Counting marine mammals from an aircraft

The researchers use a small twin-engine aircraft equipped with excellent bubble windows, allowing the two observers to look directly downwards and underneath the plane. A third person records the observations in real time and notes additional details, such as weather conditions. 

Conditions and method

The aircraft flies at a fixed altitude of 600 feet (183 metres), and surveys are only conducted when the sea state is below Beaufort 3. Counts of harbour porpoises (and other marine mammals) are carried out using the so-called Distance Sampling Line Transect Method, in which the aircraft follows pre-determined lines (transects) over the sea and the distance of each porpoise from that line is measured. 

When designing surveys and analysing data, variations in observations due to differing weather conditions, observers, and aircraft are taken into account, ensuring that the results are statistically robust. To estimate what proportion of porpoises along a transect are actually detected by observers, the racetrack method is used. In this method, the aircraft circles back after an observation to determine what proportion of the animals sighted can be seen again. 

Applications of aerial surveys

Aerial surveys of harbour porpoises are used to estimate the size of the harbour porpoise population in the Dutch North Sea. In addition, Wageningen Marine Research also conducts marine mammal surveys outside the Dutch North Sea, such as for the ObSERVE and SCANS projects.

Compared to ship-based surveys, the advantage of aerial surveys is that animals cannot swim far away before being counted. Furthermore, harbour porpoises, which are both inconspicuous and sensitive to disturbance, are easier to detect from the air than from a ship. Finally, an aircraft can cover a much larger area in the same amount of time than a vessel can. 

Quality assurance

Our researchers are experts in Distance Sampling Techniques and also provide training. They have experience with models that map the distribution of harbour porpoises. We also maintains the national database of harbour porpoise sightings.