The yellowfin
tuna is found throughout the Gulf of Mexico in the open sea
away from shore. Concentrations of the fish can seasonally
occur near diapers (lumps) in the northern Gulf of Mexico.
This fish 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.
They spawn several times a year in the open sea at temperatures
above 78° F. A 5–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 made up of tiny
traces of oil, proteins, and amino acids that wash off of
the slime layer on their bodies. When yellowfin tuna pick
up this scent trail, they 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. Tuna (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 tuna can maintain
temperatures 50°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 allows
them to swim faster and have increased endurance.
No other
fish can swim as far or as fast as tuna. Water has a great
deal of resistance or drag, so every eight-fold increase in
swimming speed takes a 100-fold increase in energy. The tuna
has physical characteristics to help it swim faster. The caudal
peduncle keels near the tail fin reduce water turbulence created
by the fin and reduce drag from that part of this large fish’s
body. Tuna 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, tuna 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.
Tuna
have been observed to swim at 28 mph for long distances. Tuna
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 tuna 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, tuna must swim at a
speed of 26 inches per second in order to provide enough water
flow to get the oxygen that they need.
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