How a Warming Future Could Threaten Baby Sharks

With climate change, epaulette sharks will likely be born hungry and smaller than normal, study finds.

epaulette sharks
Epaulette sharks are found only in the Great Barrier Reef.

E. Moothart

As climate change warms oceans, baby sharks will have new challenges. They may be born undernourished and smaller than normal, and launched into demanding environments, a new study finds.

Researchers studied how warming temperatures affected the growth, development, and performance of epaulette sharks (Hemiscyllium ocellatum), an egg-laying species found only in the Great Barrier Reef. The results were published in Scientific Reports.

They used eggs from breeding sharks at the New England Aquarium in Boston for the study.

“This collaboration was a great example of using existing resources in a public aquarium to conduct timely research without having to collect animals from the wild,” lead author Carolyn Wheeler, a PhD candidate at the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Australia and the University of Massachusetts, tells Treehugger.

Researchers exposed the eggs to three different temperatures as they were developing. The warmest temperature 31 C (87.8 F) is what is expected to be the new summer temperature for some of the shark’s range in the Great Barrier Reef by 2100 if climate change continues at its current rate. 

They tracked how the embryos grew and how quickly they consumed the yolk sac, which is the membrane-lined structure that provides nutrients to the growing shark. They watched and recorded the growth by backlighting the eggs several times each week.

“We found that rearing eggs at 31° C resulted in negative effects on development. All sharks survived the conditions, which is a good sign, but another previous study from our group found 50 percent mortality at just one degree warmer, 32° C ,” Wheeler says.

In this new study, sharks that were raised in water that was 31 C hatched several weeks earlier than those in cooler water and were slightly smaller in weight.

Shark embryos use their yolk sacs more quickly in warmer water.
Shark embryos use their yolk sacs more quickly in warmer water. M. Johnson

“The 31°C-reared hatchlings also fed extremely quickly, which may not be a good thing. Usually, sharks hatch with some yolk-sac reserved inside them so that they do not need to feed (learn how to hunt) right away,” Wheeler explains.

Because adult sharks don’t care for their eggs, shark eggs must be able to survive unprotected for as long as four months.

“The 31°C-reared hatchlings started feeding on the food we offered them within 1-2 days compared to 7-8 days for their cooler-reared counterparts. This may indicate that in nature, these warm-reared hatchlings would have less time to adjust to their new surroundings, and instead need to find food.”

Researchers found that the sharks in the warmer water generally had a lower metabolic rate, which indicates they were having a difficult time coping with the warm temperatures, Wheeler says.

“In one of our experiments, comparable to an athlete sprinting on a treadmill, sharks were exercised (chased) for several minutes,” she says. “Directly after exercise, we measured how much oxygen they were breathing, similar to how heavily we breathe after running. We found that the warm water hatchlings were less fit and would perhaps struggle if chased by a predator in the wild.”

Looking to the Future

The study suggests that in the future, sharks will enter the world into circumstances that might impede their ability to survive.

“Some of our results are alarming, but it is not necessarily all bad news for these little sharks,” Wheeler says.

In their experiments, researchers exposed shark eggs and hatchlings to constant high temperatures. However, in the wild, they would experience higher temperatures at midday and cooler temperatures at night.

“Perhaps these temperature cycles would improve their survival and fitness," Wheeler says. "So, we need to continue investigating these questions and comparing all life stages and different species to create a better picture as to how sharks and their relatives will fare under climate change.”

View Article Sources
  1. Wheeler, Carolyn R., et al. "Future Thermal Regimes for Epaulette Sharks (Hemiscyllium Ocellatum): Growth and Metabolic Performance Cease to be Optimal." Scientific Reports, vol. 11, no. 1, 2021, doi:10.1038/s41598-020-79953-0

  2. Gervais, Connor R., et al. "Population Variation In The Thermal Response To Climate Change Reveals Differing Sensitivity In A Benthic Shark." Global Change Biology, vol. 27, no. 1, 2020, pp. 108-120, doi:10.1111/gcb.15422