Large Pelagics:
By Daniel Madigan     Awards: Giese, Myers

Sharks, tuna, marlin, mahi-mahi: these are some of the most familiar marine creatures to humans, both as table fare and as charismatic megafauna featured on lunchboxes, in animated films, and dramatized in documentaries.  Beyond their natural charisma these animals play important parts in ecosystems as mid- or top-level predators.  Their importance to the human economy as well as marine ecology makes it crucial to understand where they are, what they do, and what they eat.

These large pelagic animals have remained poorly understood for a long time.  For example, simple questions such as "where does a bluefin tuna go in the North Pacific – and where doesn’t it go?" and "where do tunas feed most in the Pacific – and what in the environment makes this so?" have had only vague answers, at best.  In the last decade electronic tags have been used to track animals all over the oceans, and this has greatly improved our understanding of where big predators go.  In addition, ever-improving physiological tools have allowed us to better understand how they can survive in such a wide range of habitats – arctic waters to the tropics, warm surface waters down to the cold, dark depths. 

My studies focus on habitat use by large pelagic fish in the Eastern Pacific, and what resources they subsist on when they are there.  As such I try to use tagging technologies to tell us more about why an animal visits a certain location, or why certain physiological mechanisms evolved.  I also try to use methods complementary to electronic tagging to tell us more about the intriguing, far-ranging tracks that are generated by electronically tagged animals.

My research has several parts.  I use stable isotope values of animal white muscle to estimate foraging patterns.  Stable isotopes are rare versions of common elements (for example, a heavier carbon molecule than the average carbon you'll run into) that exist in nature in certain ratios, incorporate themselves into food, and propagate up food chains.  Since these values may vary across regions, an animal's muscle signature can tell you roughly where it’s been in the Pacific Ocean.  These signatures also increase with an organism’s place in the food web – the higher your ¹⁵N value, for example, the higher you are on the food chain.  I use this along with tagging technology and diet analysis to see how different tolerances to warmth in three tuna species translate to real-world feeding differences.  This allows me to investigate the real-world consequences of endothermy, one of the most intriguing physiological adaptations in marine fish.