Aerial survey methods

The transects were laid out from the seashore to a depth of about 30 m. In the main part of the surveyed areas the transects were laid out with an interval of 4 km to make it economically possible to cover the intended area. In two special study areas (SW Sweden at Falsterbo and in Hanöbukten) the distance between survey lines was 2 km to get a higher resolution. In some areas with a narrow zone with shallow water transect lines were parallell to the shore line.

During the first surveys in 2007 - 2011, the surey lines in the archipelagos started at the larger islands covering the zone of smaller islands and skerries out to deep water. Formerly whereas the inner parts were  covered by full-cover surveys of all sutiable waters as done at the country-wide survey in 2004. In the country-wide survey of inshore species in 2015, it was not possible to cover all inshore areas, so the transetcs were extended over the inner areas. The full transects thus goes from the mainland out to deep water, but parts of the transetcs lines (inner, outer or toral) were covered depending on the project.

Normally the surveys were done by two observers, one in the front seat beside the pilote and one in the back seat. We concentrated the surveys on a main belt covering 200 m on each side of the aircraft. Outside this zone observations were recorded in two bands (B = 200 - 500 m, and C = 500 - 1000 m). We did not use distance sampling in the archipelagos as this method does not work in that environment.  In practice a zone of about 40 m on each side below the aircraft could not be covered. The observations were recorded on a continuously running tape-recorded, whereas the track of the flight was automatically recorded on a GPS. The observations were later connected with a geographical position in the data-base.