Project planning

Number of PODs needed

Some studies of behaviour may require only one POD, as comparisons can be made over many tidal, diurnal, fishing, etc. cycles. The effect on the project of the loss of a POD to trawling, or any other cause, needs to be considered.

Studies, such as Environmental Impact Assessments (EIA), that need to compare changes at different locations, generally need several locations to give an indication of sampling error and need to allow for the fact that parts of the control and impact sites may be subjected to other uncontrolled impacts caused by fishing, dumping, vessel moorings, etc. Also, some data may be missed due to errors in setting up PODs, electronic failures, etc.

Where T-POD servicing - changing batteries and uploading data - is costly, it is advisable to have a spare set of T-PODs sufficient to allow replacement of T-PODs at sea on any one trip. This saves the time required to upload data, which can be in excess of half an hour per T-POD and avoids the need to assess the data at sea to verify that all is well. It also allows the T-POD servicing to be carried out in an easier working environment. This means that no computer or software familiarity is needed at sea and the exchange of T-PODs may be carried out by a wider range of technical staff. T-PODs are designed so that they may be set up on land and then transported ready for deployment at sea.

Statistical design

Understanding which statistics to use has progressed since the start of POD-based studies when 'encounters', defined by intervening periods with no detections, were used in an attempt to count animals. Work by the National Environmental Research Institute in Denmark showed that the waiting time between porpoise encounters was a powerful statistic and they were able to show a rapid rise in encounters when pile-driving started and a rapid return to normal behaviour among the porpoises about 4 to 5 hours after the noise ceased.

Encounter rates work at low densities but fail as densities rise and encounters commonly run together and is no longer widely used. Counting trains, as a form of 'cue counting', introduces a behavioural element (rate of head movement) that is unwanted if the aim is to measure density of animals.

The percentage of detection positive minutes has proved to be a stable and 'well-behaved' statistic that is now widely used. The time period of one minute can be raised with some benefit provided it does not often exceed 30% and is always below 60%. This measure still includes some behavioural element that needs to be considered. Behavioural metrics are not yet well defined, although it is clear from a number studies that faster click rates are indicative of feeding activity.

EIAs need to compare changes over time at the control site with changes over the same time at the impact site. The method chosen needs to be able to accommodate gaps in the data.

Moorings

Please see Deployment and retrieval.

Calibration

Where comparison between PODs at different sites is needed, some users have carried out inter-calibration tests, either in the sea or in a tank. Sea tests are preferable where the density of animals is high, as it does verify the actual detection function. Please contact us for more details.

Familiarisation time

It takes most users three or four days to become familiar with the software, setting and handling of T-PODs.

Need help?

We offer a full project planning and project management consultancy service. For more information, see the Consultancy section.