Forensic examination of the effects of oceanic environments on faunal scavenging of cadaver proxies in two contrasting marine habitats

Medico-legal or forensic entomology is commonly employed in homicide investigations and a large body of literature now exists in North America and around the world on insect colonization of carrion in terrestrial environments. However, homicide victims are frequently dumped in the ocean and many commercial and recreational deaths also occur.  Due to the obvious difficulties, very little research has been conducted on the faunal colonization of carrion in the marine environment, making forensic interpretation difficult. This presentation reports recent results from an ongoing series of marine experiments conducted in the ocean near Vancouver, BC, as part of the Victoria Experimental Network Under the Sea (VENUS), a remotely operated underwater cabled laboratory, which allows for real time observation using a number of remotely controlled cameras and sensors.

Two freshly killed 20 kg pig carcasses were deployed as human proxies on the ocean floor in spring at each of two different marine habitats. The first was in the Strait of Georgia at a depth of 300m, a well oxygenated, major commercial and recreational waterway between Vancouver and Victoria, BC.  The second site was in the Saanich Inlet at a depth of 92m. This is a deep fjord, close to Victoria, which is hypoxic for much of the year and anoxic at some times of the year.

Both sets of carcasses were deployed on a platform which allowed one of each pair to be protected by widely spaced bars to allow entrance to arthropods and most fish but to protect against sharks. The second carcass was exposed. Each set was deployed under a tripod which carried an AXIS Q6034 720 p HD webcam directly above the carcasses. The camera video recorded each carcass for 2 minutes, every 15 minutes. Various sensors were attached to the tripod legs including an Aanderaa Oxygen Optode  Seabird CTD and Wetlabs Eco-NTUS 461. These recorded oxygen, temperature, conductivity, density, pressure, salinity, SigmaT, sound speed and turbidity every 60 seconds. Carcasses were observed and monitored for 6 months when the platform was retrieved and skeletal elements collected for further analysis.

Sharks (Hexanchus griseus Bonneterre) were immediately attracted to the carcasses in the Strait of Georgia but although they fed on the exposed carcass, they did not remove much tissue and the cage successfully protected the second carcass. No sharks were observed in Saanich. Arthropods (Decapoda) were attracted immediately to carcasses in both habitats, in particular, Dungeness crabs (Metacarcinus (=Cancer) magister Dana, squat lobsters (Munida quadrispina Benedict) and Three Spot Shrimp (Pandalus platyceros Brandt). Only in the deeper, well oxygenated waters, vast numbers of Orchomene sp. amphipods colonized the carcasses within 24 h and covered the carcasses and the surrounding area in a 2-3 cm layer, competitively excluding the larger arthropods. Entering via orifices and damage created by the sharks, the amphipods completely skeletonized both carcasses from under the skin within 4 days. Once the amphipods left, the larger arthropods returned to feed on the skeleton. The carcasses in the shallower Saanich area were slowly eaten by the three larger arthropod species with no amphipods attracted. Ocean chemistry was very different in the two environs, with dissolved oxygen levels driving much of the colonization and decomposition.