Galápagos, Ecuador · Since 2020
Galápagos Yellowfin Tuna Project
Satellite tags on yellowfin tuna in the Galápagos showed that oxygen, not depth, limits their dives, keeping them close to the surface and within reach of fishing gear.
Led by Universidad San Francisco de Quito, with MMF
Yellowfin tuna (Thunnus albacares) support one of the eastern Pacific’s largest fisheries, with 260,000 tonnes landed in 2021. This study tracked them inside the Galápagos Marine Reserve to determine how they use the water column where oxygen is depleted not far below the surface.
The Galápagos Marine Reserve covers more than 140,000 km² (54,000 square miles) around the islands. Industrial fishing is banned inside it; however, a local fleet of small boats fishes for tuna, and industrial purse seiners operate just outside its boundary. Upwelling around the islands also brings cold, oxygen-poor water unusually close to the surface. In this study, led by Dr. Alex Hearn of the Galápagos Science Center and Universidad San Francisco de Quito, with Prof. Alberto Naveira Garabato of the University of Southampton, satellite tags showed how the tuna use the water column under these low-oxygen conditions. MMF’s Dr. Chris Rohner wrote up the results as lead author of the published paper. (Top photo: Jonathan Green.)
What the project has done
- Fitted pop-up satellite tags, which record movements and dives, to 14 yellowfin tuna, caught and released at several sites around the islands in August 2020, March 2021, and March 2022.
- Analyzed movement and dive records from eight tuna, tracked for 4 to 97 days, alongside modeled ocean oxygen levels.
- Published the results in the Journal of Fish Biology in 2023.
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What the study found
Depth use
The tuna spent more than half of their time (53%) in the top 10 m (33 ft) and 90% in the top 50 m (160 ft), mostly within the warm, mixed surface layer. They swam deeper by day, at an average of 38 m (125 ft), than at night, when they averaged 11 m (36 ft), and most of their deep dives occurred in daylight, probably to hunt deeper-living prey. Eight of the nine tuna that returned dive data went below 200 m (660 ft), and one reached 635 m (2,083 ft), where the water was just 7.2 °C (45.0 °F). (Photo: Galápagos Science Center.)
Oxygen sets the limit
Around the Galápagos, oxygen becomes scarce from about 50–70 m (160–230 ft) down, and at 100 m (330 ft) oxygen is low throughout the reserve. The tuna dived into this low-oxygen water hundreds of times, but only briefly, and after each of these dives they waited an average of 51 minutes before the next, presumably to recover. The same fish that reached 635 m (2,083 ft) on one dive turned back at 200–300 m (660–980 ft) on others, where oxygen ran similarly low; oxygen, rather than depth, therefore appears to limit how deep they can go.
Horizontal movements
The tagged tuna traveled an average of 13.6 km (8.5 miles) a day, dispersing across the islands and over deep water offshore. All remained in Ecuador’s waters during tracking, and two left the reserve for a fishing hotspot along its western boundary. Their tracks point to the potential importance of protected areas in maintaining healthy yellowfin tuna stocks. MMF’s work in the Galápagos follows whale sharks and silky sharks through the same waters.
Implications for fisheries
Because the tuna spend most of their time near the surface, they are within reach of the main fishing methods in the region, from small-boat trolling inside the reserve to industrial purse seiners and surface longlines outside it. Oxygen-poor water has been expanding slowly upward near the Galápagos for decades, and ocean oxygen is forecast to continue declining, which could compress the tuna’s habitat closer to the surface and make them easier to catch. The study therefore suggests that catch limits may need to account for this habitat compression.
Dr. Chris Rohner
Galapagos Silky Shark Project
Galapagos Whale Shark Project