Spotlight on Sharks

Dr Anjani Ganase discusses the latest findings from research on sharks

 

Tiger sharks roam seagrass prairie in Bahamas

Scientists have discovered the largest seagrass prairie in the world with the assistance of tiger sharks. Along the Bahamas archipelago, the area of seagrasses is estimated to be at least 66,900 km² with a upper estimate of 92,000 km², which is roughly the size of Florida. This newly discovered area increases the known area of seagrasses globally by a whopping 41 %. The prairie consists of seagrass beds of varying species and densities, such as the turtle grass (Thalassia testudinum) but includes sparse areas of pioneer shoal grass species (Halodule wrightii).

 While over 2500 surveys were conducted by divers, tiger sharks allowed researchers to extend the area of observation considerably as the tiger sharks were able to roam a much larger areas and swim to greater depth, beyond diving limits of humans (down to 90 m in the Dry Tortugas). Within the observation time, they were able to record up to 20 km of additional area. Sharks were captured using drum lines; 360 cameras and tracking devices were strapped on and the sharks were released. Tiger sharks seemed to prefer seagrass habitats for hunting, as they spent up to 70 % of the time in the seagrass areas. Previous studies used manatees and turtles, but these would not roam as far as the tiger shark. Considering the value of seagrasses in carbon sequestration and climate management, and the fidelity of this key marine organism to the seagrass beds, the conservation of such a vast area is critical.

 

Diver with a shark in seagrass Credit: Jett Britnell / Ocean Image Bank

Shark populations continue to decline

Decades of conservation and management have led to an uptick in populations of certain large pelagic predator fish species. Scientist assessed the current extinction rates of eighteen large commercial species using the IUCN red list. The assessment considers changes in populations based on catch rates, but also looks at existing stocks, maturity and reproduction rates. They found lower extinction rates for species of billfish, such as black marlin and swordfish, and several species of tuna. However, many species tuna and billfish continue to be listed on the IUCN endangered species Red List, despite the increases in populations. Conversely, scientists observed no changes in the declining trends in shark populations with many species dangerously close to being extinct. The difference between sharks and other fishes is that sharks continue to be caught as by-catch. Strategies in conservation focussed on shark species need to be prioritised to reduce the bycatch rate, as well as restricting size limits. Such strategies would go hand in hand with designated no take areas, protection of nursery areas and technology to improve current fishing methods.

  

Technology to repel shark species

Sharks are sophisticated predators capable of detecting electrical fields in the ocean from great distances, whether it is potential prey or other activity in the water. This is possible because of specialised pores located on their heads called the ampullae of lorenzini. On the remote coral cays of the Great Barrier Reef where few boats ventured, we were always welcomed by curious reef sharks that followed the sounds and the noise of the engines. It is often rumoured that sharks are the first on the scene when a plane crashes into the ocean because of the electrical impulses given off. Now scientists are trying to use this knowledge to protect both sharks and humans. In the past, researchers have developed devices that give off electrical impulses to deter sharks in areas where there is high human and shark traffic. With the continued declines in shark populations (by 71 % in the last fifty years), researchers from the UK trialled a shark by-catch mitigation device to deter sharks from the hooks of long liners of tuna fishing vessels in the south of France. Despite targeting tuna species, many of the long liners bring in mostly stingrays (61 %) and blue sharks (19 %) with less than 10 % being the target species. Initial testing of the device resulted in a reduction in blue shark by-catch by 91% and stingray by-catch by 71 %. While promising, more work needs to be done, firstly to ensure that the tuna fishing isn’t being compromised, but also expanded to different environments and different shark species.

 

Whale Shark feeding. Credit: Amanda Cotton / Ocean Image Bank

 

Detecting pregnant whale sharks

To date only one pregnant female whale shark has ever been recorded; it was captured on a commercial fishing vessel in 1995. The detection or even the understanding of the reproductive cycles of whale sharks continues to be primitive. In fact, it was often assumed that female whale sharks with large, distended bellies were pregnant; it was subsequently found that sharks have an extremely thick layer of skin and muscle. With the decline in whale shark populations, scientists have become increasingly interested in understanding the reproductive patterns of whale sharks to inform conservation. Scientists from Japan’s Okinawa Churaumi Aquarium developed a nifty tool to be able to detect pregnant whale sharks using an ultrasound imaging system in a waterproof case. The scientist would scuba dive under the bellies of the whale sharks using an underwater propulsion vehicle to keep up with the swimming sharks. Blood samples were also collected. The cumbersome technique permitted twenty-two female whale sharks around Galapagos to be successfully scanned, but not a single one was pregnant! However, they were able to determine which females were sexually mature.

 

Omnivorous whale sharks

Whale sharks like most sharks are thought to be carnivorous though they have evolved to feed on tiny zooplankton or krill. Whale sharks cruise slowly through the water column with their gaping mouth sucking water through gills to trap and eat tiny prey. Zooplankton is a major part of the ocean food web providing food for many species of marine mammals, seabirds and fish. However, whale sharks may also take in seaweed and even digest it, similar to other omnivores in the ocean. Tissue sampling of the whale sharks revealed a high amount of arachidonic acid, which is an organic molecule common in sargassum. The quantity suggests that the feeding on plants may be more than incidental, but perhaps a common part of the diet. Further research will reveal the sources of plants in their diets, but any research on whale sharks is challenging given their elusive behaviour.   

Sharks are necessary to the health of the oceans. Information about their life cycles and habitats should be shared so that these top predators in our oceans become as recognisable and familiar as beasts of the rainforest or jungle. There is need to halt their decline; and humans must realise and act on the understanding that not everything is food.

  

References

Doherty, Philip D., et al. "Efficacy of a novel shark bycatch mitigation device in a tuna longline fishery." Current Biology 32.22 (2022): R1260-R1261.

Gallagher et al. Tiger sharks support the characterization of the world’s largest seagrass ecosystem. Nature Communications. Vol. 13, November 1, 2022, p. 6328. doi: 10.1038/s41467-022-33926-1

Matsumoto et al. Underwater ultrasonography and blood sampling provide the first observations of reproductive biology in free-swimming whale sharks. Endangered Species Research. Vol. 50, March 23, 2023, p. 125. doi: 10.3354/esr01226.

Meekan, M. G., et al. "The world's largest omnivore is a fish." (2022): Ecology: 3818.  https://doi.org/10.1002/ecy.3818

https://www.sciencenews.org/article/shark-ocean-fish-population-recovery-extinction