Abstract
Acoustic monitoring of small cetaceans (harbour porpoises and dolphins) has been conducted around two platforms in the central North Sea. At one platform, there was continuous drilling activity while at the other platform, drilling did not occur during the monitoring period. The objective was to assess activity of harbour porpoises, Phocoena phocoena, around the platforms and assess the behavioural effects from sound emissions related to drilling activities.
Passive acoustic monitoring devices (PAMs – T-POD ver. 5) were deployed at both locations for a period of more than two years. The PAMs are able to detect high frequency sound from small cetaceans and thereby assess the activity of animals in the immediate area surrounding the platforms.
The results indicate that there is a relatively high activity of harbour porpoises around both the observed offshore platforms throughout the year. Clear seasonal trends in acoustic activities were not observed aside from increased activity levels during the fall of 2007 and in the late fall of 2008 at both locations. It is possible that these events are the results of a movement of harbour porpoises away from the calving grounds to the offshore areas. The offshore structures are known to attract fish and could function as feeding stations for the harbour porpoises, i.e. the platforms most likely function as artificial reefs. Hydrographic conditions at the platform locations that could influence the availability of prey in the area will naturally influence the activity level of the harbour porpoises. The study indicates that offshore platforms and drilling activities do not pose a significant threat to small cetaceans. However, the results show that short-term behavioural effects must be expected during activities that result in high sound intensity levels, e.g. during the ramming of conductors.
The study confirms that PAMs are a useful tool for monitoring behavioural changes of small cetaceans in offshore environments. However, rough weather conditions and the risk of collision with vessels present a challenge. Loss of equipment and consequently loss of data is very likely to happen. In addition, the costs of maintaining the equipment and retrieving the data are significant due to the remote location. Consequently, the use of PAMs should be carefully considered when assessing the purpose of the monitoring.
