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Studying Shark Stress

Two blacktip reef sharks swimming over coral reefs
Blacktip reef sharks (Carcharhinus melanopterus) swim in Shedd Aquarium’s Wild Reef exhibit. (Image credit: ©Shedd Aquarium/Heidi Zieger)

Dr. Lisa Naples
John G. Shedd Aquarium

As Shark Week wraps up, let’s consider the vulnerable side of these ancient species. To many people, sharks seem to be the toughest animals in the ocean—but these top predators are prone to the same physical problem that many of us experience: stress.

It’s often said that stress can kill—for sharks, that saying is all too true. In some species, stress can trigger physical responses that lead to life-threatening critical illness within minutes. This little-studied phenomenon could have significant implications for shark conservation and management: Evidence suggests that stress can cause major declines in shark populations, creating greater peril for species that are already in decline due to overfishing, bycatch and other challenges in their environment.

Understanding shark stress

Shedd Aquarium and its partners are on the leading edge of this little known—yet critically important—field. Five years ago, I began collaborating with Dr. Natalie Mylniczenko conducting field research off the Florida Keys, to increase our knowledge of how to help sharks that experience stressful events, like being caught accidentally on a fishing line. My partners and I are investigating several questions, including:

  • How do different shark species respond to and recover from stress?
  • Does a particular set of blood tests that can be run instantly shipside have value for measuring shark stress levels? Or are there other previously unidentified blood values that are useful measures of shark health and stress level?
  • Can certain medications effectively prevent stress related illnesses?

Each day in the field, we are usually on the water from dawn to dusk. We collect blood samples from sharks to help us understand their overall health by learning what normal blood values would be for each species in its particular environment. These data also help us measure how the sharks are adapting to stressors in their physical environment.

Our research process constantly evolves based on data and observations from previous years. We have learned from our earlier studies that certain blood tests are valid to use when studying elasmobranchs. Now, we use these tests not only to identify how values may differ when animals are stressed, but also to analyze how they differ between the shark species and their varying lifestyles—for example, some sharks are pelagic, spending all of their time in swimming the open ocean and experiencing very different lives than their coast-dwelling counterparts.

For example: We now know that in times of excitation or stress, sharks can experience lactic acidosis. This is a physiological condition that occurs when the body utilizes the sugar stores in the muscles to provide an energy source to replace oxygen; it’s very similar to the process that people go through during vigorous exercise. Unlike people, lactic acidosis commonly is a life-threatening condition for sharks. Now, we’re looking to see whether the blood values used to measure acidosis and other stress markers vary between shark species, and whether a species’ behavior impacts those values (e.g., would we find different blood values in shark species that swim constantly versus those that can rest on the sea floor at times).

Conservation connections

By enhancing our understanding of each species’ stress response, we can measure how different techniques and medical interventions affect the development of critical illness. Someday, this could lead to an effective medical intervention for stressed sharks that acts like a “chill pill.”

This year, we examined approximately 50 animals, taking blood samples and assessing the overall health of blacktip reef sharks, bonnethead, blacknose, lemon and nurse sharks. Each year is as exciting as the last and provides new information about these fascinating creatures: and as the years have passed, I’m reminded of how much we have left to learn about them, and how we’ve just started to understand the tremendous variations between individuals and species. We have so much left to explore and to contribute to their conservation.

The information we are acquiring is also vitally important to developing best practices for animal handling and of great value to the veterinary field. For example, when sharks are sick, and need to be examined, we can apply what we have learned with health assessments and validating shark stress responses to assure the animals remain stable during their procedures.

Shark Week may be ending, but our work continues year-round. The research that we’re conducting today could help sharks in the future. Sharks are critically at risk in the wild and rapidly dwindling in numbers. In partnership with our colleagues, Shedd hopes to help shark conservationists around the world understand more about how climate change, oil spills and other ocean issues could impact shark stress and their overall health.