Shortfin mako sharks, an endangered species, are among the fastest and most elusive predators in the ocean, and new research led by Michael Byrne, associate professor of wildlife ecology at the University of Missouri’s School of Natural Resources (SNR) sheds light on the limitations of their habitat availability caused by oceanography.
The research tracked mako sharks over vast distances using satellite telemetry. Byrne’s findings reveal a pattern in movements and distribution of the apex predators spanning thousands of miles across the Pacific Ocean, highlighting the sharks’ dependence on one very specific factor — oxygen levels in the water.
“I love mako sharks, so I spend a lot of time looking at mako shark tracking data. A decade of satellite tracking data from sharks tagged off the west coast shows what looks like an invisible fence line in the ocean, where they rarely travel south through the North Equatorial Current at around 12 degrees latitude,” Byrne said.
This pattern struck Byrne’s curiosity. He wanted to know what was keeping these otherwise highly mobile sharks with a worldwide distribution and the capacity to travel great distances from venturing further south in the east Pacific. One day, he compared the map of mako shark movements tracked by NOAA and the Guy Harvey Research Institute and Nova Southeastern University with a map of dissolved oxygen, and it suddenly clicked.
“Mako sharks are the Ferraris of the shark world,” Byrne said. “They are very fast, and they have a high metabolic rate. That high performance comes with high oxygen demands. They are also endothermic, meaning they maintain their body temperature several degrees above the surrounding water. This helps with their speed but could potentially lead to overheating if they stay in very warm water for extended periods of time.”
When examining the maps, Byrne noted that the southern distribution of mako sharks in the east Pacific corresponded nicely with the northern edge of the Pacific oxygen minimum zone (OMZ), a mass of cold, poorly oxygenated water. OMZs tend to shoal close to the ocean’s surface along the eastern boundaries of the world’s oceans because of upwelling and other oceanographic processes, and the Pacific OMZ is the world’s largest.
A phenomenon known as habitat compression can occur in these regions, where habitat available for open-ocean fish is limited as the OMZ comes closer to the surface. The fish are restricted, or “compressed”, within a narrow layer of warm water with suitable oxygen near the ocean’s surface. Could it be that extreme habitat compression created unsuitable conditions for mako sharks in the tropical Pacific?
To test this hypothesis Byrne and colleagues compared habitat conditions used by mako sharks in the east Pacific with those used by satellite-tracked mako sharks in the west Atlantic, where OMZ shoaling is not an issue. Mako sharks in the Atlantic regularly remained in very warm regions in the Caribbean Sea and Gulf of Mexico year-round, ruling out surface temperature alone as the factor limiting shark distribution in the Pacific. However, when comparing oxygen conditions in tropical regions, sharks that moved south into the North Equatorial Current in the Pacific ran into the OMZ where low-oxygen conditions could creep up to less than 100m deep, conditions which sharks in the Atlantic did not have to contend with. Meaning that, unlike in the Pacific, sharks in the tropical West Atlantic could make forays to deeper and cooler waters that were not oxygen limited. Additional analyses showed that Pacific sharks spent less time near the surface as temperature increased but were also limited in how deep they traveled by low oxygen conditions, as well as evidence that sharks actively avoided traveling over the Pacific OMZ where it was simultaneously shallow and the surface temperature was warm.
“In short, it seems that mako sharks did not travel into the tropical Pacific because they got squeezed from both directions – from the top by surface waters that were too warm to support extended occupancy, and from the bottom by low oxygen conditions that limited how deep the sharks could dive,” Byrne said.
Research has shown is that these zones are expanding, likely as a result of changing global temperatures and weather patterns and are restricting the shark’s movement in the ocean.
The study comes at a time when mako sharks are facing increased threats from overfishing and habitat degradation. Despite their status as top predators, they are vulnerable to population declines due to slow reproductive rates. Byrne’s research demonstrates how oceanography, which is ever-changing as global temperatures and weather patterns shift, creates boundaries for the wildlife that call the ocean home that are often invisible to humans.
Byrne, who specializes in the intersection of wildlife ecology and conservation biology, worked on this project with an interdisciplinary team of scientists from institutions around the country including the National Marine Fisheries Service, the Guy Harvey Research Institute at Nova Southeastern University and the University of Rhode Island. Their work not only advances the understanding of mako sharks but also sets the stage for future research into the broader environmental impacts on marine life.
These impacts could include effects on fisheries and genetic exchange of mako sharks across the equator.