Hammerhead Sharks | JONATHAN BIRD’S BLUE WORLD

Hammerhead Sharks | JONATHAN BIRD’S BLUE WORLD


In the Galapagos Islands, during certain times
of the year, large aggregations of Scalloped
Hammerheads come together to circle around
Darwin Island. Scalloped Hammerheads normally
live in the open ocean, but here at Darwin
Island they come close to shore in large numbers.
Considering the vast numbers of fish here,
it would be logical to assume that the sharks
come to feed. Yet, nobody has ever witnessed
the sharks feeding. Instead they just seem
to be swim slowly by the reef. What are these
sharks up to?
I’m hoping to find out. I have come all the
way to the Galapagos Islands to investigate
these strange and magnificent sharks.
The Galapagos Islands are a remote archipelago
600 miles off the coast of Ecuador in the
Pacific Ocean.
Many incredible animals live here, both above
and below the water. It is famous for the
largest fish on Earth—the whale shark—which
inhabits these waters. But divers also flock
here to see hammerheads—and lots of them!
Hammerheads are most common at the northern
island of Darwin, famous for its stone arch.
Rolling into the water, my assistant Gator
and I head down to the reef to wait for the
sharks.
Soon a huge school is swimming by my lens.
Hammerheads are of course known for their
strange, flattened heads. But you have to
wonder…what is that flat head for?
Shark biologists put their own heads together
to figure it out. At first, they speculated
that it gave the hammerhead wider stereoscopic
vision. But that didn’t work: hammerhead
eyes face in opposite directions, meaning
they can’t see in stereo. The wide head
probably has nothing to do with eyesight.
So maybe it has to do with the sense of smell.
Wider spaced olfactory organs would help the
hungry shark sniff out prey more accurately.
Well, not really.
Although the nostrils and olfactory organs
are widely spaced, a groove along the front
of the head effectively connects them together.
Therefore, they don’t give the hammerhead
a better sense of directional smell than any
other shark.
Scientists finally hit upon a graceful reason
for the awkward head: It helps the hammerhead
turn faster by eliminating the need for large
pectoral fins.
Some sharks, like white tip reef sharks can
rest all day if they want, just gulping water
to ventilate their gills, the shark equivalent
of breathing.
Hammerheads, on the other hand, cannot accomplish
this. Like most pelagic sharks, they must
keep swimming not only to stay up off the
bottom, but to breathe. In an ironic twist
of fate, if a hammerhead stops swimming, it
will drown. The big flat head is a wing to
keep the hammerhead up off the bottom.
Researcher Kanesa Duncan at the University
of Hawaii is studying Scalloped hammerheads.
She catches the sharks when they are just
pups, only a few weeks old, in Kaneohe Bay
on Oahu.
She uses a hand line and a barbless hook baited
with squid to catch the baby hammerheads.
By holding the shark upside down, she places
it in a kind of trance called Tonic Immobility.
Many sharks can be immobilized for handling
in this way. Nobody knows why it works, but
it makes shark handling a lot easier.
She frequently puts the pup back in the water
to aerate the gills.
Once aboard the boat, she first measures the
shark while an assistant records the data.
KANESA: PCL is 38.3. Fork Length: 42.7. Total
length: 56.5. It’s going to be tag number
2315.
Next she implants a small visual tag with
a number on it through the shark’s dorsal
fin. This does not hurt the shark at all.
The fin is almost completely made of cartilage.
Next, she weighs the pup.
KANESA: 750 on the dot.
Finally, Duncan releases the baby hammerhead
back into the ocean. Hopefully, she will catch
it again in a few months and be able to determine
how much it grew.
Back at the lab, Duncan has several captive
hammerhead pups that she is working with to
determine growth patterns. She wants to learn,
among other things, how fast baby Hammerheads
grow.
But I still want to know what they are all
doing at Darwin Island in the Galapagos, and
I finally get a clue when I see a shark being
investigated by a fish.
The strong current at Darwin Island allows
a hammerhead to swim in place, like a runner
on a treadmill, yet hold a fixed position
over the reef.
King Angelfish make their homes on this reef
and they serve as cleaner fish for the sharks.
Because the sharks get cuts and scrapes, not
to mention parasites, that need cleaning,
they come to a cleaning station where a King
Angelfish is waiting to clean the wounds and
eat the parasites.
Everyone gets something out of the deal…the
angelfish get food and the sharks get cleaned.
The sharks never eat their cleaner fish…that’s
just considered bad form.
A shark looking to be cleaned often swims
at an angle, with its white belly showing.
This body language tells the angelfish to
come on over.
Angelfish looking to clean a shark swim up
and down in the water to attract the shark’s
attention.
Often, something seems amiss to the shark
and it rebuffs the fish. The sharks feel vulnerable
while they are being cleaned, so they pick
a cleaning station carefully. If anything
doesn’t seem right, they move on.
But patience pays off and eventually the right
shark meets up with the right fish in the
right place, and everything goes according
to plan. It’s love at first bite.
Sometimes a really grungy shark needs a whole
team of angelfish to complete the task.
Darwin Island is one of my favorite places
in the whole world to dive, and hammerheads
are one of the most beautiful sharks in the
ocean. But alas, the dive must come to an
end and we head back to the surface. I’m sad
to leave but happy to have learned the secret
behind the mysterious school of hammerheads
here in the blue waters of the Galapagos.

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