Aspects of the natural history of Octopus dofleini, the giant Pacific octopus
Date
2020-09-29
Authors
Cosgrove, James Albert
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Abstract
A two part project spanning four and one half years was designed to investigate a number of aspects of the natural history of Octopus dofleini in the Northeast Pacific.
The tag-release recapture (Part I) involved weekly SCUBA dives, during daylight hours, at three sites on Southern Vancouver Island, British Columbia, Canada. A total of 320 octopuses were sighted during 616 dives (Part I) and 151 of those octopuses were tagged or recaptured.
The ratio of dives per octopus sighted was 1.93:1 and the ratio of dives per octopus tagged or recaptured was 4.08:1. A total of 98 octopuses were tagged (Part I) and 30 of those octopuses were recaptured two or more times for a recapture rate of 30.61%.
Statistical analysis of data weighted to provide uniform effort showed Site 1 (Tanner Rock) to have a significantly greater number of large octopuses while Site 2 (Tozier Rock) had a significantly greater number of very small octopuses.
An analysis of the data on octopus sex ratios found no significant difference between the number of males and females at each site even though there had been a significant difference in the ratio within Site 2.
Analysis of data on annual distribution of octopus body weight combined with the results from the recapture of individual octopuses led to the proposal of a four year lifespan for female Octopus dofleini and an undetermined, but longer, lifespan for males of the species.
An examination of the data on octopus movement showed that a peak number of small octopuses (approximately 250 g) appeared at the sites in February. The evidence showed that over the next 19 to 22 months the octopuses grew from approximately 250 grams to a pre adult body weight of 13.8 kg for males (in September) and 14.7 kg for females (in December).
No octopuses between 16 and 19.5 kg were captured suggesting a movement of octopuses of that body weight away from the sites. Small numbers of octopuses weighing between 19.5 kg and 28 kg were captured mainly during the first half of the year.
An analysis of octopus position data showed that Octopus dofleini does not maintain a constant distance from its nearest neighbour. Nearest neighbour distance was not significantly influenced by the size or sex of the neighbour nor by changes in water temperature. A highly significant correlation was found between estimated den volume and octopus body weight. There were no significant relationships between den surroundings, number of exits, den type and den depth when compared to den usage suggesting that Octopus dofleini is an opportunistic animal using whatever shelter of appropriate volume it discovers.
The major food item at all sites as determined by midden heap contents was the Red Rock Crab, Cancer productus.
An examination of the physical condition of each octopus captured revealed 21.2% of the animals had either scars or amputated arms or both.
Correlations examining the relationships between water temperature and growth rate, body weight, sex and nearest neighbour distance were nonsignificant.
Part 11 of the project was the physical measuring and describing of each den at each site. During the 126 SCUBA dives done another 127 octopuses were sighted. As octopuses were captured only when it was necessary to examine the inside of the den there were only 11 octopuses captured during the second part of the project.
In total, 447 octopuses were sighted during 742 dives. This results in an effort per octopus sighted of 1.66 dives per octopus.
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Keywords
Octopus dofleini, Cancer productus, Red Rock Crab, Tanner Rock, Tozier Rock, Vancouver Island, Northeast Pacific