Breathe deep: yellowfin tuna oxygen needs reveal marine problems

In the vast expanse of our planet’s tropical and sub-tropical waters, the yellowfin tuna (Thunnus albacares) reigns supreme. Known for their speed and agility, these large fish have long been sought after by pelagic fisheries across their global range.

A fascinating and important new study has tracked the habits of these tuna in the Galapagos region, and explains their reliance on oxygen for speed and agility.  But what if oxygen levels are changing due to climate change?

Residing among the top ten species in terms of landing weight worldwide, these tunas are the darlings of the Eastern Pacific Ocean. In 2021 alone, approximately 260,000 tons were hauled in from this region, primarily by purse-seiners, vessels designed to encircle and capture these swift swimmers. But as the catch figures ascend and the maximum sustainable yield looms closer, the future of the yellowfin tuna stands at a crossroads.

It’s worth noting that the Eastern Pacific Ocean’s yellowfin tuna population isn’t currently overexploited, according to the same study. Still, the uptick in catches over recent years raises concerns about the sustainability of this trend. At present, the stocks seem to hover around the modelled maximum sustainable yield of 288,000 tons. However, the risk of surpassing this critical threshold looms large, potentially jeopardizing the species’ equilibrium.

While the yellowfin tuna’s prowess serves it well, its habitat is facing increasing vulnerability due to shifting environmental conditions, largely spurred by climate change.

In tropical zones, where these tuna thrive, the changes in habitats are becoming evident. With their oxygen demands heightened by their vigorous gill-breathing lifestyle, yellowfin tuna find themselves increasingly impacted by expanding low-oxygen zones. A deeper understanding of the species’ ecological habits is crucial in gauging their vulnerability to this ever-evolving environmental landscape.

Previous tracking studies have revealed that yellowfin tuna often stick to their tagged areas without significant migrations. Their preferred ‘hangout’ is the mixed layer, an oceanic zone where surface temperatures remain stable, allowing them to maintain their active lifestyle. However, they do venture deeper, diving below the thermocline into colder waters to forage for prey. Here they can maintain a higher muscle temperature than their surroundings, allowing them to navigate into colder waters for brief hunting expeditions. The availability of food dictates the extent of their dives, and there’s a delicate interplay between prey presence and temperature and oxygen constraints. While they can acclimatise to lower oxygen levels through physiological adjustments, there’s a breaking point. Oxygen concentrations below certain thresholds affect their blood oxygen reserves, ultimately curtailing their ability to meet metabolic demands.

Oxygen in our oceans plays a crucial role in various biological and chemical processes. It gets into the water through two primary mechanisms: physical exchange at the water’s surface (diffusion) and biological processes within the water (such as photosynthesis). Climate change can impact oxygen levels in water through various mechanisms, such as warming waters (which hold less dissolved oxygen), disrupting ocean circulation patterns (leading to changes in the distribution of oxygen-rich and oxygen-poor water masses), exacerbating conditions that favour harmful algal blooms (which consume large amounts of oxygen and lead to oxygen depletion in affected water bodies), and ocean acidification.

In the Indian Ocean, yellowfin tuna thrive in warmer waters of 25–30°C, while their counterparts in the Gulf of Mexico prefer a cooler range of 20–28°C. Diving patterns also vary, with some individuals plunging to depths exceeding 1,000 meters. But oxygen levels fluctuate considerably across oceans, influencing their movement choices. For instance, yellowfin tuna in the Gulf of Mexico enjoy relatively oxygen-rich environments, while their counterparts in the Eastern Pacific Ocean must contend with limited oxygen and a shallower oxycline.

What if oxygen levels are changing due to climate change?

The Marine Reserve in the Galápagos has emerged as a pivotal location in the yellowfin tuna’s life. Situated in the path of the South Equatorial Current, this reserve serves as a crossroads for warm and cool waters, creating a hotspot of productivity. Both industrial and semi-industrial tuna fishing fleets gravitate to these rich waters.

Despite its equatorial location, the Galápagos experiences distinct seasons, with warm, wet periods from December to May and cooler, drier stretches throughout the rest of the year. This oscillation is largely influenced by the El Niño-Southern Oscillation.

The Eastern Tropical Pacific, where the Galápagos reside, faces unique challenges due to its relative oxygen levels. Dissolved oxygen levels at 100 meters dive below 2.4ml/l throughout the Galápagos Marine Reserve, painting a picture of limited oxygen availability. This deficiency is attributed to high surface productivity, a strong pycnocline (density boundary), and sluggish oceanic ventilation.

Yet the habitat used by tuna here remains largely unstudied, until now. New research from the Galapagos Science Center with the University of Southampton, Universidad San Francisco de Quito, and the Marine Megafauna Foundation (MMF) has sought to examine the vertical movements of yellowfin tuna.

It is part of the “Building Resilience in Galápagos Ecosystem Management to Severe Climate Change (R-GEMS)” project, funded by the Royal Society and authorized under the Galápagos National Park Directorate, and aims to uncover the physical drivers of movement patterns and spatial behaviour of important species in the region.

Dr Alex Hearn, lead investigator of the project notes, “The R-GEMS project strives to formulate a regional oceanographic model predicting the potential impacts of climate change on marine species’ behaviours over the forthcoming five decades.”

Eight yellowfin tuna were equipped with satellite tags, allowing researchers to monitor their movements both vertically and horizontally for periods ranging from 4-97 days. Surprisingly, these tuna, usually found near the surface at a median depth of 24.3 metres, occasionally ventured into colder, oxygen-deficient waters. “These occasional deep-water dives, predominantly during the day, followed by recuperative periods near the surface, suggest the tuna’s need to re-oxygenate post venturing into oxygen-deprived areas,” explained MMF’s Dr Chris Rohner, lead author of the movement study.

While short tag retention limited their tracking, the researchers believe it is evident that these tuna maintain their presence within the reserve’s boundaries. Yet, the implications of climate change are undeniable.

The habitat used by tuna here remains largely unstudied, until now.

“Our findings underscore the delicate balance marine life maintains in response to their environment,” says Dr Alberto Naveira Garabato in a press release. As oxygen levels drop and the extent of suitable habitat shrinks, the yellowfin tuna might need to make compromises.

These fish may need to frequent shallower depths or adapt their diving behaviours to accommodate their oxygen needs. Such shifts have ramifications, influencing predation dynamics, prey choices, and, ultimately, their susceptibility to fishing gear. This change could possibly make them more susceptible to surface fishing methods.

Though the study orbits around the Galápagos, its revelations hold global significance. “Our research on the yellowfin tuna in the Galápagos paints a cautionary tale of the future of global marine life in a changing climate. The urgency to fortify our marine conservation efforts has never been clearer,” says Rohner. As the world grapples with changing ocean temperatures, recognizing its cascading impacts becomes paramount.

The behaviour of the Galápagos tuna provides a snapshot into how marine species may be required to adapt to their changing environments, offering a glimpse into potential future scenarios for our global ecosystem.

In the face of these challenges, it’s clear that the yellowfin tuna’s survival hinges on our understanding of its behaviours, habitat preferences, and adaptability. As climate change reshapes their environment, scientists, conservationists, and fisheries managers must work hand in hand to ensure the long-term viability of these remarkable creatures. Whether by protecting crucial habitats, regulating fishing practices, or advancing our scientific grasp of their lifestyle, we hold the key to securing a future where the yellowfin tuna can continue to thrive amidst the shifting tides of change.