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New Sonar Method Offers Window into Squid NurseriesTechnique provides a way to monitor the health of squid fisheries |
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| Enlarge ImageHordes of squid return each spring to waters off Monterey, Calif., to spawn and lay eggs. Fishermen may be catching large numbers of squid before they can complete their complex and competitive mating behaviors and subsequently lay eggs. (Courtesy of Roger T. Hanlon, Marine Biological Laboratory) |
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| Enlarge ImageSquid (Loligo opalescens)are a staple in the diets of many fish, birds, and marine mammals. They return annually to spawn a few hundred yards off Monterey’s Cannery Row (Courtesy of Roger T. Hanlon, Marine Biological Laboratory) |
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| Enlarge ImageSquid deposit gelatinous capsules, each containing 150 to 300 embryos. The finger-sized capsules form small clumps called mops that later cluster into egg beds up to several meters in diameter. (Courtesy of Roger T. Hanlon, Marine Biological Laboratory) |
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| Enlarge ImageStrips of sonar images are mosaicked together to create a composite map of the seafloor off Monterey, Calif., where squid gather each spring to spawn. (Courtesy of Kenneth G. Foote, Woods Hole Oceanographic Institution) |
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| Enlarge ImageThe sonar map reveals a mottled pattern that represents squid egg clusters, giving scientists a way to census next year’s potential squid population. (Courtesy of Kenneth G. Foote, Woods Hole Oceanographic Institution) |
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Every spring the swallows return to Capistrano, and farther up the
California coast, hordes of squid arrive to mate and lay eggs on the
sandy seafloor off Monterey. Right on their tentacles are fishermen
seeking calamari.
California’s $30-million-a-year squid fishery has
quadrupled in the past decade—with no way to assess how much fishing is
too much. Now, a multi-institutional team of scientists has reported a
new sonar technique to locate squid eggs in the murky depths.
“This
method provides an efficient way to map distributions and estimate the
abundance of squid eggs. In effect, it censuses next year’s potential
population,” said Kenneth G. Foote, a marine acoustics expert at Woods
Hole Oceanographic Institution and lead author of an article published
in the February 2006 Journal of the Acoustical Society of America.
“It has immediate potential to give resource managers sound scientific
information to make decisions on how to sustain the fishery. Otherwise,
they’re just guessing.”
The scientific team combined the expertise of
Foote; Roger T. Hanlon from Marine Biological Laboratory in Woods Hole,
an authority on squid behavior; and Pat J. Iampietro and Rikk G.
Kvitek, seafloor mapping experts at California State University
Monterey Bay (CSUMB). The research was funded by the Sea Grant
Essential Fish Habitat Program.
Mops and clusters
Squid (Loligo opalescens)—a
staple in the diets of many fish, birds, and marine mammals—return
annually to spawn a few hundred yards off Monterey’s Cannery Row. They
deposit gelatinous capsules, each containing 150 to 300 embryos. The
finger-sized capsules form small clumps called mops that later cluster
into egg beds up to several meters in diameter. Surveying the
four-square-mile spawning grounds with divers or underwater cameras is
too difficult, time-consuming, and expensive.
“We needed a way to
detect egg clusters,” Hanlon said. “So I went across the street and
approached Ken, one of the world’s sonar experts, who said, ‘I’ll see
if I can help you.’ ”
“I wasn’t optimistic,” Foote said. “You have to be
able to distinguish weak signals echoing off small targets—the egg
clusters—from powerful signals echoing off much larger targets—the
seafloor.”
The team tested different sound wave frequencies, sonar
angles, and towing speeds and heights off the seafloor—all in an effort
to optimize sound signals reflected back from the egg clusters. They
processed the raw acoustic data and displayed the results in the form
of seafloor maps that showed characteristic patterns of mottling—squid
egg clusters—distributed on the seafloor images. Using underwater video
and the CSUMB ship’s precise positioning capabilities, they verified
the correlation between sonar images and egg clusters.
Squid's complex sex life
With the new
sonar methods, the entire Monterey spawning area could be surveyed and
analyzed in less than 40 hours at relatively low cost, with a suitably
equipped boat towing a sidescan sonar, Foote said.
Since fishing usually
commences as soon as squid appear in Monterey Bay, Hanlon said,
fishermen may be catching large numbers of squid before they can
complete their complex and competitive mating behaviors and
subsequently lay eggs.
“The next challenge is to convince the fishermen
to wait a few days for squid to complete their mating cycle,” Hanlon
said, “so that gene mixing in the population will proceed by natural,
not unnatural, selection processes. The squid need to lay enough eggs
to provide squid next year—for fishermen and for the oceans, too.”
Foote and Hanlon said the sonar techniques could be adapted and applied
to more effectively manage other squid fisheries around the world,
including major fisheries in South Africa, Japan, the Falkland Islands,
southern California, and perhaps on the United States East Coast.
— Lonny Lippsett
Posted: March 15, 2006 [top] |
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