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White Shark, Bluefin Tuna & What Makes Them Top Predators - New TCD Research

1st July 2021
White shark: A white shark (Carcharodon carcharias) swimming at the surface with a biologging package attached to dorsal fin. This package records temperature, swimming speed, depth, body movement and video footage
White shark: A white shark (Carcharodon carcharias) swimming at the surface with a biologging package attached to dorsal fin. This package records temperature, swimming speed, depth, body movement and video footage Credit: Andrew Fox

Marine biologists have long wondered why some species like the white shark and bluefin tuna are warm-blooded when most fish are not.

Now newly published research led by a Trinity College Dublin (TCD) scientist has concluded that the ability to regulate body temperatures in this way gives these fish a competitive advantage as “elite” predators.

However, the study led by PhD candidate Lucy Harding in TCD’s school of natural sciences also confirms that this advantage doesn’t allow them to live in broader temperature ranges – meaning they are just as vulnerable to climate change.

The research, which is published today (Thurs July 1st) in the journal Functional Ecology (italics), found that white shark, bluefin tuna and other warm-water species can swim approximately 1.6 (one point six) times faster than their cold-blooded counterparts.

Only 35 species of fish are warm-blooded, but this is limited to parts of their bodies, Harding explained.

“Some can warm their brain, or their eyes or muscles or a combination of same – and the reasons for this will involve more research,” she said.

Up till now, it had remained unclear what advantages being warm-blooded provided to the particular fish, she said.

“Some scientists believed being warm-blooded allowed them to swim faster, as warmer muscles tend to be more powerful, while others believed it allowed them to live in a broader range of temperatures and therefore be more resilient to the effects of ocean warming as a result of climate change,” Harding added.

Tiger shark: A tiger shark (Galeocerdo cuvier) swimming at the surface with a biologging package attached to dorsal fin. This package records temperature, swimming speed, depth, body movement and video footage. Photo: Diego Camejo (Beneath the Waves).Tiger shark: A tiger shark (Galeocerdo cuvier) swimming at the surface with a biologging package attached to dorsal fin. This package records temperature, swimming speed, depth, body movement and video footage. Photo: Diego Camejo (Beneath the Waves).

The research team assessed these two possibilities by collecting data from wild sharks and bony fish, as well as using existing databases

Biologging devices attached to the fins of animals used in the research collected information such as water temperatures encountered by the fish in their habitats.

The data also recorded the speeds at which the fish swam for most of the day, and the depths of water most frequented.

“We noticed that these warm-blooded animals have the same characteristics – in that they tend to be shaped like a bullet, and have particular fin shapes and tail beats,” Harding said.

TCD assistant professor of zoology Nick Payne noted the faster swimming speeds of the warm-blooded fishes “likely gives them competitive advantages when it comes to things like predation and migration”.

“Additionally, and contrary to some previous studies and opinions, our work shows these animals do not live in broader temperature ranges, which implies that they may be equally at risk from the negative impacts of ocean warming,” he said.

“ Findings like these – while interesting on their own – are very important as they can aid future conservation efforts for these threatened animals,” Payne added.

The research was supported by Science Foundation Ireland.

Lorna Siggins

About The Author

Lorna Siggins

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Lorna Siggins is a print and radio reporter, and a former Irish Times western correspondent. She is the author of Everest Callling (1994) on the first Irish Everest expedition; Mayday! Mayday! (2004) on Irish helicopter search and rescue; and Once Upon a Time in the West: the Corrib gas controversy (2010).

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Sharks in Irish waters

Irish waters are home to 71 species of shark, skates and rays, 58 of which have been studied in detail and listed on the Ireland Red List of Cartilaginous fish. Irish sharks range from small Sleeper sharks, Dogfish and Catsharks, to larger species like Frilled, Mackerel and Cow sharks, all the way to the second largest shark in the world, the Basking shark. 

Irish waters provide a refuge for an array of shark species. Tralee Bay, Co. Kerry provides a habitat for several rare and endangered sharks and their relatives, including the migratory tope shark, angel shark and undulate ray. This area is also the last European refuge for the extremely rare white skate. Through a European Maritime and Fisheries Fund (EMFF) project, Marine Institute scientists have been working with fishermen to assess the distribution, diversity, and monthly relative abundance of skates and rays in Tralee, Brandon and Dingle Bays.

“These areas off the southwest coast of Ireland are important internationally as they hold some of the last remaining refuges for angel shark and white skate,” said Dr Maurice Clarke of the Marine Institute. “This EMFF project has provided data confirming the critically endangered status of some species and provides up-to-date information for the development of fishery measures to eliminate by-catch.” 

Irish waters are also home to the Black Mouthed Catshark, Galeus melastomus, one of Ireland’s smallest shark species which can be found in the deep sea along the continental shelf. In 2018, Irish scientists discovered a very rare shark-nursery 200 nautical miles off the west coast by the Marine Institute’s ROV Holland 1 on a shelf sloping to 750 metres deep. 

There are two ways that sharks are born, either as live young or from egg casings. In the ‘case’ of Black Mouthed Catsharks, the nursery discovered in 2018, was notable by the abundance of egg casings or ‘mermaid’s purses’. Many sharks, rays and skate lay eggs, the cases of which often wash ashore. If you find an egg casing along the seashore, take a photo for Purse Search Ireland, a citizen science project focusing on monitoring the shark, ray and skate species around Ireland.

Another species also found by Irish scientists using the ROV Holland 1 in 2018 was a very rare type of dogfish, the Sail Fin Rough Shark, Oxynotus paradoxus. These sharks are named after their long fins which resemble the trailing sails of a boat, and live in the deep sea in waters up to 750m deep. Like all sharks, skates and rays, they have no bones. Their skeleton is composed of cartilage, much like what our noses and ears are made from! This material is much more flexible and lighter than bone which is perfect for these animals living without the weight of gravity.

Throughout history sharks have been portrayed as the monsters of the sea, a concept that science is continuously debunking. Basking sharks were named in 1765 as Cetorhinus maximus, roughly translated to the ‘big-nosed sea monster’. Basking sharks are filter feeders, often swimming with their mouths agape, they filter plankton from the water.

They are very slow moving and like to bask in the sun in shallow water and are often seen in Irish waters around Spring and early Summer. To help understand the migration of these animals to be better able to understand and conserve these species, the Irish Basking Shark Group have tagged and mapped their travels.

Remarkably, many sharks like the Angel Shark, Squatina squatina have the ability to sense electricity. They do this via small pores in their skin called the ‘Ampullae of Lorenzini’ which are able to detect the tiny electrical impulses of a fish breathing, moving or even its heartbeat from distances of over a kilometre! Angel sharks, often referred to as Monkfish have a distinctively angelic shape, with flattened, large fins appearing like the wings of an angel. They live on the seafloor in the coastal waters of Ireland and much like a cat are nocturnal, primarily active at night.

The intricate complexity of shark adaptations is particularly noticeable in the texture of their skin. Composed of miniscule, perfectly shaped overlapping scales, the skin of shark provides them with protection. Often shark scales have been compared to teeth due to their hard enamel structure. They are strong, but also due to their intricate shape, these scales reduce drag and allow water to glide past them so that the shark can swim more effortlessly and silently. This natural flawless design has been used as inspiration for new neoprene fabric designs to help swimmers glide through the water. Although all sharks have this feature, the Leafscale Gulper Shark, Centrophorus squamosus, found in Ireland are specifically named due to the ornate leaf-shape of their scales.

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