|Location:||Callister Field In the Otway Basin|
|Position:||South coast offshore Australia
38° 32' S 141° 28' E
|Water Temperature:||No data|
|Dates:||21 October - 1 November 2004|
|Gas & Oil Company||Santos|
Dr Adele J Pile, University of Sydney
The science goals for this project are to expand upon the baseline environmental surveys carried out in the area on behalf of Santos. Detailed ROV megafaunal video surveys will be carried out providing quantitative data on megafaunal ecology; particularly abundance, diversity and distribution in this area. This survey will allow a more detailed and targeted approach to be used for any subsequent surveys. In addition we will set up some artificial substrates to monitor settlement of organisms into the area.
This has been a highly successful mission as a result of 10 days of ROV operations. We completed six 100 m video transects for megafaunal abundance, diversity and distribution in the area. Transects were conducted randomly away from the BOP and extend to 100 m from the drill site. Videos will be analyzed for megafaunal diversity and habitat mapping during 2005. There is only one habitat type at Callister 1, soft bottom. There is no evidence of any rock formations or hard bottom of any type. Preliminary analysis indicates a low density and diversity of megafauna. The main component of the megafaunal community is hermit crabs, and we have collected one for identification. These crabs are responsible for the extensive bioturbation of the surface sediments as they leave trails behind. We have elected to forgo the deployment of the settlement plates as we will not be returning to this drill site. It can take months to years for a result to be obtained in this type of work so it was not appropriate to begin such a project if we were not going to be able to retrieve the experiment. The experiment is ready to deploy at the next drill site. Instead we have focused our attention to the roles that scavengers play in bioturbation of the sea bed. Our observations of the sea floor suggest that the main organisms in the area are hermit crabs. There is extensive evidence of their activities on the sea bed in the form of trails. This activity serves to mix the sediments and may be crucial in the redistribution of any drilling material. We have completed a variety of tasks designed to elucidate the effect of drilling on scavenger activity:
1. A pilot study to determine what organisms were making the trails.
2. A study that quantified scavenger activity within and way from drill spoil.
A pilot study to determine (1) what types of scavengers could be attracted to bait traps and (2) if the physical disturbance of drill spoil effected their activities was conducted 23-24 October 2004. Two types of bait traps were made. Fish fillets were placed in a laundry bag and attached to a lead weight with a lanyard for deployment. The second trap type consisted of two 600 ml water bottles attached to a lead weight with a lanyard for deploying. Bait for the second trap consisted of left over breakfast sausages and bacon. Two deployments were made, one at 66 m and a second at 12.5 m from the BOP. Each deployment consisted of two bottle traps and one bag trap and were visited each site every 2 hours to record the activity over 5 minutes. After 2 hrs of deployment there was intense activity at the 66 m sight. Five hermit crabs were attempting to get at the bait in both the bag and bottle traps. This level of activity was maintained over the next two hours. In contrast, there was no scavenger activity at the 12.5 m site over the first four hours of the deployment. Observations of the bottle traps indicated that no small scavengers such as amphipods or ophirioids (brittle stars) were present so the traps were left out over night. Observations the following day found evidence of intense activity at the 66 m sight and no scavenger activity at the 12.5 m sight. In fact, the bottle traps at the 12.5 m site had sunk into the drill spoil to a depth of nearly 30 cm and only the bag trap remained on the surface. Examination of traps collected from 66 m revealed that the crabs ripped the laundry bag to obtain access to the fish fillets. The water bottle traps were more effective as the crabs pulled the breakfast meats out of the bottles through their neck or by ripping at the cable tie attachments. There was a conspicuous lack of any small scavengers within the bottles from both the 66 m and 12.5 m sites.
We used this information to inform our experiment on the effect of drill spoil on scavenger activity. We decided to test the hypothesis that physical disturbance of the drill spoil will act as a barrier to scavenger activity. Additionally, we modified our trap design by reinforcing the attachment points with duct tape.
Locations of hermit crab activity associated with bait traps and sightings.
Experimental plots were established on 26 and 27 October 2004. Three sites were established at the headings of 210o, 120o, and 30o from the BOP. Within each site bottle traps were deployed at approximately 60 m (outside the drilling disturbance) and 3 to 5 m closer to the BOP at the demarcation of drilling spoil. Traps were examined every 24 hrs and video taped for 5 minutes. Presence of hermit crabs, their activities, and hermit crab trails were noted. If evidence of crab activity was present at the traps within the drill spoil then it was moved 5 m closer to the BOP, or further within the spoil. In addition, we noted the heading and distance from the BOP of hermit crabs around the study sight.
There was intense hermit crab activity at all of the 60 m sites. We usually saw hermit crabs at these sites during our video collection. This is incredible considering we were only there for 5 minutes each day. In addition, all the sights were well trampled by the crabs. Interestingly, there was also activity within the drill spoil. While it was not intense, most of the within the spoil traps were moved repeatedly closer to the BOP. The edge of activity appears to be related to the direction which drill castings were dispersed by the current. Sites at headings of 210o and 30o had activity at a range of 16 to 10 m from the BOP respectively. Whereas at a heading of 120o, or a current moving in a south east direction, the softness of the spoil became a physical barrier to the crabs at 45 m (Figure 1). On our final day of observations we found 5 hermit crabs at the BOP. These crabs had not been not lured by our bait traps as the nearest trap is 15 m from the BOP (Figure 2).
Preliminary analysis of the data suggests that drill spoil does not seem to be a significant barrier to hermit crab activity at Callister 1. We have video evidence that (1) hermit crabs can easily negotiate walking over spoil areas that our bottle traps became engulfed in and (2) given sufficient time crabs can recolonize areas surrounding the BOP. As the hermit crabs walk across the sediments they bulldoze the near surface layer. This activity is important for the incorporation and dispersion of the drill spoil. Hermit crabs are formable scavengers who seem not to be deterred from a potential food source by the physical disturbance associated with drilling activities.
Hermit crabs at the BOP on
1 November 2004
We will conduct the same habitat mapping and experiment on scavenger activity at the Amrit 1. This will allow us to compare biodiversity and processes between the shallow and deep locations. We will prepare a report for Santos and Transocean after analysis of all the data. We expect to publish these results in leading peer reviewed journals such as Marine Ecology Progress Series or Deep Sea Research.
For further information please contact:
Dr Adele Pile, SERPENT Project, Australia Region, University of Sydney. email@example.com
Thanks as always to our valued project partners