AMIKUQ The Giant Octopus in Prince William Sound & Cook Inlet Amikuq is the Alutiiq word for octopus

©1996 Dan Logan

Field notes

As with most things in life, when we started this project, we did not know exactly what it would involve. Field work, for sure, and a bit of time to analyze the data and write the results. Since then, however, some of us visited Old Chenega for the first time, talked with Marie Smith (the last native speaker of the Eyak language), spent a week in the Chenega Bay school house when the winds were blowing too high to fly an airplane, met sea lions and wolf eels while diving, looked at a lot of beaches, got seasick (almost!) aboard the Tempest, ate octopus from Fedora's kitchen, and even spent the night in Mike Eleshansky's pantry!

Some of the sites that we studied are shown in the study area map. We also did some work in Port Graham. This report tells you what we found. We have tried to write it without technical language (but we are scientists, so it's hard to leave out all the Latin names and other fun tools of our trade).

What was this study about? In response to interest from native villages in the Exxon Valdez oil spill impacted area, we studied Giant Octopuses (Octopus dofleini). Octopuses are a part of the subsistence lifestyle in the villages, and people had noticed that the octopuses were harder to find in years following the oil spill. To address this concern, we decided to find out some basic things about the octopuses here: Where are they found? How many? Are they reproducing in the area? What do they eat? What eats them? We completed two years of work, studying areas in the intertidal and in shallow subtidal habitats that were accessible during low tides or by SCUBA diving. Due to the limitations of SCUBA, we could not work deeper than about 20-30 meters (100 feet).

What did octopuses eat?

Octopuses bring a lot of food back to their dens to eat; and when they are done, they toss the garbage out the door into a midden pile. The items shown in this section were collected from these piles.

Like many of us, octopuses like to eat crabs. People usually feast on large crabs like Dungeness (Cancer magister) or Tanner crabs (Chionoecetes bairdi). But you wouldn't find these crabs at the dinner table of an intertidal octopus in Prince William Sound. Most of their favorite crabs never get bigger than 5 cm and are usually smaller than the spelling of their name: Cancer oregonensis (right) or Lophopanopeus bellus (carapace and claw, below). The biggest crab these octopuses munch on is the helmet crab (Telmessus cheiragonus), which can grow to be a respectable 10 cm (4 in.) wide. Subtidal octopuses don't eat as many crabs as intertidal octopuses. They consume lots of scallops, which are plentiful near their subtidal homes (below 4.5 m or 14 ft deep).

Octopuses are notoriously hungry creatures, requiring enough food to grow up to 1.8% of their body weight per day. We found that octopuses eat up to 8 crabs per day and can grow as much as 3.58 kg (7.89 lbs) in 130 days. Despite their appetite, octopuses can be picky about their food. Do spinach, brussel sprouts or liver catch in your craw? This is apparently how octopuses feel about a certain crab (Acantholithodes hispidus), which we found on our surveys, but rarely in the midden piles of octopuses.

Drawings ©1997 Tania Vincent

How does an octopus get through the hard outer shell of clams and crabs? Octopus saliva softens the shell of their prey and the octopus scrapes away the partially dissolved shell with a special organ called the radula. In this way, the octopus drills through the shell of its prey. Once a pin-hole opening has been made, more saliva kills the prey and the octopus can pry it open. For some prey, such as small scallops, the octopus simply pries it open with its arms. For other prey, such as soft shelled crabs, the octopus bites through the shell using its beak. All these marks can be found on the remains of meals that octopuses discard outside their dens. At left, top is a drawing of a drill mark (scale bar is one millimeter), and left, bottom is a bite mark left by an octopus on a leg of the helmet crab (scale bar one centimeter). Remains of the Prey

Where did we find octopuses?

Most of the octopuses that we found (79%) were in the intertidal zone between +2 and -1.5 m deep. Octopuses were also more abundant on our shallow dives than they were on deep dives (top graph). In the intertidal (bottom graph), we found octopuses mostly found on soft substrates (mud, sand or gravel) rather than hard (bedrock, rock outcrops, large cobble fields); and where boulders were present rather than where they were absent. This makes sense, as the octopuses love to excavate their dens under the boulders. We also discovered that octopuses were associated with areas of dense kelp (still in the bottom graph. Open bars are the least kelp, solid bars the most kelp). We did not find more octopuses near eel grass beds.

We put sonic tags on five octopuses. The sonic tag emits a 'ping' and allows us to track the octopus with a directional hydrophone. After being tagged, the octopuses went to deeper water. However, they came back to the intertidal pretty quickly (see tracking map). Note that they spend most of their time in very shallow water.

These three things, the numbers of octopuses in the intertidal, the higher numbers on shallow dives compared to deep dives, and the return of disturbed individuals to the intertidal, all suggest that octopuses in Prince William Sound prefer intertidal habitats and use them regularly.

What eats octopuses?

Just about anything that can find them. For example, octopus parts were found in the stomachs of 26% of 43 fish species examined in a 1995 study in the Sound (see graph). Harbor seals in the Sound also eat octopuses, as do sea otters.

We found more octopuses in shallow water and near heavy kelp cover. Sonic-tagged octopuses that left the intertidal after we disturbed them returned to intertidal areas with heavy kelp. This suggests that dense broad-leaf kelps were an important feature of octopus habitat. Why did octopuses like kelp?

We found no evidence that there was more food in shallow water nor in the kelp. However, kelp may provide cover to octopuses, keeping them out of view of their predators. Living in the intertidal might be a way to avoid predators because predators like the big fishes do not spend time in very shallow areas. Even sea otters, although they can feed in the intertidal, feed about 95% of the time in water from 8 to 34 meters deep.

Octopus densities in our study were only one tenth as high as densities in British Columbia (where the only data for comparison were from). Most predators that we have here are also down there, yet in British Columbia the octopuses are not found near kelp beds or in particularly shallow water. However, while sea otters were exterminated in both areas, the two sites differ in that otters have returned to Prince William Sound, but were still absent from British Columbia. We think that intertidal habitats in the Sound provide refuge from sea otter predation, and octopuses here may be limited in distribution to depths where otters do not forage.

CONCLUSIONS

Several results from this project might be considered by fishermen who harvest octopuses. First, about 80% of the octopuses that we found occurred in shallow water habitats from the intertidal to -5 m; only 20% were found in deeper water to -30 m. Second, octopuses in the shallow habitats stayed there: they may only move a few hundred meters over the course of a year. So, intertidal and very shallow habitats may provide the necessary rearing habitat for octopuses that settle there. Harvesting in the intertidal may draw from a confined strip of habitat extending only as deep as -5 m. However, octopuses made short trips from the intertidal to the very shallow subtidal in our study, so that octopuses harvested from intertidal dens may be quickly replaced by octopuses from adjacent subtidal habitat. It is unlikely, however, that this pool of octopuses extends to depths from -5 to -30 m.

All stages of the octopus life cycle, except for eggs, have been reported from Prince William Sound, so we think it likely that a breeding population exists there. Given the prevailing notion that octopuses have become scarce recently, we suggest that fishermen refrain from harvesting individuals 15 kgs and larger, as these animals may be close to breeding size.

Sea otters compete with octopus for both bivalves and crabs and sea otters also prey on octopuses. Therefore, sea otters may be limiting both the distribution and abundance of octopuses in Prince William Sound and Port Graham. If so, this would suggest that octopuses living below otter foraging depths (rarely deeper than -30 m) may be more plentiful than at depths from -5 to -30 m where most otters forage.

Where are all the octopuses?

Finally, the low octopus densities recorded in this study deserve consideration. Native villagers in the Sound and Cook Inlet have noted a decline in Octopus dofleini abundance, and voiced concern about the status of octopus following the Exxon Valdez oil spill. Elsewhere along the east Pacific coast, several aquariums, where the Giant Octopus is a popular exhibit, complained about being unable to get octopuses. Declines during the early 1990s in the Japanese fishery have raised concerns about O. dofleini in that country also. Taken alone, none of these data are more than suggestive. For example, densities from Alaska and British Columbia are unlikely to be directly comparable. Cumulatively however, these observations raise the question of whether O. dofleini has been declining throughout its range during the 1990s. This question could not be answered by this study, as no long term data were available for comparison.

Who is the Octopus Project?

This project was supported by the Exxon Valdez Oil Spill Trustee Council and Project Aware. A complete report has been provided to the EVOS Restoration Office and will be available from the Oil Spill Public Information Center after review.