Sharks, Orcas and Corals in Latest Ocean Research

As we enter 2024, Dr Anjani Ganase presents some of recent findings from ocean studies.

 

A school of hammerhead sharks, Mikimoto, Japan. Credit: Masayuki Agawa / Ocean Image Bank

Shark mortality rises even with legislative measures

 

A recent study highlighted the continued increase in shark mortality from fishing, up from 76 million to 80 million between 2012 and 2019, despite widespread regulation changes. About 25 million of those catches are threatened shark species. In this time, there was a ten-fold increase in international regulations on shark fishing across 29 countries and overseas territories. Unfortunately, the majority of the measures focused on the banning of shark finning rather than policies that focused on reducing shark mortality. The ban on shark finning created a broader incentive in the shark fishing industry for the fishing and marketing of the whole shark for different purposes.

 

Shark fishing continues to increase in coastal waters as compared to the open ocean. Hotspots for high shark mortality continue to be the Atlantic coasts of North and South America, North Indian Ocean and the Coral Triangle. Sharks are important predators of the sea and are important governors of the ocean food web. When we think about sharks, we typically imagine the apex predators – the Great White, Tiger and the Bull sharks - but there are thousands of other species in our oceans adapted to a wide range of marine environments. Their diversity highlights the evolutionary versatility and adaptability of sharks that allowed them to dominate the oceans for 400 million years. Think about the giant whale sharks in the warm tropics. Greenland sharks have the potential of living up to 500 years. The Bluntnose sixgill sharks live near the deep ocean floor down to 8,000 feet. These are species that are being threatened. 

 

Coral reef, French Polynesia. Credit: Hannes Klostermann / Ocean Image Bank

 

Coral species can remember and may adapt to heat waves

 

In Hawai’i, a long-term study within Kaneohe Bay monitored the impact of multiple bleaching events on certain species of corals. The results showed that while many colonies bleached and died, some colonies displayed no heat stress at all and did not bleach, while others bleached and recovered. For the colonies that recovered, scientists noticed that they bleached less and less in subsequent heat wave events, effectively acclimatising to the heat stress. Why or how the coral are able to acclimatise, may be partially answered by another long-term research study conducted in Mo’orea in French Polynesia.

 

The study revealed that certain coral species may be capable of remembering marine heat waves, thanks to their microbial counterparts. While corals are known to have a symbiotic relationship with micro- algae that live in their tissues and provide the coral host with food through photosynthesis, corals are also hosts to a diverse microbial community composed of bacteria, viruses and archaea. These associations facilitate several functions that the corals benefit from, such as sulphur, carbon and nitrogen cycling. The microbial communities are likely to be impacted by marine heat waves with consequences to the coral host. In instances, the bacterial communities may bloom and result in coral diseases, However, one study tracked changes in the microbes in three coral species exposed to back-to-back marine heat waves. Outcomes show that coral species displayed some form of ecological memory that corresponded to shifts in the microbial communities before, during and after that allowed for acclimatisation to the second heat wave. Such experimental studies give insight into the natural adaptation potential for some coral species.

 

Orca, Indian Ocean. Credit: Vincent Kneefel / Ocean Image Bank

Orcas the ultimate ocean predators

 

Of late, orcas have been in the media for their devious attacks on sailing boats in the strait of Gibraltar. These attacks, while rare, started to be reported in 2020. The attacks occur as the orcas attempt to bite or break the rudder by ramming it. While scientists put this to either defensive behaviour following a traumatic experience with a rudder of a boat or simply fun, the world is learning how clever these beings are.

 

Orcas make up one species in the family of dolphins and whales. However, there are about ten sub species or ecotypes that occur around the world. While their ranges overlap, they do not interbreed which may be because of the unique physical and behaviours traits that have developed within the subgroup. Orcas have always been vicious hunters. In the Indian Ocean, orcas learned to poach fish from longliners, affectng the local fisheries around the Crozet Islands. In south Africa, two male orcas were known for hunting and killing great white sharks to feed on their livers.

 

In 2019, a pod of orcas was observed to hunt and kill blue whale pups by pushing and holding them underwater until they drowned. The tongue of the whale was eaten first. Similar behaviour had been documented in the whaling days of South Australia where the orcas would work with whalers to corral large baleen whales into bays to be harpooned. The orcas would nip at the tails of the whales to tire them out allowing the whalers to harpoon them. The orcas would eat the tongue before leaving the carcass for the whalers. Old Tom was a legendary male orca with an aggressive streak. He lived near the port of Eden in New South Wales, and the pod was called the Killers of Eden. Old Tom was thought to be over 90 years old when he died. Orcas have always been this way; we are only now learning about them.

 

 Cyclone trends increase the risk at coastal areas

 

Tropical cyclones, known as hurricanes in the western hemisphere, are formed in the tropical oceans in areas of low pressure and warm surface temperatures. The damaging impacts of cyclones result from high wind speeds, heavy rainfall, severe storm surge and wave action. Here in the Caribbean, we are familiar with hurricanes and the threats they pose. A 44-year analysis (1979 – 2022) of cyclone data found that cyclones follow a trend of stronger wave hazards because of increased wave height and larger areas of impact. The trend of increased wave heights is consistent in all ocean basins, but the largest increases occurred in the North Atlantic Ocean. Stronger wave action is a serious concern for coastal habitats. Here in the Caribbean most of our communities and cities occur in these low-lying areas. Climate change is thought to drive cyclone activities. We should identify vulnerable areas and prioritise adaptation of coasts, using natural defences such as mangrove systems and reefs.

 

References:

Boris Worm et al. Global shark fishing mortality still rising despite widespread regulatory change.Science383,225-230(2024).DOI:10.1126/science.adf8984

 

Brown, Kristen T., et al. "Divergent bleaching and recovery trajectories in reef-building corals following a decade of successive marine heatwaves." Proceedings of the National Academy of Sciences 120.52 (2023): e2312104120.

 

Shi, J., Feng, X., Toumi, R. et al. Global increase in tropical cyclone ocean surface waves. Nat Commun 15, 174 (2024). https://doi.org/10.1038/s41467-023-43532-4

 

Totterdell, J. A., Wellard, R., Reeves, I. M., Elsdon, B., Markovic, P., Yoshida, M., Fairchild, A., Sharp, G., & Pitman, R. L. (2022).

 The first three records of killer whales (Orcinus orca) killing and eating blue whales (Balaenoptera musculus). Marine Mammal Science, 38(3), 1286–1301. https://doi.org/10.1111/mms.12906

 

Vompe, A. D., Epstein, H. E., Speare, K. E., Schmeltzer, E. R., Adam, T. C., Burkepile, D. E., Sharpton, T. J., & Vega Thurber, R. (2023). Microbiome ecological memory and responses to repeated marine heatwaves clarify variation in coral bleaching and mortality. Global Change Biology, 30, e17088. https://doi.org/10.1111/gcb.17088

 

https://www.scientificamerican.com/article/orcas-are-learning-terrifying-new-behaviors/