Meet the bluefin tuna the toughest fish in the sea Grantly Galland and Raiana McKinney
What’s as big as a polar bear,
swallows its prey whole,
and swims at 40 miles an hour?
It’s not a shark or a killer whale.
It’s the Atlantic bluefin tuna.
The largest and longest-lived
of the 15 tuna species,
the Atlantic bluefin has a unique set
of adaptations
that make it one of the most dominant
predators in the ocean.
It starts as a tiny hatchling in the
Gulf of Mexico or the Mediterranean Sea,
no bigger than a human eyelash.
Within its first year of life,
It develops something known as
regional endothermy—
the ability to regulate
its body temperature.
An Atlantic bluefin gets oxygen
from cold ocean water using its gills.
This process cools its blood.
Then, heat the tuna generates swimming
and hunting warms the blood.
In most fishes, this heat would be lost
back out into the ocean through the gills.
But in the Atlantic bluefin,
a mechanism called countercurrent exchange
traps the heat.
Cold blood on its way
to the large swimming muscles
passes close to warm blood
leaving those muscles
in a specialized network of blood vessels
known as a rete mirabile.
Here the heat “jumps” to the cold blood
and stays in the body.
This makes bluefin one of the few
warm-blooded fishes,
a huge advantage
in the marine environment.
Cold-blooded animals whose
body temperature
depends entirely on the environment become
sluggish in colder waters.
But a bluefin’s ability to keep warm
means it has sharper vision,
can better process information,
and can swim faster than its prey.
It thrives in cold, deep, subarctic water.
Thanks to their warm bloodedness,
their powerful muscles,
and their streamlined torpedo shape
with fins that fold into grooves
to reduce drag,
bluefin tuna can reach speeds
few other animals can match.
Their maximum speed of 40 miles per hour
is faster than that of a great white shark
or orca whale,
and even at their comfortable
cruising speed,
they can cross the Atlantic
in a couple months.
All this swimming requires
a great deal of oxygen,
but the bluefin is adapted
for this as well.
The faster it swims, the more water
passes over its gills,
and the more oxygen it can absorb
from that water.
This need for a constant flow of water
means the tuna must always remain
on the move.
It also means bluefin cannot suck prey
into their mouths
the way most other fishes do.
Instead, they must chase down
their prey with their mouths open.
They eat smaller prey than most predators
their size,
including squid, crustaceans,
and smaller fish species like mackerel.
The bluefin’s temperature-regulating
ability
doesn’t just make it a superior hunter—
it gives it nearly unlimited range.
As soon as they’re strong enough to swim
against the current,
Atlantic bluefin leave the warm waters
of their spawning grounds
and spend their lives hunting
all over the Atlantic Ocean.
Tunas from both the Gulf of Mexico
and the Mediterranean Sea
frequent the same feeding grounds
and range from Brazil and Texas
to Iceland and Senegal and beyond.
But when the time comes
to reproduce around age 10,
they always return to their sea of origin.
Here, groups of males and females release
millions of eggs and sperm
into the water.
They’ll migrate back and forth between
feeding and spawning grounds
annually for the rest of their lives.
Atlantic bluefin can live
for over 40 years, growing all the while.
The largest specimens are tens of millions
of times heavier than when they hatched.
The same huge size that makes
bluefin tuna indomitable in the ocean
has made them vulnerable
to one predator in particular: us.
Humans have a long history
of fishing Atlantic bluefin—
it’s even stamped on ancient Greek coins.
But in recent decades,
demand has skyrocketed
as bluefin are hunted for sashimi, sushi,
and tuna steaks.
An individual fish can sell
for $10,000 or more,
promoting overfishing and illegal fishing.
But if recent conservation efforts are
redoubled and quotas are better enforced,
bluefin populations can begin to recover.