Photo by Dan Logan ©1996 |
In the
Octopus's Garden
by Julie Zeidner Russo
The sighting
of fantastic sea creatures like the giant octopus have been
portrayed throughout history. Probably the first recorded mention
of something that resembles an octopus was the monster Scylla
in Homer's Odyssey. At the dawn of a new millennium,
the giant octopus is not so much a folkloric figure as a culinary
delicacy for the Japanese, a staple in some Native American
diets, and a subject of rare scientific scrutiny.
There is still much to learn about the giant octopus beyond
its role in fiction or as a food source. Taste for the giant
octopus could make it subject to an eastern Pacific commercial
fishery before long. This increasing demand comes well in
advance of an understanding of how and where the giant octopus
lives.
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A wealth of information on what is known about the giant octopus
can be found at marine ecologist David Scheel's web site at http://www.pwssc.gen.ak.us/~dls.
Reading through the site, one learns a great deal about the largest
species of octopus in the world known as Enteroctopus dofleini.
Although the giant octopus rarely weighs more than 100 pounds (45
kg), a few large individuals have been recorded up to 400 pounds
(182 kg). While there are more than 100 species of octopuses in
the world, our knowledge of octopuses comes almost entirely from
a few species, Scheel said.
An intensive effort to learn more about the giant octopus commenced
after the Exxon Valdez oil spill impacted the area. The Prince William
Sound Science Center (PWSSC), a nonprofit research institute where
Scheel is an associate scientist, was established in response to
the environmental disaster.
| The institute's focus would be on ecological
research associated with the Prince William Sound and Copper
River Watersheds in Alaska. Their work would be important since
the Sound is one of the last major glacial carved embayments
on the northwestern edge of a temperate coastal zone with rainforest
biodiversity. "It's a really amazing area with a very complex
history of coastal peoples," Scheel said. |
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A coastal tradition was upset following the 1989 oil spill. The native
villagers in Tatitlek and Chenega Bay eat amikuq (the Alutiiq word
for octopus) as part of their subsistence lifestyle. They reported
that octopuses became scarce in the years following the 1989 oil spill,
Scheel said. A two-year study was launched by Scheel and co-investigators
to survey octopuses from the shoreline to 30 feet. The study was paid
for by the Exxon Valdez Oil Spill Trustee Council. The researchers
adopted the technique used by native villagers when harvesting octopus
that den under rocks and are exposed at low tide as a means of counting
and characterizing octopuses. They found densities an order of magnitude--10
to one--less than a survey done in British Columbia by scientist Brian
Hartwick in the '70s and early '80s, Scheel said. Researchers did
not have enough information about the giant octopus to attribute this
difference to the more northern study area in Prince William Sound,
or to a decline in abundance from man-made causes or from natural
fluctuations in abundance. "It's not clear there was any decline in
octopuses," said Scheel, noting more research would have to be done
to determine if there was a problem.
In 1996, researchers used SCUBA dives to further investigate where
and how the octopus lives and moves, and what it eats. They tagged
individuals with sonic tracking devices, and monitored their movements.
"SCUBA surveys proved that octopuses were more abundant on shallow
dives (to five m.) than on deep dives," Scheel said. The researchers
also found an abundance of juvenile octopuses, and very few adults.
Where had they gone? To determine whether depth is significant in
the ecology of the giant octopus researchers would need to dive
deeper.
NURP's Delta submersible by the ship Medeia
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The chance to go deeper in search of the adult
giant octopus occurred in May of 1998. This would be the first
time researchers ever provided descriptions of octopus habitat
below SCUBA depths. Scheel and ecologist Tania Vincent, also
of the PWSSC, made 27 dives in the Delta submersible with support
from the NOAA's Undersea Research Program (NURP). |
"Using the submersible, we were able to go down and find octopuses
too small to be caught by pots or long-line," Scheel said, "as well
as survey the habitats, the predators, and the prey base." Scheel
and Vincent would conduct 20 1000 m long transect surveys, three dives
tracking a sonic-tagged octopus, and four dives to resurvey areas
explored during the earlier SCUBA research and other sites of interest.
Giant octopuses have a unique life history that is only partly
understood. They live across the broad continental shelf of the
north Pacific ocean in a range extending from southern California
to the Pacific Northwest, across the Aleutians, and south to Japan,
Scheel said. They exist in shallow waters to 200 m, and may occur
deeper. What happens to them during their short life span of three
to five years is not fully known.
| Females lay as many as 100,000 eggs inside of
a rocky den and tend to them. "They senesce and die at about
the time the eggs hatch," Scheel said. The males, who move around
during the mating period, also die a few months later. On hatching,
baby octopuses enter the plankton till they get big enough to
settle down to the bottom again. After settlement, dens are
an important resource for the octopus. Dens are not only used
by the females for tending to their eggs, but serve as a place
to hide from other predators. |
A midden or refuse heap outside octopus den
Photo by David Scheel ©1998 |
"Marine mammals that feed on octopuses include seals, sea lions,
sea otters, and killer whales, at least," Scheel said. Their dens-nicknamed
the Octopus's Garden-also reveal a lot about them. Researchers study
the middens or refuse heaps outside the octopus den to learn more
about their diets, which include crustaceans, small crabs, scallops,
bivalves, snails, fish, and even other octopuses.
Prior to the NURP study, the researchers understanding of the
ecology of the giant octopus in rocky habitats of the eastern Pacific
came from surveys to 30 m. Determining the abundance of octopuses
below 30 m would be a key part of this study. Researchers theorized
that since octopuses were nearly absent from depths between 10 and
30 m. in Prince William Sound, but abundant in shallower waters,
that the very shallow areas might be important rearing habitat,
Scheel said. They would need data from deeper areas to compare.
There also appeared to be an abundance of octopuses living in heavy
kelp beds in shallow areas, which may provide shelter from predators.
"We thought octopuses may be restricted by high predation risk from
using habits at intermediate depths to 40 meters," Scheel said.
"If this were true, we anticipated that larger octopuses would be
more common in deeper water; and that there might be more octopuses
of all sizes."
A camouflaged octopus observed by researchers
Photo by David Scheel ©1998 |
What researchers discovered puzzled them. During
the Delta dives from10 to 200 meters, Scheel and Vincent only
found a total of 19 octopuses and only one was of adult size.
"Contrary to our expectations, we found that larger octopuses
were rare at any depth," Scheel said, "and although we found
octopuses at all depths to 200 m, we found no indication of
greater numbers of octopuses as we went deeper." However, they
did note changes in the denning habits and diets of the octopuses
at greater depths. |
In order to manage the octopus fishery, resource managers will
have to learn more about the natural fluctuations in octopus populations.
Japan already has a large commercial fishery for octopus. While
no commercial east Pacific fishery exists yet, interest is growing
in developing one by parties in Japan, Africa, Greece, Alaska, and
British Columbia.
"Fisheries scientists believe it would only take small changes
in market demand for octopuses to trigger an increased commercial
fishing effort," Scheel said. "This creates a possibility of over-exploitation
of the stock, a matter of particular concern in the Gulf of Alaska
coastal areas in light of damage to octopus habitat during the Exxon
Valdez oil spill in 1989 and the subsistence harvests of octopuses
by native peoples."
| Centuries later, the giant octopus still has the
capacity to intrigue us, but whether it can endure time before
it is fully understood is questionable. There must have been
other giant sea creatures that existed long ago that would have
certainly marveled us today had they not perished. This makes
understanding great animals that we know exist like the giant
octopus seem all the more precious, if not critical. "The solitary
nature and excellent camouflage abilities of the octopuses make
them hard to count," Scheel said. While this helps protect them
against natural predators, it makes it difficult for fisheries
managers to conduct stock assessments to determine how they
are faring and how they might be properly managed. |
Octopuses have excellent vision
Photo by David Scheel © |
"Managers are considering whether an assessment based on mapping suitable habitat would provide a means to assess the population," Scheel said. "However, even habitat mapping is limited by our understanding of octopus habit ecology.
This research should improve what is known about the giant octopus, so that enough information is available to prevent octopuses from being overexploited as a fisheries resource.
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