier to detect.
In the Pacific, where El Niño and La Niña can reshape ocean conditions from one year to the next, that longer perspective matters. Without it, it’s hard to separate natural variability from longer-term trends.
“Particularly if you want to look at the impacts of climate change, it starts to be of real interest when you have a long time difference,” Dr Allain explains.
A ship built for night work
Dr Allain describes the Kaiyo-Maru as a “massive” vessel compared to other research vessels she has worked on.
“I get seasick,” she admits. The difference between a small rolling boat and a large, steady platform can be the difference between functioning and failing when the sampling gets intense. That tends to happen in the evening or the small hours of the morning. Workday begins as the sun sets.
The team would typically start around 5–6pm, then cycle through a sequence of instruments and nets, working sometimes until 3 or 4am, depending on how much the sea delivered and how long it took to process on deck and in the lab.
“We are working at night because micronekton and zooplankton are coming up close to the surface during the night,” said Dr Allain.
Dr Valerie Allain (right) with a Solomon Islands researcher and Japanese research crew sorting specimens aboard the Kaiyo-Maru (Source The Pacific Community)
Micronekton are small (2 – 20cm) marine animals, typically fish, squid, and shrimp that inhabit the twilight zone down to around 1000 metres. They are a key link between zooplankton, like krill and fish larvae that drift in water currents, and fish species like tuna.
Sampling was organised around “short stations” and “long stations”. A short station might last a single night, focused on collecting zooplankton and the tiniest tuna larvae, just a few millimetres long, alongside measurements that describe the surrounding water mass – temperature, currents, and other parameters, plus acoustic work.
Long stations stretched across three nights at the same location and added a more time-consuming operation – micronekton trawls, deployed and towed at different depths between the surface and about 200 metres, then emptied, sorted, and repeated.
The expedition also collected environmental DNA, sampling genetic traces in seawater, work Allain notes, is still in early stages because of the intensive laboratory processing needed to turn those samples into identifications.
The geography of a moving lab
The two-month cruise itself was expansive, covering thousands of kilometres across national waters and the high seas. The first leg departed Japan and took about ten days to reach equatorial waters, with sampling that included Kiribati waters and later stops in the Solomon Islands, where Dr Allain boarded for the second leg.
Then the vessel moved north to sample in the waters of the Federated States of Micronesia (FSM) and adjacent international waters, before the team disembarked in Saipan in the Northern Mariana Islands.
That “ship as moving laboratory” model is central to what the cruise was trying to achieve: building a clearer map of where tuna begin life, and how those nursery grounds shift in space and time.
What they’re chasing: tuna in early life
The Japanese side of the collaboration is primarily focused on tuna themselves, especially larvae and juveniles, and what those early-life numbers might say about “recruitment” – the pipeline of young fish that may later enter the fishery. For Allain and The Pacific Community, the same voyage is also an opportunity to study tuna habitat and food.
That’s where micronekton comes in. It represents food for tuna and many other predators. Tuna juveniles around 10–12 centimetres can sit within that micronekton size range, which means the nets and trawls used to sample the food web can also capture juvenile tuna. SPC’s broader interest is to understand densities and distributions of micronekton using a mix of acoustic methods and pelagic trawls, building a richer picture of the environment tuna rely on.
Once samples come back onboard, the work becomes painstaking and methodical: sorting specimens into groups, weighing them, measuring them, and then preserving them, often freezing larger samples, while very small tuna larvae may be preserved in high-grade alcohol for later examination and genetic analysis.
Much of the detailed identification happens later, and in this collaboration, it extends beyond the voyage itself. The Pacific Community staff working on taxonomy are set to travel to Japan’s Kochi University to work alongside Japanese colleagues processing micronekton identification.
Track of the Kaiyo-Maru during September – November 2025 (Source: Japan Fisheries Research and Education Agency, FRA)
Collaboration at the heart of science
Dr Allain says the size of the Kaiyo-Maru didn’t just mean a lot less nausea-inducing rolling and pitching. The ship is equipped with “really high tech” instrumentation and automation. The trade-off with a larger vessel is the need to plan further ahead because big ships can’t easily pivot when weather shifts or an instrument breaks.
English may be a scientific lingua franca, but Dr Allain says the subtleties that deepen collaboration were sometimes lost in translation with a multinational crew. Still, learning flowed in practical ways. SPC staff worked closely with Japanese experts on identifying tiny specimens, difficult work when larvae can be hard to distinguish by eye, and picking up techniques such as photographing specimens effectively using an aquarium setup.
Why it matters to the Pacific – and Japan
These cruises are expensive, and Allain is clear about why they’re worth doing: they provide observations that “ground truth” the models that fisheries managers increasingly depend on.
Models can test scenarios and help forecast what might happen, but they must be tethered to reality – real fish in real water – if Pacific nations are to manage tuna sustainably and with confidence.
“With mathematical models, the fish in your computer sometimes act differently from your fish in the real world, so it’s still important to have some real observations to back up those models,” Dr Allain says.
The ultimate value, she argues, is improving the science that supports fisheries management, because tuna underpin food security and economies across the Pacific.
Long-term ambition is also about change. Inter-annual variability, especially the influence of El Niño and La Niña, is so strong in the region that you can see big differences in larval densities from one year to the next, and the team has already observed substantial differences between consecutive years in similar sampling legs.
That variability, paradoxically, becomes part of the experiment. Repeated sampling across different climate phases helps scientists understand how tuna early life responds to shifting conditions, while the 20-year gap to earlier Japanese surveys may help reveal whether the baseline itself is moving.
Monitoring change over time
Back on land, it will take time for this voyage’s findings to filter into the scientific literature. A research cruise can be months at sea, then years of analysis afterwards, and meaningful interpretation tends to require multiple years in a time series rather than a single expedition in isolation.
In the meantime, the team is preparing its annual reporting to the Western and Central Pacific Fisheries Commission (WCPFC), offering what Allain describes as preliminary results and an account of what was done, with deeper conclusions to come as the dataset grows.
Life at sea, though, offers its own immediate payoff. Allain says she still loves going out on the water, with “less office time and getting closer to the subject of study”. A highlight of the trip was welcoming aboard Solomon Islands university students, and hosting two early-career ocean professionals, both young women from Solomon Islands. Visitors at each port toured the ship, meeting with the scientists, but also the ship’s captain and engineers.
In a region whose future is tied to the ocean, that kind of exposure is also part of the mission: building the human capability to understand, protect, and manage the Pacific’s most valuable fish.
Visitors from the Solomon Islands National University touring the Kaiyo-Maru and learning about how the samples are taken (source: The Pacific Community)
