Patrolling the border: Billfish exploit the hypoxic boundary created by the world's largest oxygen minimum zone.

Area of Science:

• Marine Ecology
• Oceanography
• Behavioral Ecology

Background:

• Pelagic predators face challenges with scarce and unpredictable prey distribution.
• Horizontal fronts are known foraging hotspots, but the role of vertical fronts is less understood.
• Vertical fronts, such as thermoclines and oxyclines, create persistent habitats aggregating prey.

Purpose of the Study:

• To investigate how top pelagic predators exploit vertical fronts in the eastern tropical Pacific oxygen minimum zone.
• To document predator foraging behavior in relation to oxygen and temperature gradients.
• To identify novel predatory behaviors associated with vertical habitat boundaries.

Main Methods:

• Utilized high-resolution biologging data, including oxygen saturation and video.
• Tracked blue marlin (Makaira nigricans) and sailfish (Istiophorus platypterus) movements and behaviors.
• Analyzed prey search patterns in relation to dive shape and vertical oceanographic features.

Main Results:

• Prey search behavior intensified near the thermocline (blue marlin) and hypoxic boundary (sailfish).
• A novel behavior of repeated dives below the thermocline and hypoxic boundary was observed.
• This behavior is hypothesized to be an ambush strategy for prey aggregated at these fronts.

Conclusions:

• Vertical fronts, particularly those associated with oxygen minimum zones, are critical foraging habitats for pelagic predators.
• The observed diving behavior suggests specialized predation strategies at oceanographic boundaries.
• Findings highlight the importance of understanding these fronts in the context of climate change and expanding oxygen minimum zones.