Shark Tagging Study Uncovers Hidden Movements
of Sharks
Fishers Collaborate by Returning Tags
(Credit Graham Shark Lab at SIO)
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Sharks inspire fear. Yet truly, they are among the sea's more vulnerable
species because they are so easily over fished. While science can
help protect these awesome creatures, many sharks are difficult
to study because their movements are basically invisible, and their
homes so vast. As part of a broad national effort to learn more
about sharks, California Sea Grant is funding researchers to tag
thresher sharks – a highly migratory species that "threshes"
small fish with its powerful sickle-like tail. The tagging data
is, for the first time, allowing marine biologists to learn where
thresher sharks live at different stages of their life, where they
go and what factors control their movements. This information has
shown, among other things, that fishing regulations designed to
protect marine mammals and sea turtles also alleviate pressure on
sharks.
(Credit Graham Shark Lab at SIO)
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The type of tagging data being collected during this Sea Grant
project is important because so much basic biological information
about sharks is missing, said NOAA Fisheries biologist Suzanne Kohin,
who is familiar with the project and involved in shark management.
Thresher sharks are believed to migrate seasonally, but only part
of their migration route is known. Nobody is sure where adults breed,
either. The habitat requirements of juveniles are yet another mystery.
“We know very little about their vertical and horizontal distributions,"
she said.
The lack of basic information on the life histories of sharks is
particularly significant because historically most commercial shark
fisheries have been unsustainable, said Jeffrey Graham of Scripps
Institution of Oceanography, the lead researcher on the California
Sea Grant project. Concerns about the sustainability of thresher
sharks, a secondary target of the commercial swordfish drift gill
net fishery, have been heightened recently by closures of fishing
grounds in the north Pacific Ocean to reduce "takes" of
endangered sea turtles. Biologists, including Graham, are concerned
the regulation will shift fishing effort to the Southern California
Bight, believed to be a nursery ground for thresher and mako sharks.
(Credit Graham Shark Lab at SIO)
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Graham is not at odds with fishers, however. In fact, he and Sea
Grant Trainee Dan Cartamil, a doctoral student in Graham's shark
laboratory, are relying on cooperation with the fishing community
to retrieve "archival" tags that have been placed on the
pectoral fins of about 70 thresher sharks. The pecan-sized tags
record a shark's depth and the temperature of water through which
it swims. Most importantly, the tags are able to collect data for
periods long enough to shed light on sharks’ seasonal habitat
preferences. The only downside: the only way to recover the data
stored on the tag's tiny waterproof chip is to retrieve the tag
and download its contents to a computer.
This is where fishers come in. Since they are the ones who catch
sharks, and the ones to see the tags, they are the ones who are
being asked to return the tags. Luckily, many fishers are willing
to help. "We are as interested as the scientists in learning
about sharks but for a different reason," said Mike Garret,
a sport fisher in Dana Point who recently earned a $100 reward for
returning a tag. (Each tag is inscribed with instructions to call
the shark lab.) "We want to target them more efficiently."
Garret said he believes better science can both help the resource
and fisheries. For his part, he is interested in learning more about
what threshers eat seasonally to better lure them with the right
bait. "We already know they feed on bait fish, but I've also
caught them on squid," he said.
Steve Fosmark, a longtime gillnetter from Pebble Beach, who sits
on the Pacific Fisheries Management Council's Highly Migratory Species
Advisory Subpanel, said he would like to know thresher’s migration
patterns to optimize his fishing operations and reduce fuel costs.
Although the tagging data is not yet sufficient to plot out the
sharks’ migration routes, Cartamil said has been able to reconstruct
their diving behaviors and to correlate these to the ocean's vertical
thermal structure. His analyses show that during the day, sharks
make repeated dives through the thermocline (an area where ocean
water temperatures change rapidly with depth) to depths up to 1,000
feet. At night, they stay within the "mixed layer," a
layer of relatively warm water at the surface, so named because
the water is well mixed by waves and wind. They do not dive at night
because, as visual predators, they presumably cannot see small schooling
fish on which they feed. Mako sharks have similar diving patterns,
though threshers may dive to greater depths, he said.
Acoustic tag. (Credit Graham Shark Lab at SIO)
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Computations based on the diving data suggest thresher sharks spend
about 95 percent of their nocturnal life at depths less than 36
feet, Cartamil said. The 36-foot mark is significant because it
is the minimum depth at which drift gill nets may be set in federal
waters. The regulation is designed to create a safe corridor for
marine mammals and sea turtles that otherwise are easily entangled
in the sometimes mile-long nets. The sharks’ nocturnal behavior
is key because gill netting is done at night.
(Credit Graham Shark Lab at SIO)
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The good news is that the net regulation also protects thresher
sharks, Cartamil said. But, it also means that raising nets slightly
could significantly increase the numbers of thresher sharks harvested.
As the research progresses, Cartamil said he hopes to learn more
about where young thresher sharks live. So far, all of the sharks
they have tagged have been sub-adults or adults. The absence of
juveniles in deeper waters has been conspicuous. One theory is that
baby thresher sharks live in nearshore waters, which if true means
that beach water quality, runoff, habitat degradation, and other
aspects of urban life could be impacting these animals. |
