A zigzag survey design for continuous transect sampling with guaranteed equal coverage probability

Abstract

Marine resource surveys in large areas have high cost, and to find an optimal survey design with regard to efficiency and scientific outcome is an important issue. A randomized zigzag design for straight line and curved transects is developed that guarantees equal coverage probability, i.e., each point in the study area has the same probability of being covered by the transect. The basic idea is to fit automatically either the smallest rectangle, or the smallest circular sector enclosing the actual area. Then a recipe for the location of zigzag legs that provide equal coverage probability everywhere in the rectangle or circular sector is outlined, and thereby also at any location within the study area, which simplifies unbiased abundance estimation. The cost of this approach is the unwanted distance to be traveled from the point where a transect leg leaves the study area to the point where the next leg enters. A comparison of a randomized parallel, straight line zigzag, and curved zigzag approach applied to 7 sandeel areas with great variety revealed an average off-effort traveling distance of 28%, 9% and 6%, respectively. Thus, it appears that the developed zigzag design is far more efficient than the parallel design.