Octopuses are clever, reclusive, dexterous, strong and slippery as heck—especially those belonging to the very largest species: the giant Pacific octopus (Enteroctopus dofleini). So how are researchers to catch and track them? Certainly not with traditional nets and tags, which the octopuses can (respectively) squeeze out of and rip off.
Instead, try enlisting the help of Alaska's cod fishermen, building a bunch of small plywood boxes, and mixing up some colorful, injectable silicone.
These are some of the unlikely ingredients that are part of ingenious new ways to conduct octopus population research.
As well studied as octopuses are, their population sizes, travel habits, lifespans and growth rates remain relative mysteries. And even though the giant Pacific octopuses are enormous (regularly caught at 10 to 20 kilograms—and occasionally reported to weigh more than 45 kilos), they are exceedingly difficult to count and to study in the wild. And even though they range from the Pacific Northwest northward and over to northern Japan, very little is known for sure about their population numbers.
To complicate matters more, giant Pacific octopuses are solitary characters that prefer to live in hidden dens along the seafloor. Fisheries biologists can use bottom trawls to collect other animals to count, but this method often misses octopuses altogether. So not only are they nearly impossible to tag with traditional external marking tags, the adults are also quite difficult to sample.
All of this means that "octopus in Alaska are giving us a big headache" when it comes to managing them, says Liz Conners, a National Oceanic and Atmospheric Administration researcher at the Alaska Fisheries Science Center.
This is where the fishermen, plywood boxes and brightly colored silicone come in. These unwieldy animals often come up as bycatch in the cage-like pods local fishermen use to catch cod. The fishermen either toss the big octopuses back to sea or keep them (to sell as bait or to processors to export as food). Reid Brewer, a marine biologist at the University of Alaska, Fairbanks, found that he could use these coincidentally caught octopuses for a larger tracking project thanks to the variously colored injectable silicone (called Visible Implant Elastomer), which he lodges in various patterns just under the mantle skin. These unique color combinations are then logged as an individual marker for the animal after it is measured and weighed and its location is noted—all before it gets tossed back into the water. The fishermen then know to look for and note the tags and locations of any marked octopus in the future.
But even with this subtle tagging method, wrestling a giant octopus is not always easy—even for practiced fishermen and researchers. "They come up fightin' mad and ready to put up a wrestling match," Conners says. "They're quite an armful." One ornery specimen stuck one of its strong arms down a hole on the boat, and it took four people to finally dislodge it, she recalls.
To execute more targeted studies of octopus, Conners has helped come up with a crafty method of bringing in large octopuses without having to rely on fishing bycatches. She and her colleagues have tried tossing out a variety of fake dens (attached to long lines) for octopuses to find and crawl into. They tried old tires, plastic boxes, plywood boxes and other enclosures. The local giant Pacific octopuses liked the plywood boxes best, Conners reports. "Now we at least have a tool we can use to go after more specific studies," she says, such as creating more accurate estimates for overall abundance and to find out how often octopuses that were caught and thrown overboard survive the ordeal.
None of the several octopus species in Alaskan waters are a targeted commercial species there—yet. So in the meantime, knowing more about them will help fisheries biologists know more about the catches that are bringing in the money, such as cod, crabs and pollock.
"All of the different species in the ecosystem are connected, so we need to keep track of the non-target species," Conners says, because the "dynamics of the non-target species can impact the target species."
Conners and others in Alaska anticipate that with growing demand for octopus as food, however, there will soon be regulations about how much even cod fishermen can keep among their incidental catches. The fisheries groups also need to know more about the local octopus population if people are ever going to start fishing for octopus specifically. "We're scrambling to get knowledge and regulation of octopus before direct fishing is allowed," she says.
The fishermen are generally happy to help out with the tagging and tracking effort, Conners says, because they know that the results will eventually help inform regulations for their octopus bycatches—and thus their ability to fish for cod, their main cash catch.
The researchers are hoping to get better information on these large, mysterious animals' underwater lives for basic science's sake, as well. Current estimates place the giant Pacific octopus's typical lifespan at only three to six years, but that is "mostly based on captive studies," Conners says. "It's pretty hard to get that information out of an octopus in the wild." Researchers can pretty reliably age fish by counting ring deposits on small bones called otoliths (much like counting the rings on a tree). But these bones in octopuses are quite small and in the giant Pacific octopus tend to be too soft to study with much certainty. Conners says they are working on ways to count rings deposited on the beak and stylet (part of the vestigial shell), but they haven't settled on a definitive growth rate for the giant Pacific octopus yet. These new studies should help settle these basic questions—and might be able to be extended to study other octopus populations and species around the globe.
"Octopuses have been widely studied in terms of physiology, but studying the dynamics of populations of octopus in the wild is still a wide-open field," Conners says.
The first results of the tracking study should be available in the next year or two, Conners says, and she hopes to have findings from her capturing work later this year.
Read more about NOAA Fisheries Service's giant Pacific octopus research here.
Illustration courtesy of Ivan Phillipsen