Johansen, K.L., Boertmann, D., Mosbech, A. & Hansen, T.B. 2012. Manual for seabird and marine mammal survey on seismic vessels in Greenland. 3rd revised edition, May 2012. Aarhus University, DCE – Danish Centre for Environment and Energy, 74 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 38 http://dce2.au.dk/pub/TR38.pdf
The method involves 300 m (or 500 m) wide strip transects operated only on one side the ship. Longitudinally, the transects are subdivided into so-called observation periods (e.g. 2, 5 or 10 minute intervals) and the sightings (flocks of birds/marine mammals) are grouped under these periods.
Due to the probability of detection decreasing with distance away from the observer, the transect strip is subdivided into narrower distance bands parallel to the track line of the ship. All birds on the water within 300 m (or 500 m) perpendicular to track line of the ship are counted and designated with a distance code according to the band in which they are observed.
To avoid an overestimate of flying birds, these are counted only by means of the snapshot technique. That is, at a certain moment in time (snapshot time), all birds flying over the transect strip within a certain distance ahead of the ship (snapshot distance) are counted quickly and designated with a special snapshot distance code. After making the snapshot count, the observer does not record flying birds again until the ship has travelled the snapshot distance at which point another snapshot count is made. In effect, the whole transect strip is covered with respect to flying birds, but they are counted only in instances.
As sightings of marine mammals are comparatively rare, they are treated a little differently. No matter where marine mammals are sighted (inside or outside the transect strip), they are always recorded. However, during active survey focus is directed to the 180° ahead of the ship. Further, the angle relative to the course of the ship and the direct distance in meters to marine mammals are always recorded.
This procedure constitutes the core of the methodology and results in systematic data suitable for estimating densities and spatial modelling. However, unsystematic observations of rare/interesting bird species outside the transect strip/snapshot may always be recorded if they are designated with the distance code corresponding to “off transect” (see section 5.2). In special situations the survey method is combined with so-called total counting (see section 5.1).