body is deep, but streamlined. It has a metallic, deep blue
back, changing to yellow and silver on the belly. A yellow
band extends down the side, and the belly often has about
20 vertical broken lines, a characteristic found on no other
tuna, but not always noticeably present on yellowfin tuna.
Large fish are easily identified by the long crescent-shaped
extensions of the anal and second dorsal fins.
Yellowfin tuna are spawning and eating machines. In spite
of being fairly long-lived, some reach sexual maturity during
their first year, although most are age 2 or 3 when they first
spawn. The spawn several times a year in the open sea at temperatures
above 78º F. A 5½-foot long female can lay 8 million
eggs. Yellowfin tuna feed on a huge variety of finfish, squid,
shrimp, and crabs. They are very efficient sight hunters,
but can also actually smell their prey. Fish leave a scent
in the water made up of oils, proteins, and amino acids from
the slime layer on their bodies. Tiny traces of this wash
off of the fish. When yellowfin tuna pick up this scent trail,
they actually track down their prey.
Yellowfin tuna make both seasonal and daily migrations. In
the Pacific Ocean, they are often found on the edges of island
coral reefs during the day. Each night, they travel up to
9 miles offshore to feed and then return to the exact same
spot the next day. That's the equivalent, in human terms,
of walking 37 miles each night for supper. Tagging studies
on tunas in the open sea show similar behavior. A tuna will
hang around a floating log or other debris during the day,
travel long distances at night, and return to the exact same
log the next day.
All species of tuna share some interesting biological characteristics.
Fish in general are thought of as "cold-blooded".
That means that their body temperature is the same as that
of their environment. Tunas (and a few sharks) have developed
the ability to control their body temperature through a network
of veins and arteries called a "rete mirabile" that
traps (and dumps) body heat. Even smaller tunas can maintain
temperatures 50º F higher than surrounding water temperatures.
This is a huge advantage. For most cold-blooded fish, the
colder the water and therefore their body is, the slower and
more sluggish they are. Tunas' warmer body temperatures speed
up the chemical reactions in their body that produce energy
and allows their muscles to contract more quickly. This provides
faster swimming speeds and increases their endurance.
No other fish can swim as far or as fast as tunas. Water has
a great deal of resistance or drag, so every eight-fold increase
in swimming speed takes a 100-fold increase in energy. One
structure that reduces drag are the caudal peduncle keels
found near the tail fin. These keels reduce water turbulence
created by the tail fin and lower the drag by that part of
the body. Tunas also have a series of sail-like finlets on
the top and bottom of their body behind their fins. These
are thought to prevent the development of swirls of water
that would spin off the body and tail, allowing the tail fin
to work more efficiently in undisturbed water. The first dorsal
fin also folds down into a groove on the body to reduce drag
when the fish does not need it to maneuver.
Compared to other less active fish, tunas have hearts that
are ten times larger for their body weight, pump three times
more blood, and have blood pressure three times higher. They
also have a much higher proportion of red muscle in their
bodies than the average fish, which allows them to cruise
at higher speeds more efficiently. Tunas have been observed
to swim at 28 mph for long distances. Tunas also have gills
that are up to 30 times larger in surface area than those
of other fish. Additionally, tuna cannot open and close their
gill covers with their opercular muscles to force water over
their gills. Flaring gill covers would create drag. This means,
however, that tunas must swim or suffocate. They are "obligate
ram ventilators." They must swim through the water with
their mouths open to stay alive. Oxygen-bearing water is swept
over their gills purely due to the movement of the fish. In
fact, tunas must swim at a speed of 26 inches per second in
order to provide enough water flow to get the oxygen that