Warming oceans raise concern for shark health

Epaulette sharks (Hemiscyllium ocellatum) inhabit the Great Barrier Reef and are known to be tolerant of extreme conditions (Wheeler et al 2021). Female sharks lay eggs in the coral of the reef and leave them unprotected, where they hatch after approximately four months (Gamillo 2021).

Figure 1. Epaulette shark (Hemiscyllium ocellatum) (Image courtesy of Smithsonian Magazine)

Epaulette sharks are native to the Great Barrier Reef. They hunt in isolated tidal pools and lay their eggs in the coral of the reef. These sharks are known to be able to survive extreme conditions which makes them a good study species when looking at the potential effects of climate change.

Oceans “soak up” heat trapped in the atmosphere. When excess energy is added to the climate through things like burning fossil fuels, the ocean will absorb some of that energy (WHOI). Water can hold more heat than land, so it warms more slowly; even so, since the Industrial Revolution (around the 1860s) the ocean has warmed approximately 1.5 degrees Celsius (Deng 2024). These warming waters cause sea level rise, weather changes, coral bleaching, altered ecosystems, etc.

Warming waters raise concerns for species like sharks because they are ectotherms meaning water temperatures affect the body’s biological and physiological processes (how the body works: development rates, metabolism) (Wheeler et al 2021). Many shark species are threatened because they have slow generation times and low reproductive output; meaning they do not frequently have babies and it takes a long time for those babies to grow up. For species like Epaulette sharks that are known to be more tolerant of extreme conditions, their embryos are still at risk because they are unprotected (Wheeler et al 2021).

A study was done where scientists reared 27 epaulette sharks at different temperatures (27,29, and 31 degrees Celsius). These temperatures were chosen based on their current average habitat temperature and predicted future ocean temperatures (Wheeler et al 2021). After birth, they tracked the growth, development, and metabolic costs of the sharks. The results of the study were that sharks reared at higher temperatures consumed their yolk faster, hatched earlier, weighed less, and exhibited reduced metabolic performance (Wheeler et al 2021). These results are concerning because sharks born at higher temperatures were smaller and weaker, putting them at risk for survival. The reduction in metabolic performance at increased temperatures is especially concerning because it means these sharks will be weaker, lose energy faster, and could have a harder time hunting for food successfully (Wheeler et al 2021).

Considering this is a known tolerant species, and they are negatively impacted by increased temperatures, that raises concerns for species that are not as tolerant. If species that were thought to be able to handle extremes actually can’t, then what does that mean for everything else? More research needs to be done on how increased temperatures could affect not only this species but other marine animals. More research also needs to be done on potential alternative solutions for this species because moving habitats may not be an option for them since they use the coral reef environment for hunting and reproduction. If the effects of ocean warming become too extreme, this species may need to find a way to move to colder waters or risk dying off.

References

Deng W (2024) Ocean warming and warning. Nature Climate Change, https://doi.org/10.1038/s41558-023-01921-z

Gamillo E (2021) Ocean warming threatens baby sharks in the Great Barrier Reef. https://www.smithsonianmag.com/smart-news/ocean-warming-threatens-baby-sharks-great-barrier-reef-180976788/ (date last accessed 2 April 2024).

Wheeler C, Rummer J, Bailey B, Lockwood J, Vance S, Mandelman J (2021) Future thermal regimes for epaulette sharks (Hemiscyllium ocellatum): growth and metabolic performance cease to be optimal. Scientific Reports, https://doi.org/10.1038/s41598-020-79953-0

WHOI, Ocean Warming. https://www.whoi.edu/know-your-ocean/ocean-topics/climate-weather/ocean-warming/ (date last accessed 3 April 2024).