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Decomposition in the Ocean

By Dr. Gail Anderson, (SFU School of Criminology)

The decomposition of a body on land has been well researched and insect colonization patterns and rates are frequently used in homicide investigations to estimate elapsed time since death. However, very little research has been conducted on bodies in the ocean. VENUS offers a unique opportunity to not only observe a carcass after death in the ocean, but to actually be able to observe and analyze it in real time. Consequently, we are conducting a series of experiments on carcasses in the Saanich Inlet. Pig carcasses are used to model human bodies as they are similar in size to the human torso, have similar skin, are hairless and have gut flora and fauna that are very similar to our own. Therefore, they decompose similarly to humans, so this work can be directly applied to homicides and other deaths which occur in these waters.

Three pig carcasses have been deployed in Saanich Inlet at depths of 94-99 m. the carcass is weighted to keep it within camera range. Pig 1 was deployed in August 2006. It immediately attracted a number of animals including squat lobsters, Dungeness crabs and spot shrimp. Two days after it was placed on the ocean floor, a large piece of tissue was removed from the rump. This was not directly observed, but the bite mark left behind suggests that the culprit was a six-gill shark. From that moment on, the open bite wound became the site of feeding for most animals as it provided easy access to the flesh and organs. The carcass was hollowed out rapidly and the lower half of the body was eaten within days of deployment. The rear legs became disarticulated. No typical signs of decomposition were seen, but rather the carcass was consumed. No bloat or refloat would be expected at this depth due to pressure. Unfortunately, when the lower half of the pig was eaten, the weights slipped off and the pig was then moved by animal action. The remains were moved from site of the camera by Day 22. In November, a search was made for any remaining bones but nothing was recovered.

Pig 2 was deployed one year and a month later, in September 2007, at the same site. This study mirrored the previous study showing that the results were replicable. Again, the main scavengers were squat lobsters, Dungeness crabs and three spot shrimp. This time, there was no early shark bite, but the larger crabs rapidly attacked the stomach area and broke into the abdomen within 24 h of placement. Once the Dungeness crabs had broken through the skin, they and the smaller squat lobsters were seen to rip tissue and organs from the gut area. As before, once the tissue was opened, this area became the main site of feeding and little attention was paid to other areas. By Day 14, vast numbers of small amphipods were seen covering all open areas of the carcass. On Day 18, half the carcass, together with one weight, was again removed. It is suspected that this was again a six-gill shark. However, this time, due to better weighting, the rest of the carcass remained in camera view for the duration of decomposition. The animal was hollowed out by animal feeding and was left with skin loosely over the bones, like a shirt. This was eventually pulled over the head by squat lobsters to expose the bare bones. The carcass was skeletonized by Day 24. The last of the cartilage was removed from the bones by Day 42. The bones were later recovered and are undergoing further study.

The most obvious difference between the third study and the first two is the level of dissolved oxygen. The Saanich Inlet is known for its hypoxic or low oxygen conditions. Levels of dissolved oxygen below 2 mL/L are considered hypoxic and levels below 1 mL/L are considered stressful for most animals. In the Pig 1 study, the oxygen levels were above 2 mL/L for the first few days then dropped below 2 mL/L, but did not drop below 1 mL/L until part way through the study. In the Pig 2 study, the oxygen levels remained below 1 mL/L for most of the study. This was clearly not too low to prevent Dungeness crabs and shrimp from feeding on the remains. Towards the end of the study, when oxygen levels dropped very low, only the squat lobsters remained. These animals are better adapted to low oxygen conditions. But clearly, as the oxygen levels were higher at the start of both experiments, the larger scavengers were still attracted and contributed greatly to the breakdown of the body. The big difference with Pig 3 is that the carcass was deployed a little later, at the end of September so that the oxygen levels were very low when the experiment began. At the start of the study, the oxygen levels were at 0.5 mL/L, dropping below 0.1 mL/L at times. It is probable that these extremely low oxygen conditions have prevented the large scavengers from reaching the remains, and the few squat lobsters present are incapable of breaking the skin to feed on the tissue. By Day 31, a thick bio-film began to develop on the carcass, possibly made up of filamentous sulfur bacteria, due to the production of H2S from decomposition. Skin was sloughed off in places, probably due to the movement of slender sole. Suddenly, in late December, dissolved oxygen levels rapidly increased above the 2 mL/L level and large numbers of fish were seen in camera view. It took some days before arthropods returned, and the tissue by this time was not as attractive as fresh tissue, however, all the previously noted large arthropods were attracted and skeletonized the remains. Clearly, when conditions prevent the larger arthropods from breaking open the remains, the smaller arthropods, although more tolerant of very low oxygen conditions, were not able to feed on the carcass.

It is hoped that the next experiments will include an acoustic camera which will allow us to identify animals that are attracted to the carcass at any time, without having to turn on the lights. This would allow us to positively identify such visitors as the shark. We also plan to conduct further experiments at different depths and in higher oxygen levels. This work is ongoing. We are planning to deploy a fourth pig in the deeper more oxygenated waters of Georgia Strait, as we go to print.

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