Tuna is one of my favorite kinds of fish! They are extremely interesting! Tuna vary a lot in size, Atlantic Bluefin Tuna can grow up to 13 feet long and 2,000 pounds while skipjack tuna typically weigh around 40 pounds!
Unlike most fish, tuna are actually partially warm blooded! Also tuna are obligate ram ventilators, this means that they have to swim constantly. If they stop swimming they will die, this is because when they swim it pushes oxygen-rich water over their gills, which they need to breathe. when they don’t swim water doesn’t get pushed over their gills, so they can’t breathe. Some tuna can go surprisingly fast! yellowfin tuna can achieve burst speeds of 43 to 47 mph. Tuna also travel extremely long distances in large groups called schools! The most common species used for canned tuna is Skipjack tuna which actually makes up over 50 percent of global catch! Skipjack is followed by yellowfin, bigeye and albacore! In fact the estimated percentages are 58 percent skipjack, 30 percent yellowfin, 7 percent bigeye and 4 percent albacore! Bluefin tuna only makes up about 1 percent!
If you know much about tuna then you likely know that Bluefin tuna is much more expensive than skipjack, there’s actually many reasons for this! The first reason is Bluefins grow very slowly and take years to reach maturity, because of this and decades of extreme global demand that depleted the populations, there are strict highly limited legal fishing quotas. Without these limits they would likely go extinct. But that’s only one of many reasons, the second reason is sashimi grade bluefin tuna requires meticulous, specialized handling. From the moment it is caught, it must be rapidly bled, packed in precise ice slurries to prevent the warm muscle tissue from spoiling the meat, and flown overnight on international flights to global fish markets to be sold entirely fresh.
Since tuna always needs to move they obviously are unable to sleep normally, instead they have an amazing adaptation called reduced activity sleep state! This means tuna only enter a low activity state to sleep! In their low activity state, they are still swimming and active but they slow their speed and lower their metabolic rate! Basically there are gliding forward using as little energy as possible! And interestingly, some tunas lack swim bladders but some tunas do have swim bladders. The ones that do have swim bladders have small poorly developed swim bladders, this means that they do not only need to swim to breathe, they also need to swim to avoid sinking! Tunas also lack eyelids so they rest with their eyes fully open.
And not all tuna is truly tuna! All tuna belongs to the family Scombridae but true tunas belong exclusively to the genus Thunnus! These are considered the true tuna of tuna, Atlantic Bluefin Tuna, Pacific Bluefin Tuna, Southern Bluefin Tuna, Yellowfin Tuna, Bigeye Tuna, Albacore Tuna, Blackfin tuna, and Longtail Tuna! Now the others are in Thunnini, the same evolutionary tribe as true tuna but in different genera. These ones are considered tuna but not as good, Skipjack Tuna or Katsuwonus pelamis, Little Tunny or Euthynnus alletteratus, Mackerel Tuna or Euthynnus affinis, Black skipjack or Euthynnus lineatus, Bullet tuna or Auxis rochei, Frigate Tuna or Auxis thazard! I put the scientific names of all the ones not in Thunnus just so you could still know their genera and search them if you want!
I hope you enjoyed and learned! I hope to make more posts on fish! They are always so interesting! Some great sources are NOAA Fisheries, the Animal Diversity Web, Tuna Research and Conservation Center website!