There’s a fish swimming around the oceans that almost every person (and cat) has heard of. Whether the word “tuna” brings back memories of lunch sandwiches, your favourite dive, or horrific customers at the restaurant where you used to work, there’s a lot more to know about these large schooling fish than how they look and taste. An important food source all over the world, tuna make huge migrations, can dive to depths greater than 1000m, and can withstand temperature changes that would stop a human heart.
Tuna is derived from the ancient Greek thyno meaning to dart and rush along, and the term is used to describe a number of genera including skipjack, Bluefin and yellowfin tunas. Their sizes range from 60cm to 4.6 meters. The largest tuna recorded was 881 pounds (400 kilos). Large, bluefin tuna can go for astronomical amounts of money in fish markets. One fish can cost over a million US dollars because of the high demand for tuna meat. With such a literally high price to pay, most Bluefin tuna species are at least at Endangered status on the IUCN list.
These fish can swim incredibly fast. Meaning, they can swim so fast that the water around them essentially boils. It forms vapor bubble cavities – called cavitation. Some tuna have been found with lesions on their tails caused by the bubbles hitting them. Usually they swim at a lower speed, not because this hurts them (they have bony fins with no nerve endings) but because the cavitation bubbles create a vapor film around their fins that limits their speed.
Tuna are among only a few fish species that are endothermic (warm blooded). They are able to conserve the heat that their muscles produce when swimming, allowing tuna to move through temperature changes and maintain high metabolic temperatures. They can use this to protect their reproductive organs from fluctuating temperature extremes while they dive. It also probably helps them swim so fast.
However, their hearts are still exposed to the cold ambient temperatures of the water around them. Tuna make use of their rete mirabile (wonderful net) to conserve heat during blood and gas exchange. Blood vessels flow in opposite directions very close to each other so warm blood can heat the cooler blood and maintain a central body temperature up to 20°C above ambient. They are the only bony fish to have this capacity. The heart however, receives blood straight from the gills which have been exposed to the cold water temperatures, so while tuna pass through cold water, the heart cools as well, sometimes up to 15°C in minutes! The drastic temperature changes would stop the heart of many animals, so scientists are still trying to find out how the tuna heart keeps pumping despite regular thermal shocks. They have found that tuna heart beat changes due to temperature and that adrenaline is involved when temperatures change quickly.
This helps the tuna during their great migration across the ocean. Many species of tuna are born on one side of an ocean and migrate to the other within their first year, exposing them to a wide range of water temperatures. The inherent nomadic behaviour of tuna also makes them very difficult to protect. They travel through many countries waters on their migrations (see diagram below), exposing them to changing regulations in catch protocol. They are highly regulated in one country, Australia for example, but once they travel into Japanese waters, they can be snatched up with much less concern for fishing practices or catch numbers and no other country can come in to stop it. International regulations are tricky and hard to enforce.
Some are calling for “Ocean sanctuaries” in areas across the oceans to give tuna time to recover their populations. But the only way multiple countries are going to agree to any regulations and enforcement is if people get involved to demand it. If consumers start demanding more sustainable practices and voters start demanding marine protection, only then will countries start to protect a widely loved, still highly unknown, physiologically unique fish species.
‘What you didn’t know about Tuna’ was written by Roya
Photo Credit: www.worldwildlife.org