The Elusive Giant Pacific Octopus
GIANT PACIFIC OCTOPUS (Enteroctopus dofleini) is the largest species of octopus in the world. It is found in the northern Pacific Ocean from the northwest coast of the continental United States to Japan, including waters off Alaska.
Octopus are not directly targeted by fishermen in Alaska, but they are occasionally captured in nets and fishing gear. They are found most often in pots used to fish for Pacific cod, where they often weigh more than 50 pounds. Most of the octopus caught in pot gear are thrown back alive into the sea, but some are kept for use as bait or sold to processors for human consumption.
As part of an effort to better understand marine ecosystems, scientists with NOAA Fisheries have recently started to monitor and manage all of the species sold by commercial fisheries, even those such as octopus that are not directly targeted. To properly set fishing catch limits for a species, scientists need to know two key things: how many of the species are present in the ecosystem and how fast they grow and reproduce. Unfortunately, we know very little about either of these things for octopus.
Capture and Tagging Studies
To learn more about this elusive animal, researchers from NOAA's Alaska Fisheries Science Center and the University of Alaska are studying ways to capture and tag octopus. Part of the research focuses on developing a fishing method directly tailored for octopus, since they are seldom caught by the usual research trawl nets.
The scientists exploit the octopus' love of hiding places by putting out artificial dens clipped to a long fishing line. The octopus crawl into these buckets or boxes and stay in them as the boxes are pulled to the surface. In an experiment funded by the North Pacific Research Board (NPRB Project 906), a variety of pot shapes and materials were tried out to see which capture the most octopus. Developing species-specific fishing gear will allow scientists to conduct further experiments and to monitor whether the octopus population changes over time.
|Check out some spectacular underwater footage of octopus. Click image to view the video. (For more cool video visit the NOAA Fisheries YouTube channel.)
|A small pot filled with a large octopus. Photo by Christina Conrath.
Biologist Reid Brewer of the University of Alaska Fairbanks works with a commercial fishing boat to study the movement and growth of octopus. He tags the octopus by injecting several dots of brightly-colored plastic under its skin. Each octopus gets a unique color sequence, then is carefully weighed and released. When a tagged animal is recaptured, the animal is weighed again and the time and place are recorded.
Results will allow him to estimate how many octopus are present in his study area, and what their natural rate of mortality is. Tagging information will also show where octopus move about during the year, and how fast they grow in the wild.
NOAA scientists at the AFSC's Kodiak laboratory are also studying the reproductive biology of Giant Pacific octopus and the effects of being captured and discarded. Animals contributed by pot fishermen or captured during gear experiments are taken to the laboratory, where measurements determine how close they are to reproducing.
The laboratory studies also look at how big the octopus get before maturing and how many eggs are produced by a single female. The condition of the octopus' reproductive organs at different times of the year is observed in order to find out which seasons are best for octopus to mate and lay their eggs.
Octopus reproduce only once in their life. Males die after mating and passing a sperm packet to the female; the female may wait several months before using the packet to fertilize her eggs. She attaches the eggs in strings to hang from the roof of a den and watches over them until they hatch, then she, too, dies.
By learning more about this elusive animal, scientists will be better able to provide management advice for commercial fisheries in Alaska, and prevent this mysterious species from being overexploited.