Whale sharks killed, displaced by Gulf Oil
Brian Handwerk
The Gulf oil spill occurred in
crucial habitat for the world’s largest fish
The Gulf oil spill fouled a vital stretch of feeding habitat for
whale sharks, possibly killing some of the world’s largest fish, new
research suggests.
An estimated 4.9 million barrels of oil (one barrel equals 42
gallons, or 159 liters) flowed into an area south of the Mississippi
River Delta, where of one-third of all northern Gulf of Mexico whale
shark sightings have occurred in recent years, scientists say.
The 45-foot-long (14-meter-long) fish, still largely a mystery to
scientists, is considered a vulnerable species by the International
Union for Conservation of Nature.
A whale shark filters prey amid a school of cleaner fish file
photo Photograph by Colin Parker, My Shot |
“This spill’s impact came at the worst possible time and in the worst
possible location for whale sharks,” biologist Eric Hoffmayer, who
studies whale sharks at the University of Southern Mississippi’s Gulf
Coast Research Laboratory said.
Sightings confirmed that the animals were unable to avoid the slick
at the surface, where the giant fish may feed for seven to eight hours a
day. The oil may have clogged the fish’s gills, suffocating them, or it
might have contaminated their prey - though no dead whale sharks have
been found, Hoffmayer noted.
“We’ve seen aerial photos with animals within a few miles of the
wellhead and swimming in thick oil,” said Hoffmayer, a National
Geographic Society Waitt grantee. “At the end of the day, if these
animals were feeding in an area where there was surface oil, and if they
ingested oil, there is a good possibility that they died and sank to the
bottom. At this point we have no idea how many animals have been
impacted.”
Oil toxic to filter-feeding sharks Though much of the Gulf oil has
disappeared from the surface, the spill isn’t going away - and
scientists are still trying to uncover the extent of its invisible
effects on Gulf wildlife.
For instance, certain toxic ingredients of oil - and even the
chemical dispersants used during the cleanup - could potentially cause
long-term problems for whale sharks and many other species. Those may
include compromised endocrine or immune response systems, scientists
note.
Whale sharks filter a lot of water through their mouths and gills -
almost 160,000 gallons (605,000 liters) of water an hour - as they feed
on tiny plankton and fish. These sharks swim with their wide mouths open
to suck in plankton-rich waters, which they then force back out their
gills, retaining only tiny morsels of food.
“They would no doubt absorb contaminants even in dispersed form. Does
that build up in their tissues and affect their health?” Director of the
Center for Shark Research at Mote Marine Laboratory in Sarasota, Florida
and biologist Bob Hueter said.
To answer that question, many scientists are now searching for the
presence of polycyclic aromatic hydrocarbons (PAH) and other oil
contaminants in the blood and tissues of Gulf whale sharks.
“It will probably take years to see what the signature of this oil
does to the health and physiology of these animals,” Hueter said.
Oil driving sharks to new territories Scientists hope that tagging
animals can help them learn if the spill impacts whale shark behavior in
the years ahead.
But some observations have already suggested the whale sharks have
changed some of their habits.
Sightings of sharks near Florida’s Gulf coast have led to speculation
that the sharks and other large marine species may have been displaced
by the oil and moved on to a more pristine neighborhood.
During the summer months following the oil spill, Mote scientists
began chronicling repeated near-shore observations of large marine
animals, such as whale sharks, that are typically found in far deeper
waters out near the eastern Gulf’s continental shelf.
“This summer unusually high numbers and species of sharks were here
on the West Florida Shelf, and that includes whale sharks in much larger
numbers than we are accustomed to seeing,” Hueter said.
Those animals may have headed east to escape the oil, though no one
can say for sure. The team tagged several fish to track their future
movements in oiled waters and see whether the disaster causes lifestyle
changes in the whales.
Oil still unknown threat One problem is that no one is exactly sure
where the bulk of dispersed oil has gone, or in what form it exists. For
example, preliminary results suggest it’s settled on the seafloor or is
still suspended in remnant undersea plumes.
What’s more, whale sharks can be found everywhere in the water
column, from the surface to the depths, so pinpointing their possible
exposure to oil can be difficult.
“In some form or fashion, 60 to a hundred million gallons of oil are
still out there, and all we know is it’s not at the surface,” the
University of Southern Mississippi’s Hoffmayer said. “With this idea of
submerged oil out there, we don’t know what threats exist to the
animals.”
For instance, no one knows if the sharks will start to avoid the rich
feeding grounds to which the migratory animals have returned regularly
so far. “In coming years we’ll hopefully be able to say something about
the sightings, either that whale sharks appear to be impacted heavily,
or, we were lucky here and they haven’t missed a beat,” University of
Southern Mississippi’s Hoffmayer said.
Whale sharks undertake great migrations As scientists learn more
about the elusive whale shark, they’ve already discovered that the
impacts of the oil spill disaster could stretch farther than anyone
would have suspected just a few years ago.
That’s because seemingly disparate whale shark populations ranging
from the Caribbean and Central America to the Gulf of Mexico are
actually deeply connected, according to lead shark scientist with the
Wildlife Conservation Society’s Ocean Giants Program Rachel Graham.
National Geographic News |