Climate Change Spells Trouble For Reef Sharks

It may be a new year, but that doesn’t mean we can wipe away our planet’s problems just because the calendar now says “2021.” Unfortunately, the new year did not bring a stop to climate change. As greenhouse gases continue to trap excess trapped heat that comes from anthropogenic emissions, our oceans are facing not only an increase in sea level but sea surface temperatures. Changes in ocean temperatures and currents brought about by climate change will not only impact worldwide climate patterns but bring unprecedented changes to our aquatic ecosystems and the critters who call them home.

Tropical ecosystems are thought to be the hardest impacted, as many species are adapted to narrow temperature ranges and most are ectotherms, with water temperatures regulating their biological and physiological processes that are crucial for survival. “Thermal performance curves allow scientists to identify the temperatures where a certain trait (e.g., growth, development, reproduction) starts to be negatively impacted. At some low temperature and at some high temperature, for any species, certain biological functions become non-ideal and eventually start failing,” said scientist Carolyn R. Wheeler, co-tutelle Ph.D. student between the School for the Environment at the University of Massachusetts Boston and the ARC Centre of Excellence for Coral Reef Studies at James Cook University. “Identifying these critical temperatures allows us to predict how these species will fair with warming waters from climate change over the next century.”  

One such species that will be severely affected by rising water temperatures are Chondrichthyan fishes (the sharks, rays, skates, and chimaeras) which are mostly ectothermic. “Adaptation, in conjunction with relocation, represent the only (hopeful and positive) options for marine life at this point. Therefore, while making it a global mission to reduce our reliance on fossil fuels, decrease greenhouse gas emissions, and improve the way we generate and dispose of waste, we also need to identify the most vulnerable species and populations and make sure they are not experiencing additional stressors in their habitats and lifetimes,” says Dr. Jodie Rummer, an Associate Professor at the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies at James Cook University. “That is why we (my team) focus – from an evolutionary and ecological perspective, but with conservation implications – on the physiological adaptations that marine fishes require to cope with future climate change conditions, thresholds, limits, and trigger points for various stressors, and in the cases where adaptation may not be possible, their capacity to move to and thrive in more favourable habitats.”

The Chondrichthyans present a challenge in relation to ocean warming vulnerability, since many of the over 1,000 species are globally threatened, poorly characterized in terms of basic life history, and are slow to mature and reproduce. Not only is it hard to study them in controlled laboratory settings, but what is true for one species is not true for another. That is why a team of researchers, led by Wheeler, decided to focus on the epaulette shark (Hemiscyllium ocellatum), a small oviparous shark that is endemic only to the Great Barrier Reef in Australia. “Epaulette sharks are not a perfect representation of all sharks. For one, they lay eggs; whereas, the majority of sharks and rays give birth,” said Wheeler. “Nonetheless epaulette sharks are a good start, and can perhaps help us build models for larger animals.” Not only have epaulette sharks been the focus of many climate change-related laboratory studies, but they also possess unique traits that allow them to hunt in isolated tidal pools and survive extreme conditions. In other words, if epaulette sharks cannot cope with, in this case, thermal stress, how will other, less tolerant species fare?

 “We sourced all of the epaulette shark eggs in this study from breeding epaulette sharks at The Anderson Cabot Center for Ocean Life at the New England Aquarium in Boston. 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,” explained Wheeler. “Eggs were exposed to three different temperatures throughout development. The highest temperature, 87.8°F (31°C) is expected to be a new summer water temperature for sections of the epaulette shark’s range on the Great Barrier Reef by 2100 if climate change proceeds at the current rate. We tracked the growth of the embryos and how quickly they consumed their yolk-sac by backlighting the eggs several times a week.” The scientists investigated how growth, development, and metabolic performance traits of embryonic and neonate H. ocellatum were affected by increased temperatures relevant to mid and end-of-century ocean warming scenarios.

The results were pretty alarming. “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 (Gervais et al. 2016) found 50% mortality at just one degree warmer, 32°C. In our study, sharks reared at 31°C hatched several weeks earlier than their cooler condition counterparts and were slightly smaller in weight having hatched at the warmest temperature too,” said Wheeler. These hatchlings also fed extremely quickly and had a lower metabolic rates, indicating sharks were perhaps overwhelmed by the warm temperatures and would perhaps struggle if chased by a predator in the wild. But it is not necessarily all bad news for these little sharks. “In our experiments, we exposed eggs and hatchlings to a constant high temperature throughout the study. In the wild, they would experience higher temperatures at midday and cool off at night. Perhaps these temperature cycles would improve their survival and fitness. 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 fair under climate change.” 

This information is timely—especially given that the home of these sharks has seen a rapid succession of mass coral bleaching events, heatwaves, and multiple severe tropical cyclones. This study is just something else to consider when doing vulnerability assessments for the Great Barrier Reef and future management actions. In fact, it’s something global coral reef ecosystems should take heed of! “Ultimately, our findings suggest that, under ocean warming scenarios for the middle and end of the twenty-first century, tropical oviparous chondrichthyan species will likely be exposed to their upper thermal limits for critical activities such as growth and development, which causes concern for the future health of the ecosystems they help to support,” the scientists state in their study. The team hopes that this latest study adds to the evidence that we need fast and effective possible actions to reduce global greenhouse gas emissions… and perhaps worldwide partnerships can help reduce cumulative impacts on this iconic reef!