By EurekAlert
Tuesday, November 3, 2009
The white shark may be the ultimate loner of the ocean, cruising
thousands of miles in a solitary trek, but a team of researchers
has discovered that the sharks have maintained such a consistent
pattern of migration that over tens of thousands of years the white
sharks in the northeastern Pacific Ocean have separated themselves
into a population genetically distinct from sharks elsewhere in the
world.
"White sharks are a large, highly mobile species," said Salvador
Jorgensen, a postdoctoral scholar at Stanford's Hopkins Marine
Station. "They can go just about anywhere they want in the ocean,
so it's really surprising that their migratory behaviors lead to
the formation of isolated populations."
Scientists with the Tagging of Pacific Predators (TOPP) program
combined satellite tagging, passive acoustic monitoring and genetic
tags to study white sharks (Carcharodon carcharias) –
popularly known as great white sharks – in the North Pacific.
The team consisted of researchers from Stanford University,
University of California-Davis, Point Reyes Bird Observatory and
the Pelagic Shark Research Foundation, and the details of their
study are to be published online Nov. 3 by the Proceedings of
the Royal Society B.
The fact that the northeastern Pacific white sharks undergo such
a consistent, large-scale migration, and that they are all closely
related and distinct from other known white shark populations,
suggests that it is possible to conduct long-term population
assessment and monitoring of these animals.
Barbara Block, professor of marine sciences at Stanford and a
coauthor of the paper, said, "Catastrophic loss of large oceanic
predators is occurring across many ecosystems. The white sharks'
predictable movement patterns in the northeastern Pacific provide
us with a super opportunity to establish the census numbers and
monitor these unique populations. This can help us ensure their
protection for future generations."
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
The researchers used a combination of satellite and acoustic
tags to follow the migrations of 179 individual white sharks
between 2000 and 2008. These sharks were adults or sub-adults that
ranged in size up to 4,000 pounds, and were individually tagged at
sites along the central California coast, including the Gulf of the
Farallones, Tomales Bay and Año Nuevo. The electronic tags
reveal that the sharks spend the majority of their time in three
areas of the Pacific: the North American shelf waters of
California; the slope and offshore waters around Hawaii; and an
area called the "White Shark Café," located in the open
ocean approximately halfway between the Baja Peninsula and the
Hawaiian Islands.
"The thing we've learned about white sharks," said Scot
Anderson, a marine biologist with the National Park Service, who
has been involved in white shark research in Northern California
for more than two decades, "is that they're not swimming around
aimlessly – they are very selective predators." Anderson is a
coauthor of the paper.
Based on years of data demonstrating that the white sharks were
homing with high fidelity back to California, the research team
placed acoustic listening receivers on the ocean floor at sites
thought to be high residency areas, or "hot spots."
By attaching 78 acoustic tags that create a unique "ping" or
acoustic code for each tagged shark, the researchers were able to
detect when the white sharks came within 250 meters (820 feet) of a
receiver. This allowed the researchers to discern their pattern of
coastal movements in high detail. The acoustic-tagged sharks spent
time at four key sites along the central and northern California
coast, each of which supports large colonies of seals and sea
lions: Southeast Farallon Island, Tomales Point, Año Nuevo
Island and Point Reyes. The tags revealed that often sharks had
favorite sites where they would remain resident for up to 107 days,
although they occasionally would make brief visits to the other
nearshore hot spots.
"The sharks were detected frequently at their chosen site,"
Jorgensen said, "which means that they are patrolling around there
nearly constantly, for long periods of time. They will occasionally
visit one of the adjacent sites, but they always come back."
The team also was surprised to learn about new movements that
the acoustic tags revealed in some nearshore locations. They found
five white sharks were detected on acoustic receivers beneath the
Golden Gate Bridge that originally were installed to listen for
salmon, which migrate from the bay to the sea and back again. There
are currently no detectors in San Francisco Bay, so there are no
data to indicate how far or why the sharks crossed into the bay;
however, seals and sea lions are in the region and could be
potential prey for the large sharks. Five sharks also were
acoustically detected close to shore in Hawaii off Waialua Bay and
Kualoa Point on Oahu, and off the coast of Kona.
Genetics techniques were used to examine the relationships of
the California sharks to all other white sharks examined globally.
Studies of maternally inherited mitochondrial DNA sequences show
that the populations are distinct, and suggest that the
northeastern Pacific population may have been founded by a
relatively small number of sharks in the late Pleistocene –
within the last 200,000 years or so. The other populations of white
sharks are concentrated near Australia and South Africa.
Molecular geneticist Carol Reeb, a research associate at
Stanford, said, "If you had asked us a few years ago, we would have
said white sharks found in California probably migrated throughout
the Pacific. Now, even though we know they travel great distances,
their paths are surprisingly constrained to specific routes. This
explains how a highly migratory marine species becomes a
genetically isolated population. This also makes it much easier to
appreciate how vulnerable the northeastern Pacific white shark
population could become if too many individuals were taken as
either catch or bycatch, since these sharks do not appear to
interbreed with other shark populations."
SOURCE